View source for Engineering:Light-emitting diode

{{short description|Semiconductor and solid-state light source}}
{{Infobox electronic component
| name              = Light-emitting diode
| image             = 3 color LEDs.jpg
| upright           =  1.136
| caption           = LEDs in red, green, and blue. The LEDs in this picture are housed in "regular" cases, but there are many variants of LEDs.
| working_principle = [[Physics:Electroluminescence|Electroluminescence]]
| invented          = {{ubl|[[Biography:H. J. Round|H. J. Round]] (1907)<ref>{{cite web|url=http://www.myledpassion.com/History/hj-round.htm|title=HJ Round was a pioneer in the development of the LED|website=www.myledpassion.com|access-date=April 11, 2017|archive-date=October 28, 2020|archive-url=https://web.archive.org/web/20201028074225/http://www.myledpassion.com/History/hj-round.htm|url-status=dead}}</ref>|[[Biography:Oleg Losev|Oleg Losev]] (1927)<ref name="100-YEAR HISTORY">{{cite news| url=http://holly.orc.soton.ac.uk/fileadmin/downloads/100_years_of_optoelectronics__2_.pdf| title=The life and times of the LED — a 100-year history| date=April 2007| agency=The Optoelectronics Research Centre, University of Southampton| access-date=September 4, 2012| url-status=dead| archive-url=https://web.archive.org/web/20120915034646/http://holly.orc.soton.ac.uk/fileadmin/downloads/100_years_of_optoelectronics__2_.pdf| archive-date=September 15, 2012| df=mdy-all}}</ref>|James R. Biard (1961)<ref>[http://www.freepatentsonline.com/3293513.pdf US Patent 3293513], "Semiconductor Radiant Diode", James R. Biard and Gary Pittman, Filed on Aug. 8th, 1962, Issued on Dec. 20th, 1966.</ref>|[[Biography:Nick Holonyak|Nick Holonyak]] (1962)<ref name="LEMELSON-MIT">{{cite news|url=http://web.mit.edu/invent/n-pressreleases/n-press-04LMP.html |title=Inventor of Long-Lasting, Low-Heat Light Source Awarded $500,000 Lemelson-MIT Prize for Invention |date=April 21, 2004 |agency=Massachusetts Institute of Technology |access-date=December 21, 2011 |location=Washington, D.C. |url-status=dead |archive-url=https://web.archive.org/web/20111009111042/http://web.mit.edu/invent/n-pressreleases/n-press-04LMP.html |archive-date=October 9, 2011 }}</ref>}}
| first_produced    = {{start date and age|1962|10|}}
| symbol            = [[File:IEEE 315-1975 (1993) 8.5.4.2.svg]]
| pins              = [[Physics:Anode|Anode]] and [[Physics:Cathode|cathode]]
}}
[[File:LED, 5mm, green (en).svg|thumb|Parts of a conventional LED. The flat bottom surfaces of the anvil and post embedded inside the epoxy act as anchors, to prevent the conductors from being forcefully pulled out via mechanical strain or vibration.]]
[[File:Surface mount LED close up image.png|thumb|Close-up image of a [[Engineering:SMD LED|surface mount LED]]]]
[[File:LED Operation.ogg|thumb|Close-up of an LED with the voltage being increased and decreased to show a detailed view of its operation.]]
[[File:Br20 1.jpg|thumb|alt=Modern LED [[Physics:Green retrofit|retrofit]] with E27 screw in base|A bulb-shaped modern retrofit LED lamp with aluminum heat sink, a light diffusing dome and E27 screw base, using a built-in power supply working on mains voltage]]

A '''light-emitting diode''' ('''LED''') is a [[Physics:Semiconductor|semiconductor]] [[Engineering:Electronics|device]] that [[Physics:Light#Light sources|emits light]] when [[Physics:Electric current|current]] flows through it. [[Physics:Electron|Electron]]s in the semiconductor recombine with [[Physics:Electron hole|electron hole]]s, releasing energy in the form of [[Physics:Photon|photon]]s. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the [[Physics:Band gap|band gap]] of the semiconductor.<ref>{{cite web |url=http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGLED.pdf |work=[[Organization:University of California, Irvine|University of California, Irvine]] |access-date=12 January 2019 |title=Light Emitting Diodes |last=Edwards |first=Kimberly D. |page=2 |archive-date=February 14, 2019 |archive-url=https://web.archive.org/web/20190214175634/http://faculty.sites.uci.edu/chem1l/files/2013/11/RDGLED.pdf |url-status=dead }}</ref> White light is obtained by using multiple semiconductors or a layer of light-emitting [[Physics:Phosphor|phosphor]] on the semiconductor device.<ref>{{cite web |url=https://www.lrc.rpi.edu/programs/nlpip/lightinganswers/led/whitelight.asp |title=How is white light made with LEDs? |work=[[Organization:Rensselaer Polytechnic Institute|Rensselaer Polytechnic Institute]] |author=Lighting Research Center |access-date=12 January 2019 |archive-date=May 2, 2021 |archive-url=https://web.archive.org/web/20210502084248/https://www.lrc.rpi.edu/programs/nlpip/lightinganswers/led/whiteLight.asp |url-status=dead }}</ref>

Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity [[Physics:Infrared|infrared]] (IR) light.<ref name=FirstPracticalLED /> Infrared LEDs are used in [[Engineering:Remote control|remote-control]] circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red.

Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in [[Engineering:Seven-segment display|seven-segment display]]s. Later developments produced LEDs available in [[Physics:Visible spectrum|visible]], [[Physics:Ultraviolet|ultraviolet]] (UV), and infrared wavelengths with high, low, or intermediate light output, for instance, white LEDs suitable for room and outdoor lighting. LEDs have also given rise to new types of displays and sensors, while their high switching rates are useful in advanced communications technology with applications as diverse as [[Engineering:Navigation light|aviation lighting]], [[Engineering:Christmas lights|fairy lights]], strip lights, [[Engineering:Automotive lighting#Light-emitting diodes (LED)|automotive headlamps]], advertising, [[Earth:Lighting|general lighting]], traffic signals, camera flashes, [[Engineering:LED wallpaper|lighted wallpaper]], [[Earth:Grow light|horticultural grow lights]], and medical devices.<ref name="Aguilar">{{Cite book|pmid=18002450|doi= 10.1109/IEMBS.2007.4352784|year= 2007|last1= Peláez|first1= E. A|volume= 2007|pages= 2296–9|last2= Villegas|first2= E. R|title= 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society|chapter= LED power reduction trade-offs for ambulatory pulse oximetry|isbn= 978-1-4244-0787-3|s2cid= 34626885}}</ref>

LEDs have many advantages over incandescent light sources, including lower power consumption, a longer lifetime, improved physical robustness, smaller sizes, and faster switching. In exchange for these generally favorable attributes, disadvantages of LEDs include electrical limitations to low voltage and generally to DC (not AC) power, the inability to provide steady illumination from a pulsing DC or an AC electrical supply source, and a lesser maximum operating temperature and storage temperature.

As a [[Engineering:Transducer|transducer]] of electricity into light, LEDs operate in reverse of [[Physics:Photodiode|photodiode]]s.

==History==
===Discoveries and early devices===
[[File:SiC LED historic.jpg|thumb|Green electroluminescence from a point contact on a crystal of SiC recreates Round's original experiment from 1907.]]

[[Physics:Electroluminescence|Electroluminescence]] as a phenomenon was discovered in 1907 by the English experimenter [[Biography:H. J. Round|H. J. Round]] of [[Company:Marconi Company|Marconi Labs]], using a crystal of [[Chemistry:Silicon carbide|silicon carbide]] and a cat's-whisker detector.<ref>{{Cite journal|author=Round, H. J.|year=1907|title=A note on carborundum|journal=Electrical World|volume=19|page=309|url=https://books.google.com/books?id=TAZRAAAAYAAJ&pg=PA309}}</ref><ref>{{cite web|url=http://www.jmargolin.com/history/trans.htm|author=Margolin J|website=jmargolin.com|title=''The Road to the Transistor''}}</ref> Russian inventor [[Biography:Oleg Losev|Oleg Losev]] reported the creation of the first LED in 1927.<ref name="Losev1927">{{cite journal|last1=Losev|first1=O. V.|title=Светящийся карборундовый детектор и детектирование с кристаллами|journal=Телеграфия и Телефония без Проводов [Wireless Telegraphy and Telephony]|date=1927|volume=5|issue=44|pages=485–494|trans-title=Luminous carborundum detector and detection with crystals|language=ru}} English translation:  {{cite journal|last1=Losev|first1=O. V.|title=Luminous carborundum detector and detection effect and oscillations with crystals|journal=Philosophical Magazine|date=November 1928|volume=5|issue=39|pages=1024–1044|doi=10.1080/14786441108564683|series=7th series}}</ref> His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades, partly due to the very inefficient light-producing properties of silicon carbide, the semiconductor Losev used.<ref name="Zheludev_100yearhistory">{{Cite journal |author = Zheludev, N. |year = 2007 |title = The life and times of the LED: a 100-year history |journal = Nature Photonics |volume = 1 |issue = 4 |pages = 189–192 |url = http://www.nanophotonics.org.uk/niz/publications/zheludev-2007-ltl.pdf |doi = 10.1038/nphoton.2007.34 |bibcode = 2007NaPho...1..189Z |access-date = April 11, 2007 |archive-url = https://web.archive.org/web/20110511132457/http://www.nanophotonics.org.uk/niz/publications/zheludev-2007-ltl.pdf |archive-date = May 11, 2011 |url-status     = dead |df = mdy-all}}</ref><ref>{{cite book|author=Lee, Thomas H. |title=The design of CMOS radio-frequency integrated circuits|url=https://archive.org/details/designcmosradiof00leet_307 |url-access=limited |publisher={{wipe|Cambridge University Press}}|year=2004|isbn=978-0-521-83539-8|page=[https://archive.org/details/designcmosradiof00leet_307/page/n18 20]}}</ref>
 
In 1936, [[Biography:Georges Destriau|Georges Destriau]] observed that electroluminescence could be produced when zinc sulphide (ZnS) powder is suspended in an insulator and an alternating electrical field is applied to it. In his publications, Destriau often referred to luminescence as Losev-Light. Destriau worked in the laboratories of Madame [[Biography:Marie Curie|Marie Curie]], also an early pioneer in the field of luminescence with research on [[Chemistry:Radium|radium]].<ref name="ChemiePhysique">{{Cite journal|author=Destriau, G.|year=1936|title=Recherches sur les scintillations des sulfures de zinc aux rayons|journal=Journal de Chimie Physique|volume=33|pages=587–625|doi=10.1051/jcp/1936330587
}}</ref><ref name="ConciseEncyclopediaOfPhysics">McGraw-Hill Concise Encyclopedia of Physics: electroluminescence. (n.d.) McGraw-Hill Concise Encyclopedia of Physics. (2002).</ref>

Hungarian Zoltán Bay  together with György Szigeti  pre-empted LED lighting in Hungary in 1939 by patenting a lighting device based on silicon carbide, with an option on boron carbide, that emitted white, yellowish white, or greenish white depending on impurities present.<ref>{{Cite web|url=https://www.iitk.ac.in/solarlighting/files/brief_history_of_LEDs.pdf|title=Brief history of LEDs}}</ref> [[Biography:Kurt Lehovec|Kurt Lehovec]], Carl Accardo, and Edward Jamgochian explained these first LEDs in 1951 using an apparatus employing [[Chemistry:Silicon carbide|SiC]] crystals with a current source of a battery or a pulse generator and with a comparison to a variant, pure, crystal in 1953.<ref>{{Cite journal|year=1951|title=Injected Light Emission of Silicon Carbide Crystals|journal=Physical Review|volume=83|issue=3|pages=603–607|url=http://www.campevans.org/_CE/html/tpr-08-01-1951p604-lehovec.html|doi=10.1103/PhysRev.83.603|url-status=dead|archive-url=https://web.archive.org/web/20141211015103/http://www.campevans.org/_CE/html/tpr-08-01-1951p604-lehovec.html|archive-date=December 11, 2014|df=mdy-all|bibcode=1951PhRv...83..603L|last1=Lehovec|first1=K|last2=Accardo|first2=C. A|last3=Jamgochian|first3=E}}</ref><ref>{{Cite journal|year=1953|title=Injected Light Emission of Silicon Carbide Crystals|journal=Physical Review|volume=89|issue=1|pages=20–25|doi=10.1103/PhysRev.89.20|bibcode=1953PhRv...89...20L|last1=Lehovec|first1=K|last2=Accardo|first2=C. A|last3=Jamgochian|first3=E}}</ref>

[[Biography:Rubin Braunstein|Rubin Braunstein]]<ref>{{cite web |url=http://personnel.physics.ucla.edu/directory/faculty/braunstein/ |title=Rubin Braunstein |publisher=UCLA |archive-url=https://web.archive.org/web/20110311150933/http://personnel.physics.ucla.edu/directory/faculty/braunstein |archive-date=March 11, 2011 |access-date=January 24, 2012 |url-status=dead }}</ref> of the Radio Corporation of America reported on infrared emission from [[Chemistry:Gallium arsenide|gallium arsenide]] (GaAs) and other semiconductor alloys in 1955.<ref>{{Cite journal|author=Braunstein, Rubin
|year=1955 |title=Radiative Transitions in Semiconductors |journal=Physical Review|volume=99|pages=1892–1893 |doi=10.1103/PhysRev.99.1892 |issue=6 |bibcode = 1955PhRv...99.1892B }}</ref> Braunstein observed infrared emission generated by simple diode structures using [[Chemistry:Gallium antimonide|gallium antimonide]] (GaSb), GaAs, [[Chemistry:Indium phosphide|indium phosphide]] (InP), and [[Engineering:Silicon-germanium|silicon-germanium]] (SiGe) alloys at room temperature and at 77&nbsp;[[Kelvin|kelvin]]s<!-- lower-case and pluralized! See Kelvin#Usage_conventions -->. In 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance.  As noted by Kroemer<ref>{{cite journal |last1=Kroemer |first1=Herbert |title=The Double-Heterostructure Concept: How It Got Started |journal=Proceedings of the IEEE|date=Sep 16, 2013 |volume=101 |issue=10 |pages=2183–2187 |doi=10.1109/JPROC.2013.2274914|s2cid=2554978 }}</ref> Braunstein "…had set up a simple optical communications link: Music emerging from a record player was used via suitable electronics to modulate the forward current of a GaAs diode. The emitted light was detected by a PbS diode some distance away. This signal was fed into an audio amplifier and played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of fun playing with this setup." This setup presaged the use of LEDs for [[Engineering:Optical communication|optical communication]] applications.

[[File:SNX100.jpg|thumb|A 1962 Texas Instruments SNX-100 GaAs LED contained in a TO-18 transistor metal case]]

In September 1961, while working at [[Company:Texas Instruments|Texas Instruments]] in Dallas, {{wipe|Texas}}, James R. Biard and Gary Pittman discovered near-infrared (900&nbsp;nm) light emission from a [[Engineering:Tunnel diode|tunnel diode]] they had constructed on a GaAs substrate.<ref name=FirstPracticalLED>{{cite web|author1=Okon, Thomas M. |author2=Biard, James R. |title=The First Practical LED|url=http://edisontechcenter.org/lighting/LED/TheFirstPracticalLED.pdf|website=EdisonTechCenter.org|publisher=Edison Tech Center|date=2015|access-date=2016-02-02}}</ref> By October 1961, they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically isolated semiconductor photodetector.<ref>Matzen, W. T. ed. (March 1963) [https://web.archive.org/web/20180729085826/http://www.dtic.mil/dtic/tr/fulltext/u2/411614.pdf "Semiconductor Single-Crystal Circuit Development,"] Texas Instruments Inc., Contract No. AF33(616)-6600, Rept. No ASD-TDR-63-281.</ref> On August 8, 1962, Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc-diffused [[Physics:P–n junction|p–n junction]] LED with a spaced [[Physics:Cathode|cathode]] contact to allow for efficient emission of infrared light under forward bias. After establishing the priority of their work based on engineering notebooks predating submissions from G.E. Labs, [[Company:RCA|RCA]] Research Labs, [[Company:IBM|IBM]] Research Labs, [[Organization:Bell Labs|Bell Labs]], and Lincoln Lab at MIT, the U.S. patent office issued the two inventors the patent for the GaAs infrared light-emitting diode (U.S. Patent [http://www.freepatentsonline.com/3293513.pdf US3293513]), the first practical LED.<ref name=FirstPracticalLED /> Immediately after filing the patent, [[Company:Texas Instruments|Texas Instruments]] (TI) began a project to manufacture infrared diodes. In October 1962, TI announced the first commercial LED product (the SNX-100), which employed a pure GaAs crystal to emit an 890&nbsp;nm light output.<ref name=FirstPracticalLED /> In October 1963, TI announced the first commercial hemispherical LED, the SNX-110.<ref>{{cite journal| title=One-watt GaAs p-n junction infrared source |author1=Carr, W. N.  |author2=G. E. Pittman | journal=Applied Physics Letters |date=November 1963 | volume=3 | issue=10 | pages = 173–175 | doi=10.1063/1.1753837|bibcode=1963ApPhL...3..173C }}</ref>

In the 1960s, several laboratories focused on LEDs that would emit visible light. A particularly important device was demonstrated by [[Biography:Nick Holonyak|Nick Holonyak]] on October 9, 1962, while he was working for [[Company:General Electric|General Electric]] in Syracuse, New York. The device used the semiconducting alloy gallium phosphide arsenide (GaAsP). It was the first semiconductor laser to emit visible light, albeit at low temperatures. At room temperature it still functioned as a red light-emitting diode. GaAsP was the basis for the first wave of commercial LEDs emitting visible light. It was mass produced by the Monsanto and Hewlett-Packard companies and used widely for displays in calculators and wrist watches.<ref>{{cite web |title=Nick Holonyak, Jr., six decades in pursuit of light |date=May 4, 2012 |last=Kubetz |first=Rick |publisher=University of Illinois |url=https://grainger.illinois.edu/news/2012-05-04-nick-holonyak-jr-six-decades-pursuit-light |access-date=2020-07-07 |archive-date=July 10, 2020 |archive-url=https://web.archive.org/web/20200710150131/https://grainger.illinois.edu/news/2012-05-04-nick-holonyak-jr-six-decades-pursuit-light |url-status=dead }}</ref><ref>{{cite journal | url=http://apl.aip.org/resource/1/applab/v1/i4 | title=Coherent (Visible) Light Emission from Ga(As<sub>1−x</sub> P<sub>x</sub>) Junctions | author=Holonyak Nick | journal=Applied Physics Letters | date=December 1962 | issue=4 | doi=10.1063/1.1753706 | last2=Bevacqua | first2=S. F. | volume=1 | page=82 | bibcode=1962ApPhL...1...82H | url-status=dead | archive-url=https://web.archive.org/web/20121014035733/http://apl.aip.org/resource/1/applab/v1/i4 | archive-date=October 14, 2012 | df=mdy-all }}</ref><ref name="chisuntimes">{{cite news| title = U. of I.'s Holonyak out to take some of Edison's luster| author = Wolinsky, Howard| date = February 5, 2005| url = http://findarticles.com/p/articles/mi_qn4155/is_20050202/ai_n9504926| archive-url = https://web.archive.org/web/20060328001535/http://findarticles.com/p/articles/mi_qn4155/is_20050202/ai_n9504926| url-status = dead| archive-date = March 28, 2006| newspaper = Chicago Sun-Times| access-date = July 29, 2007 }}</ref>

M. George Craford,<ref>{{Cite journal |year=1995 |volume=32 |pages= 52–55 |doi=10.1109/6.343989 |title=M. George Craford [biography] |last1=Perry |first1=T.&nbsp;S. |journal=IEEE Spectrum| issue=2}}</ref> a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972.<ref>{{cite web|url=http://www.technology.gov/Medal/2002/bios/Holonyak_Craford_Dupuis.pdf|title=Brief Biography&nbsp;— Holonyak, Craford, Dupuis|publisher=Technology Administration|access-date=May 30, 2007|url-status=dead|archive-url=https://web.archive.org/web/20070809062214/http://www.technology.gov/Medal/2002/bios/Holonyak_Craford_Dupuis.pdf|archive-date=August 9, 2007|df=mdy-all}}</ref> In 1976, T. P. Pearsall designed the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.<ref>{{Cite journal |title=Efficient, Lattice-matched, Double Heterostructure LEDs at 1.1 mm from Ga<sub>''x''</sub>In<sub>1−''x''</sub>As<sub>''y''</sub>P<sub>1−''y''</sub> by Liquid-phase Epitaxy |journal=Appl. Phys. Lett. |volume=28 |page=499 |year=1976 |doi=10.1063/1.88831 |last1=Pearsall |first1=T. P. |last2=Miller |first2=B. I. |last3=Capik |first3=R. J. |last4=Bachmann |first4=K. J. |issue=9 |bibcode = 1976ApPhL..28..499P }}</ref>

=== Initial commercial development ===
[[File:TI-30-LED-Display-3682e1.jpg|thumb|LED display of a TI-30 scientific calculator ({{circa|1978}}), which uses plastic lenses to increase the visible digit size]]
[[File:8-digit-multiplex-7-segment-LED-X-Ray.tif|thumb|alt=Eight small rectangular blobs, which are the digits, connected by fine hair-like wires to tracks along a circuit board|X-Ray of a 1970s 8-digit LED calculator display]]

Until 1968, visible and infrared LEDs were extremely costly, on the order of {{wipe|United States dollar}}200 per unit, and so had little practical use.<ref name="Schubert" /> The first commercial visible-wavelength LEDs used GaAsP semiconductors and were commonly used as replacements for [[Physics:Incandescence|incandescent]] and [[Engineering:Neon lamp|neon indicator lamps]], and in [[Engineering:Seven-segment display|seven-segment display]]s, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as calculators, TVs, radios, telephones, as well as watches.<ref>{{cite journal | url=http://www.datamath.org/Display/Monsanto.htm | title=LEDs cast Monsanto in Unfamiliar Role | author=Rostky, George | journal=Electronic Engineering Times |date=March 1997 | issue=944}}</ref>

The [[Company:Hewlett-Packard|Hewlett-Packard]] company (HP) was engaged in [[Research and development|research and development]] (R&D) on practical LEDs between 1962 and 1968, by a research team under Howard C. Borden, Gerald P. Pighini at HP Associates and HP Labs.<ref name="Borden">{{cite journal |last1=Borden |first1=Howard C. |last2=Pighini |first2=Gerald P. |title=Solid-State Displays |journal=Hewlett-Packard Journal |date=February 1969 |pages=2–12 |url=http://hparchive.com/Journals/HPJ-1969-02.pdf |archive-url=https://web.archive.org/web/20231105061725/http://hparchive.com/Journals/HPJ-1969-02.pdf |archive-date=2023-11-05 |url-status=live}}</ref> During this time HP collaborated with Monsanto Company on developing the first usable LED products.<ref name="Kramer">{{cite book |last1=Kramer |first1=Bernhard |title=Advances in Solid State Physics |date=2003 |publisher=Springer Science & Business Media |isbn=9783540401506 |page=40 |url=https://books.google.com/books?id=oREQToB2q2UC&pg=PA40}}</ref> The first usable LED products were HP's [[Engineering:LED display|LED display]] and Monsanto's [[Engineering:LED lamp|LED indicator lamp]], both launched in 1968.<ref name="Kramer"/>

Monsanto was the first organization to mass-produce visible LEDs, using [[Chemistry:Gallium arsenide phosphide|Gallium arsenide phosphide]] (GaAsP) in 1968 to produce red LEDs suitable for indicators.<ref name="Schubert">{{Cite book|author=Schubert, E. Fred |title=Light-Emitting Diodes|publisher=Cambridge University Press|year=2003|chapter=1|isbn=978-0-8194-3956-7}}</ref> Monsanto had previously offered to supply HP with GaAsP, but HP decided to grow its own GaAsP.<ref name="Schubert" /> In February 1969, Hewlett-Packard introduced the HP Model 5082-7000 Numeric Indicator, the first LED device to use [[Engineering:Integrated circuit|integrated circuit]] (integrated [[Engineering:LED circuit|LED circuit]])  technology.<ref name="Borden" /> It was the first intelligent LED display, and was a revolution in digital display technology, replacing the [[Engineering:Nixie tube|Nixie tube]] and becoming the basis for later LED displays.<ref>{{cite web |title=Hewlett-Packard 5082-7000 |url=http://www.decadecounter.com/vta/articleview.php?item=1052 |website=The Vintage Technology Association |access-date=15 August 2019 |archive-date=November 17, 2014 |archive-url=https://web.archive.org/web/20141117052948/http://www.decadecounter.com/vta/articleview.php?item=1052 |url-status=dead }}</ref>

In the 1970s, commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the [[Engineering:Planar process|planar process]] (developed by Jean Hoerni,<ref>{{cite patent | title=Method of Manufacturing Semiconductor Devices |country=US |number=3025589 | inventor=Hoerni, J. A. |fdate=May 1, 1959 |gdate=Mar 20, 1962}}</ref><ref>[http://www.google.com/patents/US3025589 Patent number: 3025589] Retrieved May 17, 2013</ref> ). The combination of planar processing for chip fabrication and innovative packaging methods enabled the team at Fairchild led by optoelectronics pioneer Thomas Brandt to achieve the needed cost reductions.<ref>{{Cite news| author=Bausch, Jeffrey |title =The Long History of Light Emitting Diodes |url=http://www.electronicproducts.com/Optoelectronics/LEDs/The_long_history_of_light-emitting_diodes.aspx|publisher =Hearst Business Communications  |date= December 2011 }}</ref>  LED producers continue to use these methods.<ref>{{Cite journal |last1 = Park |first1 = S. -I. |last2 = Xiong |first2 = Y. |last3 = Kim |first3 = R. -H. |last4 = Elvikis |first4 = P. |last5 = Meitl |first5 = M. |last6 = Kim |first6 = D. -H. |last7 = Wu |first7 = J. |last8 = Yoon |first8 = J. |last9 = Yu |first9 = C. -J. |last10 = Liu |doi = 10.1126/science.1175690 |first10 = Z. |last11 = Huang |first11 = Y. |last12 = Hwang |first12 = K. -C. |last13 = Ferreira |first13 = P. |last14 = Li |first14 = X. |last15 = Choquette |first15 = K. |last16 = Rogers |first16 = J. A. |title = Printed Assemblies of Inorganic Light-Emitting Diodes for Deformable and Semitransparent Displays |journal = Science |volume = 325 |issue = 5943 |pages = 977–981 |year = 2009 |pmid = 19696346 |url = http://vcsel.mntl.illinois.edu/Pubs/Pubs+2009/2009+Sci+325+977+flex+substrate+LED.pdf |url-status = dead |archive-url = https://web.archive.org/web/20151024212822/http://vcsel.mntl.illinois.edu/Pubs/Pubs%202009/2009%20Sci%20325%20977%20flex%20substrate%20LED.pdf |archive-date = October 24, 2015 |df = mdy-all|bibcode= 2009Sci...325..977P |osti = 1876039 |citeseerx = 10.1.1.660.3338|s2cid = 8062948 }}</ref>

The early red LEDs were bright enough for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors became widely available and appeared in appliances and equipment.

Early LEDs were packaged in metal cases similar to those of transistors, with a glass window or lens to let the light out. Modern indicator LEDs are packed in transparent molded plastic cases, tubular or rectangular in shape, and often tinted to match the device color. Infrared devices may be dyed, to block visible light. More complex packages have been adapted for efficient heat dissipation in  high-power LEDs. Surface-mounted LEDs further reduce the package size. LEDs intended for use with fiber optics cables may be provided with an optical connector.

===Blue LED===
The first blue-violet LED using magnesium-doped [[Chemistry:Gallium nitride|gallium nitride]] was made at [[Organization:Stanford University|Stanford University]] in 1972 by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering.<ref>[https://spectrum.ieee.org/tech-talk/geek-life/history/rcas-forgotten-work-on-the-blue-led "Nobel Shocker: RCA Had the First Blue LED in 1972"]. ''IEEE Spectrum''. October 9, 2014,</ref><ref>[http://www.oregonlive.com/silicon-forest/index.ssf/2014/10/oregon_tech_ceo_says_nobel_pri.html "Oregon tech CEO says Nobel Prize in Physics overlooks the actual inventors"]. ''The Oregonian''. October 16, 2014</ref> At the time Maruska was on leave from [[Company:RCA|RCA Laboratories]], where he collaborated with Jacques Pankove on related work. In 1971, the year after Maruska left for Stanford, his RCA colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc-doped gallium nitride, though the subsequent device Pankove and Miller built, the first actual gallium nitride light-emitting diode, emitted green light.<ref>Schubert, E. Fred (2006) ''Light-emitting diodes'' (2nd ed.), Cambridge University Press. {{ISBN|0-521-86538-7}} pp. 16–17</ref><ref>Maruska, H. (2005). [http://www.sslighting.net/news/features/maruska_blue_led_history.pdf "A Brief History of GaN Blue Light-Emitting Diodes"]. ''LIGHTimes Online – LED Industry News''. {{webarchive |url=https://web.archive.org/web/20120611224723/http://www.sslighting.net/news/features/maruska_blue_led_history.pdf |date=June 11, 2012 }}</ref> In 1974 the U.S. Patent Office awarded Maruska, Rhines, and Stanford professor David Stevenson a patent for their work in 1972 (U.S. Patent [http://www.google.com/patents/US3819974 US3819974 A]). Today, magnesium-doping of gallium nitride remains the basis for all commercial blue LEDs and [[Engineering:Laser diode|laser diode]]s. In the early 1970s, these devices were too dim for practical use, and research into gallium nitride devices slowed.

In August 1989, Cree introduced the first commercially available blue LED based on the [[Physics:Direct and indirect band gaps|indirect bandgap]] semiconductor, silicon carbide (SiC).<ref>[http://www.cree.com/about/milestones.asp Major Business and Product Milestones]. Cree.com. Retrieved on March 16, 2012. {{webarchive |url=https://web.archive.org/web/20110413031614/http://www.cree.com/about/milestones.asp |date=April 13, 2011 }}</ref> SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.<ref>{{Cite journal|last1=Edmond|first1=John A.|last2=Kong|first2=Hua-Shuang|last3=Carter|first3=Calvin H.|date=1993-04-01|title=Blue LEDs, UV photodiodes and high-temperature rectifiers in 6H-SiC|journal=Physica B: Condensed Matter|volume=185|issue=1|pages=453–460|doi=10.1016/0921-4526(93)90277-D|issn=0921-4526|bibcode=1993PhyB..185..453E}}</ref><ref>{{cite web |title=History & Milestones |url=http://www.cree.com/About-Cree/History-and-Milestones |website=Cree.com |publisher=Cree |access-date=2015-09-14 |archive-date=February 16, 2017 |archive-url=https://web.archive.org/web/20170216121011/http://cree.com/About-Cree/History-and-Milestones |url-status=dead }}</ref>

In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping<ref>{{cite web|title = GaN-based blue light emitting device development by Akasaki and Amano|website = Takeda Award 2002 Achievement Facts Sheet|publisher = The Takeda Foundation|date = April 5, 2002|url = http://www.takeda-foundation.jp/en/award/takeda/2002/fact/pdf/fact01.pdf|access-date = November 28, 2007}}</ref> ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, [[Biography:Theodore Moustakas|Theodore Moustakas]] at Boston University patented a method for producing high-brightness blue LEDs using a new two-step process in 1991.<ref>Moustakas, Theodore D. {{US patent|5686738A}} "Highly insulating monocrystalline gallium nitride thin films" Issue date: March 18, 1991</ref> In 2015, a US court ruled that three Taiwanese companies had infringed Moustakas's prior patent, and ordered them to pay licensing fees of not less than US$13 million.<ref>{{cite news |last=Brown |first=Joel |title=BU Wins $13 Million in Patent Infringement Suit |url=http://www.bu.edu/today/2015/bu-wins-13-million-in-patent-infringement-suit/ |date=7 December 2015 |work=BU Today |access-date=7 December 2015 }}</ref>

Two years later, in 1993, high-brightness blue LEDs were demonstrated by [[Biography:Shuji Nakamura|Shuji Nakamura]] of Nichia Corporation using a gallium nitride (GaN) growth process.<ref name="Nakamura">{{cite journal | title=Candela-Class High-Brightness InGaN/AlGaN Double-Heterostructure Blue-Light-Emitting-Diodes | last1=Nakamura | first1=S. | last2=Mukai | first2=T. | last3=Senoh | first3=M. | journal=[[Physics:Applied Physics Letters|Applied Physics Letters]] | year=1994 | volume=64 | page=1687 | bibcode=1994ApPhL..64.1687N | doi=10.1063/1.111832 | issue=13 }}</ref><ref>{{cite web |last1=Nakamura |first1=Shuji |title=Development of the Blue Light-Emitting Diode |url=http://spie.org/x115688.xml |publisher=SPIE Newsroom |access-date=28 September 2015}}</ref><ref>Iwasa, Naruhito; Mukai, Takashi and Nakamura, Shuji {{US patent|5578839}} "Light-emitting gallium nitride-based compound semiconductor device" Issue date: November 26, 1996</ref> These LEDs had efficiencies of 10%.<ref>{{cite book | url=https://books.google.com/books?id=algLBgAAQBAJ | title=Light-Emitting Diodes (2nd Edition, 2006) | isbn=978-0-9863826-1-1 | last1=Fred Schubert | first1=E. | date=January 2006 | publisher=E. Fred Schubert }}</ref> In parallel, [[Biography:Isamu Akasaki|Isamu Akasaki]] and [[Biography:Hiroshi Amano|Hiroshi Amano]] of Nagoya University were working on developing the important [[Chemistry:Gallium nitride|GaN]] deposition on [[Chemistry:Sapphire|sapphire]] substrates and the demonstration of p-type doping of GaN. This new development revolutionized LED lighting, making [[Physics:Blue laser|high-power blue light sources]] practical, leading to the development of technologies like [[Engineering:Blu-ray|Blu-ray]].<ref>{{cite web|url=https://ssleec.ucsb.edu/news/2023/01/12/professor-shuji-nakamura-was-key-invention-blu-ray-technology|archive-url=https://web.archive.org/web/20230324123538/https://ssleec.ucsb.edu/news/2023/01/12/professor-shuji-nakamura-was-key-invention-blu-ray-technology|title=Professor Shuji Nakamura was key to the Invention of Blu-Ray Technology|publisher=[[Organization:University of California, Santa Barbara|University of California, Santa Barbara]]|date=12 January 2023|archive-date=24 March 2023|access-date=4 June 2023}}</ref><ref>{{cite web|url=https://www.nae.edu/128641/Dr-Shuji-Nakamura-|archive-url=https://web.archive.org/web/20190411125926/https://www.nae.edu/128641/Dr-Shuji-Nakamura-|title= Dr. Shuji Nakamura|publisher=National Academy of Engineering|archive-date=11 April 2019|access-date=4 June 2023}}</ref>

Nakamura was awarded the 2006 Millennium Technology Prize for his invention.<ref>[https://www.news.ucsb.edu/2006/012148/2006-millennium-technology-prize 2006 Millennium technology prize awarded to UCSB's Shuji Nakamura]. Ia.ucsb.edu (June 15, 2006). Retrieved on August 3, 2019.</ref>
Nakamura, [[Biography:Hiroshi Amano|Hiroshi Amano]] and [[Biography:Isamu Akasaki|Isamu Akasaki]] were awarded the Nobel Prize in Physics in 2014 for the invention of an efficient gallium nitride blue LED.<ref name="NYT-20141007-DO">{{cite news |last=Overbye |first=Dennis |title=Nobel Prize in Physics |url=https://www.nytimes.com/2014/10/08/science/isamu-akasaki-hiroshi-amano-and-shuji-nakamura-awarded-the-nobel-prize-in-physics.html |date=7 October 2014 |work={{wipe|The New York Times}} }}</ref>

In 1995, Alberto Barbieri at the [[Organization:Cardiff University|Cardiff University]] Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED using [[Chemistry:Indium tin oxide|indium tin oxide]] (ITO) on (AlGaInP/GaAs).

In 2001<ref>{{Cite journal | last1 = Dadgar | first1 = A. | last2 = Alam | first2 = A. | last3 = Riemann | first3 = T. | last4 = Bläsing | first4 = J. | last5 = Diez | first5 = A. | last6 = Poschenrieder | first6 = M. | last7 = Strassburg | first7 = M. | last8 = Heuken | first8 = M. | last9 = Christen | first9 = J. | last10 = Krost | first10 = A.| title = Crack-Free InGaN/GaN Light Emitters on Si(111) | doi = 10.1002/1521-396X(200111)188:1<155::AID-PSSA155>3.0.CO;2-P | journal = Physica Status Solidi A | volume = 188 | pages = 155–158 | year = 2001 }}</ref> and 2002,<ref>
{{Cite journal | last1 = Dadgar | first1 = A. | last2 = Poschenrieder | first2 = M. | last3 = BläSing | first3 = J. | last4 = Fehse | first4 = K. | last5 = Diez | first5 = A. | last6 = Krost | first6 = A. | doi = 10.1063/1.1479455 | title = Thick, crack-free blue light-emitting diodes on Si(111) using low-temperature AlN interlayers and in situ Si\sub x]N\sub y] masking | journal = Applied Physics Letters | volume = 80 | issue = 20 | page = 3670 | year = 2002 |bibcode = 2002ApPhL..80.3670D }}</ref> processes for growing [[Chemistry:Gallium nitride|gallium nitride]] (GaN) LEDs on [[Chemistry:Silicon|silicon]] were successfully demonstrated. In January 2012, Osram demonstrated high-power InGaN LEDs grown on silicon substrates commercially,<ref>{{cite web |url=http://www.osram-os.de/osram_os/EN/Press/Press_Releases/Company_Information/2012/_documents/OSRAM_PI_Production_GaNonSi_e.pdf |title=Success in research: First gallium-nitride LED chips on silicon in pilot stage |access-date=2012-09-15 |url-status=dead |archive-url=https://web.archive.org/web/20120915034646/http://www.osram-os.de/osram_os/EN/Press/Press_Releases/Company_Information/2012/_documents/OSRAM_PI_Production_GaNonSi_e.pdf |archive-date=September 15, 2012 |df=mdy }}. www.osram.de, January 12, 2012.</ref> and GaN-on-silicon LEDs are in production at [[Company:Plessey#Plessey Semiconductors Ltd|Plessey Semiconductors]]. As of 2017, some manufacturers are using SiC as the substrate for LED production, but sapphire is more common, as it has the most similar properties to that of gallium nitride, reducing the need for patterning the sapphire wafer (patterned wafers are known as epi wafers). [[Company:Samsung|Samsung]], the [[Organization:University of Cambridge|University of Cambridge]], and [[Company:Toshiba|Toshiba]] are performing research into GaN on Si LEDs. Toshiba has stopped research, possibly due to low yields.<ref>Lester, Steve (2014) [https://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lester_substrate-pkg_tampa2014.pdf Role of Substrate Choice on LED Packaging] {{Webarchive|url=https://web.archive.org/web/20140712100725/https://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lester_substrate-pkg_tampa2014.pdf |date=July 12, 2014 }}. Toshiba America Electronic Components.</ref><ref>[https://www.gan.msm.cam.ac.uk/projects/silicon "GaN on Silicon"]. Cambridge Centre for Gallium Nitride. Gan.msm.cam.ac.uk. Retrieved July 31, 2018.</ref><ref>Bush, Steve (June 30, 2016). [https://www.electronicsweekly.com/blogs/led-luminaries/toshiba-gets-out-of-gan-on-si-leds-2016-06/ "Toshiba gets out of GaN-on-Si LEDs"]. ''Electronics Weekly''. Retrieved July 31, 2018.</ref><ref>{{Cite book |doi=10.1109/IEDM.2013.6724622|chapter=LED manufacturing issues concerning gallium nitride-on-silicon (GaN-on-Si) technology and wafer scale up challenges|title=2013 IEEE International Electron Devices Meeting |pages=13.2.1–13.2.4 |year=2013 |last1=Nunoue |first1=Shin-ya |last2=Hikosaka |first2=Toshiki |last3=Yoshida |first3=Hisashi |last4=Tajima |first4=Jumpei |last5=Kimura |first5=Shigeya |last6=Sugiyama |first6=Naoharu |last7=Tachibana |first7=Koichi |last8=Shioda |first8=Tomonari |last9=Sato |first9=Taisuke |last10=Muramoto |first10=Eiji |last11=Onomura |first11=Masaaki |isbn=978-1-4799-2306-9|s2cid=23448056}}</ref><ref>Wright, Maury (May 2, 2016).  [https://www.ledsmagazine.com/articles/2016/05/samsung-s-tarn-reports-progress-in-csp-and-gan-on-si-leds.html "Samsung's Tarn reports progress in CSP and GaN-on-Si LEDs"]. ''LEDs Magazine''.</ref><ref>[https://compoundsemiconductor.net/article/99020/Increasing_The_Competitiveness_Of_The_GaN-on-silicon_LED "Increasing the Competitiveness of the GaN-on-silicon LED"]. ''Compound Semiconductor'' (30 March 2016).</ref><ref>[https://www.ledinside.com/news/2015/3/samsung_to_focus_on_silicon_based_led_chip_technology_in_2015 "Samsung To Focus on Silicon-based LED Chip Technology in 2015"]. ''LED Inside'' (17 March 2015).</ref> Some opt for [[Physics:Epitaxy|epitaxy]], which is difficult on [[Chemistry:Silicon|silicon]], while others, like the University of Cambridge, choose a multi-layer structure, in order to reduce (crystal) lattice mismatch and different thermal expansion ratios, to avoid cracking of the LED chip at high temperatures (e.g. during manufacturing), reduce heat generation and increase luminous efficiency. Sapphire substrate patterning can be carried out with [[Engineering:Nanoimprint lithography|nanoimprint lithography]].<ref>Keeping, Steven. (2013-01-15) [https://www.digikey.com/en/articles/techzone/2013/jan/material-and-manufacturing-improvements-enhance-led-efficiency "Material and Manufacturing Improvements"]. ''DigiKey''. Retrieved on 2018-07-31.</ref><ref>Keeping, Steven (December 12, 2014) [https://www.digikey.com/en/articles/techzone/2014/dec/manufacturers-shift-attention-to-light-quality-to-further-led-market-share-gains "Manufacturers Shift Attention to Light Quality to Further LED Market Share Gains"]. ''DigiKey''. Retrieved July 31, 2018.</ref><ref>Keeping, Steven. (September 24, 2013). [https://www.digikey.com/en/articles/techzone/2013/sep/will-silicon-substrates-push-led-lighting-into-the-mainstream "Will Silicon Substrates Push LED Lighting Into the Mainstream?"]. ''DigiKey''. Retrieved July 31, 2018.</ref><ref>Keeping, Steven (March 24, 2015). [https://www.digikey.com/en/articles/techzone/2015/mar/improved-silicon-substrate-leds-address-high-solid-state-lighting-costs "Improved Silicon-Substrate LEDs Address High Solid-State Lighting Costs"]. ''DigiKey''. Retrieved July 31, 2018.</ref><ref>[http://www.toshiba-machine.co.jp/en/NEWS/product/20110518_04.html "Development of the Nano-Imprint Equipment ST50S-LED for High-Brightness LED"]. ''Toshiba Machine'' (May 18, 2011). Retrieved July 31, 2018.</ref><ref>[https://electroiq.com/2015/11/the-use-of-sapphire-in-mobile-device-and-led-industries-part-2/ "The use of sapphire in mobile device and LED industries: Part 2"]. Solid State Technology (September 26, 2017). Retrieved July 31, 2018.</ref><ref>[http://www.appliedmaterials.com/semiconductor/products/epitaxy "Epitaxy"]. ''Applied Materials''. Retrieved July 31, 2018.</ref>

GaN-on-Si is difficult but desirable since it takes advantage of existing semiconductor manufacturing infrastructure. It allows for the wafer-level packaging of LED dies resulting in extremely small LED packages.<ref name="Toshiba">{{citation |title=Role of Substrate Choice on LED Pacakaging |last=Lester |first=Steve |publisher=Toshiba America Electronic Components |url=https://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lester_substrate-pkg_tampa2014.pdf |archive-url=https://web.archive.org/web/20140712100725/https://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lester_substrate-pkg_tampa2014.pdf |archive-date=12 July 2014}}</ref>

GaN is often deposited using Metalorganic vapor-phase epitaxy (MOCVD),<ref>Semiengineering: MOCVD vendors eye new apps<!--https://semiengineering.com/mocvd-vendors-eye-new-apps/--></ref> and it also utilizes [[Engineering:Lift-off (microtechnology)|Lift-off]].

===White LEDs and the illumination breakthrough===
Even though white light can be created using individual red, green and blue LEDs, this results in poor [[Earth:Color rendering|color rendering]], since only three narrow bands of wavelengths of light are being emitted. The attainment of high efficiency blue LEDs was quickly followed by the development of the first white LED. In this device a {{chem|Y|3|Al|5|O|12}}:Ce (known as "[[Chemistry:Yttrium aluminium garnet|YAG]]" or Ce:YAG phosphor) [[Chemistry:Cerium|cerium]]-doped [[Physics:Phosphor|phosphor]] coating produces yellow light through [[Biology:Fluorescence|fluorescence]]. The combination of that yellow with remaining blue light appears white to the eye. Using different phosphors produces green and red light through fluorescence. The resulting mixture of red, green and blue is perceived as white light, with improved color rendering compared to wavelengths from the blue LED/YAG phosphor combination.<ref>{{Cite journal |last1=Izotov |first1=Sergey |last2=Sitdikov |first2=Anton |last3=Soldatkin |first3=Vasily |last4=Tuev |first4=Vasily |last5=Olisovets |first5=Artem |date=2014 |title=Study of Phosphors for White LEDs |journal=Procedia Technology |language=en |volume=18 |pages=14–18 |doi=10.1016/j.protcy.2014.11.005|doi-access=free }}</ref>

[[File:Haitz-law.svg|thumb|upright=1.45|Illustration of [[Haitz's law|Haitz's law]], showing improvement in light output per LED over time, with a logarithmic scale on the vertical axis]]

The first white LEDs were expensive and inefficient. The light output then increased [[Exponential growth|exponentially]]. The latest research and development has been propagated by Japanese manufacturers such as [[Company:Panasonic|Panasonic]], and Nichia, and by Korean and Chinese manufacturers such as [[Company:Samsung|Samsung]], Solstice, Kingsun, Hoyol and others. This trend in increased output has been called [[Haitz's law]] after Roland Haitz.<ref>{{Cite journal|doi=10.1038/nphoton.2006.78|title=Haitz's law|year=2007|journal=[[Physics:Nature Photonics|Nature Photonics]]|volume=1|page=23|issue=1|bibcode = 2007NaPho...1...23. |doi-access=free}}</ref><ref>{{Cite web | url=http://www.ledcornbulbs.com/LEDlightsapplication/List-of-Top-10-LED-light-manufacturer-in-China-19.html |title = List of Top 10 LED light manufacturer in China|archive-url = https://web.archive.org/web/20141009180039/http://www.ledcornbulbs.com/LEDlightsapplication/List-of-Top-10-LED-light-manufacturer-in-China-19.html|archive-date = 9 October 2014}}</ref>

Light output and efficiency of blue and near-ultraviolet LEDs rose and the cost of reliable devices fell. This led to relatively high-power white-light LEDs for illumination, which are replacing incandescent and fluorescent lighting.<ref>{{cite web | url=http://www.electrooptics.com/features/junjul06/junjul06leds.html | title=LED there be light, Nick Morris predicts a bright future for LEDs | website=Electrooptics.com | date=1 June 2006 | first=Nick | last=Morris | access-date=March 4, 2009 | archive-date=November 23, 2011 | archive-url=https://web.archive.org/web/20111123210448/http://www.electrooptics.com/features/junjul06/junjul06leds.html | url-status=dead }}</ref><ref>{{Cite magazine|url=https://www.forbes.com/2008/02/27/incandescent-led-cfl-pf-guru_in_mm_0227energy_inl.html|title=The LED Illumination Revolution|magazine={{wipe|Forbes}}|date=February 27, 2008}}</ref>

Experimental white LEDs were demonstrated in 2014 to produce 303 lumens per watt of electricity (lm/W); some can last up to 100,000 hours.<ref>[https://www.nobelprize.org/nobel_prizes/physics/laureates/2014/press.html "The Nobel Prize in Physics 2014"] (press release). Nobel Prize Committee, 7 October 2014</ref><ref>[http://www.cree.com/news-media/news/article/cree-first-to-break-300-lumens-per-watt-barrier "Cree First to Break 300 Lumens-Per-Watt Barrier"]. {{Webarchive|url=https://web.archive.org/web/20180728221801/http://www.cree.com/news-media/news/article/cree-first-to-break-300-lumens-per-watt-barrier |date=July 28, 2018 }}. Cree.com (Match 26, 2014). Retrieved July 31, 2018.</ref> Commercially available LEDs have an efficiency of up to 223 lm/W as of 2018.<ref>[https://www.samsung.com/led/lighting/mid-power-leds/3030-leds/lm301b/ LM301B | SAMSUNG LED | Samsung LED Global Website]. Samsung.com. Retrieved on 2018-07-31.</ref><ref>[https://www.samsung.com/led/about-us/news-events/news/news-detail-37/ Samsung Achieves 220 Lumens per Watt with New Mid-Power LED Package]. Samsung.com (2017-06-16). Retrieved on 2018-07-31.</ref><ref>[http://www.luxnlum.in/led-breakthrough-promises-ultra-efficient-luminaires/ LED breakthrough promises ultra-efficient luminaires | Lux-n-Lum].Retrieved on 2018-04-06.</ref> A previous record of 135 lm/W was achieved by Nichia in 2010.<ref>{{Cite web|url=https://phys.org/news/2010-08-white-super-high-luminous-efficacy.html|title=White LEDs with super-high luminous efficacy could satisfy all general lighting needs|website=phys.org}}</ref> Compared to incandescent bulbs, this is a huge increase in electrical efficiency, and even though LEDs are more expensive to purchase, overall lifetime cost is significantly cheaper than that of incandescent bulbs.<ref>[https://www.eia.gov/todayinenergy/detail.cfm?id=15471 LED bulb efficiency expected to continue improving as cost declines]. U.S. Energy Information Administration (March 19, 2014)</ref>

The LED chip is encapsulated inside a small, plastic, white mold. It can be encapsulated using resin ([[Chemistry:Polyurethane|polyurethane]]-based), silicone, or epoxy containing (powdered) Cerium-doped YAG phosphor. After allowing the solvents to evaporate, the LEDs are often tested, and placed on tapes for [[Engineering:SMT placement equipment|SMT placement equipment]] for use in LED light bulb production. Encapsulation is performed after probing, dicing, die transfer from wafer to package, and wire bonding or flip chip mounting, perhaps using [[Chemistry:Indium tin oxide|Indium tin oxide]], a transparent electrical conductor. In this case, the bond wire(s) are attached to the ITO film that has been deposited in the LEDs.
Some "remote phosphor" LED light bulbs use a single plastic cover with YAG phosphor for several blue LEDs, instead of using phosphor coatings on single-chip white LEDs.<ref>{{cite web |url=http://www.lamptech.co.uk/Spec%20Sheets/LEDi%20Philips%20806K58RP827-B22d%20Prince.htm |title=Philips LED 60W 806lm Retrofit with Remote Phosphor |website=lamptech.co.uk |access-date=9 January 2022 }}</ref>

The temperature of the phosphor during operation and how it is applied limits the size of an LED die. [[Engineering:Wafer-level packaging|Wafer-level packaged]] white LEDs allow for extremely small LEDs.<ref name="Toshiba" />

== Physics of light production and emission ==
{{main|Engineering:Light-emitting diode physics}}
In a light emitting diode, the recombination of electrons and electron holes in a semiconductor produces light (be it infrared, visible or UV), a process called "[[Physics:Electroluminescence|electroluminescence]]". The wavelength of the light depends on the energy [[Physics:Band gap|band gap]] of the semiconductors used. Since these materials have a high index of refraction, design features of the devices such as special optical coatings and die shape are required to efficiently emit light.<ref name="pears1">{{cite book|last1=Pearsall|first1=Thomas|title=Photonics Essentials, 2nd edition|publisher=McGraw-Hill|date=2010|url=https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html|isbn=978-0-07-162935-5|access-date=February 25, 2021|archive-date=August 17, 2021|archive-url=https://web.archive.org/web/20210817005021/https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html|url-status=dead}}</ref>

Unlike a [[Physics:Laser|laser]], the light emitted from an LED is neither spectrally [[Physics:Coherence|coherent]] nor even highly monochromatic. Its [[Spectrum#Electromagnetic spectrum|spectrum]] is sufficiently narrow that it appears to the [[Color vision|human eye]] as a pure (saturated) color.<ref>{{Cite web|title=LED Basics {{!}} Department of Energy|url=https://www.energy.gov/eere/ssl/led-basics|access-date=2018-10-22|website=www.energy.gov}}</ref><ref>{{cite web|author=<!--Staff writer(s); no by-line.-->|date=2013-07-25|title=LED Spectral Distribution|url=https://optiwave.com/resources/applications-resources/optical-system-led-spectral-distribution/|access-date=20 June 2017|website=optiwave.com}}</ref> Also unlike most lasers, its radiation is not [[Physics:Coherence#Spatial coherence|spatially coherent]], so it cannot approach the very high [[Physics:Radiance|intensity]] characteristic of [[Physics:Laser|laser]]s.

== Single-color LEDs ==
[[File:Blue light emitting diodes over a proto-board.jpg|thumb|upright|[[Blue|Blue]] LEDs]]

By [[Engineering:Light-emitting diode physics#Materials|selection of different semiconductor materials]], single-color LEDs can be made that emit light in a narrow band of wavelengths from near-infrared through the visible spectrum and into the ultraviolet range.  As the wavelengths become shorter, because of the larger band gap of these semiconductors, the operating voltage of the LED increases. 
{{anchor|blue LED}}

Blue LEDs have an active region consisting of one or more InGaN [[Physics:Quantum well|quantum well]]s sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber.

[[Chemistry:Aluminium gallium nitride|Aluminium gallium nitride]] (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for [[Physics:Ultraviolet|ultraviolet]] LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If unalloyed GaN is used in this case to form the active quantum well layers, the device emits near-ultraviolet light with a peak wavelength centred around 365&nbsp;nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.{{citation needed|date=March 2016}}

With [[Chemistry:Aluminium gallium nitride|AlGaN]] and AlGaInN, even shorter wavelengths are achievable. Near-UV emitters at wavelengths around 360–395&nbsp;nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in documents and bank notes, and for [[Physics:UV curing#LEDs|UV curing]]. Substantially more expensive, shorter-wavelength diodes are commercially available for wavelengths down to 240&nbsp;nm.<ref>{{cite journal |url=http://www.semiconductor-today.com/features/SemiconductorToday%20-%20Going%20deep%20for%20UV%20sterilization%20LEDs.pdf |journal=Semiconductor Today |title=Going Deep for UV Sterilization LEDs |page=82 |volume=5 |issue=3 |author=Cooke, Mike |date=April–May 2010 |url-status=dead |archive-url=https://web.archive.org/web/20130515030549/http://www.semiconductor-today.com/features/SemiconductorToday%20-%20Going%20deep%20for%20UV%20sterilization%20LEDs.pdf |archive-date=May 15, 2013 }}</ref> As the photosensitivity of microorganisms approximately matches the absorption spectrum of [[Biology:DNA|DNA]], with a peak at about 260&nbsp;nm, UV LED emitting at 250–270&nbsp;nm are expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365&nbsp;nm) are already effective disinfection and sterilization devices.<ref name="water sterilization">{{Cite journal | last1 = Mori | first1 = M. | last2 = Hamamoto | first2 = A. | last3 = Takahashi | first3 = A. | last4 = Nakano | first4 = M. | last5 = Wakikawa | first5 = N. | last6 = Tachibana | first6 = S. | last7 = Ikehara | first7 = T. | last8 = Nakaya | first8 = Y. | last9 = Akutagawa | first9 = M. | doi = 10.1007/s11517-007-0263-1 | last10 = Kinouchi | first10 = Y. | title = Development of a new water sterilization device with a 365 nm UV-LED | journal = Medical & Biological Engineering & Computing | volume = 45 | issue = 12 | pages = 1237–1241 | year = 2007 | pmid = 17978842 | s2cid = 2821545 | doi-access = free }}</ref>
UV-C wavelengths were obtained in laboratories using [[Chemistry:Aluminium nitride|aluminium nitride]] (210&nbsp;nm),<ref name="aln">{{Cite journal | last1 = Taniyasu | first1 = Y. | last2 = Kasu | first2 = M. | last3 = Makimoto | first3 = T. | doi = 10.1038/nature04760 | title = An aluminium nitride light-emitting diode with a wavelength of 210 nanometres | journal = Nature | volume = 441 | issue = 7091 | pages = 325–328 | year = 2006 | pmid = 16710416 | bibcode = 2006Natur.441..325T| s2cid = 4373542 }}</ref> [[Chemistry:Boron nitride|boron nitride]] (215&nbsp;nm)<ref name="BN">{{Cite journal | last1 = Kubota | first1 = Y. | last2 = Watanabe | first2 = K. | last3 = Tsuda | first3 = O. | last4 = Taniguchi | first4 = T. | title = Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure | doi = 10.1126/science.1144216 | journal = Science | volume = 317 | issue = 5840 | pages = 932–934 | year = 2007 | pmid =  17702939| bibcode = 2007Sci...317..932K | doi-access = free }}</ref><ref name="bn2">{{Cite journal | last1 = Watanabe | first1 = K. | last2 = Taniguchi | first2 = T. | last3 = Kanda | first3 = H. | doi = 10.1038/nmat1134 | title = Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal | journal = Nature Materials | volume = 3 | issue = 6 | pages = 404–409 | year = 2004 | pmid = 15156198 |bibcode = 2004NatMa...3..404W | s2cid = 23563849 }}</ref> and [[Chemistry:Diamond|diamond]] (235&nbsp;nm).<ref name="dia">{{Cite journal | last1 = Koizumi | first1 = S. | last2 = Watanabe | first2 = K. | last3 = Hasegawa | first3 = M. | last4 = Kanda | first4 = H. | title = Ultraviolet Emission from a Diamond pn Junction | doi = 10.1126/science.1060258 | journal = Science | volume = 292 | issue = 5523 | pages = 1899–1901 | year = 2001 | pmid = 11397942| bibcode = 2001Sci...292.1899K| s2cid = 10675358 }}</ref>

== White LEDs ==

There are two primary ways of producing [[White|white]] light-emitting diodes. One is to use individual LEDs that emit three [[Primary color|primary color]]s—red, green and blue—and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, similar to a [[Physics:Fluorescent lamp|fluorescent lamp]].  The yellow phosphor is [[Chemistry:Cerium|cerium]]-doped [[Chemistry:Yttrium aluminium garnet|YAG]] crystals suspended in the package or coated on the LED. This YAG phosphor causes white LEDs to appear yellow when off, and the space between the crystals allow some blue light to pass through in LEDs with partial phosphor conversion. Alternatively, white LEDs may use other phosphors like manganese(IV)-doped [[Chemistry:Potassium fluorosilicate|potassium fluorosilicate]] (PFS) or other engineered phosphors. PFS assists in red light generation, and is used in conjunction with conventional Ce:YAG phosphor. In LEDs with PFS phosphor, some blue light passes through the phosphors, the Ce:YAG phosphor converts blue light to green and red (yellow) light, and the PFS phosphor converts blue light to red light. The color, emission spectrum or color temperature of white phosphor converted and other phosphor converted LEDs can be controlled by changing the concentration of several phosphors that form a phosphor blend used in an LED package.<ref>{{Cite web | url=https://www.ledinside.com/news/2014/11/seeing_red_with_pfs_phosphor |title = Seeing Red with PFS Phosphor}}</ref><ref>{{Cite web | url=https://www.ledsmagazine.com/architectural-lighting/retail-hospitality/article/16696629/ge-lighting-manufactures-pfs-red-phosphor-for-led-display-backlight-applications | title=GE Lighting manufactures PFS red phosphor for LED display backlight applications| date=March 31, 2015}}</ref><ref>{{cite journal | url=https://sid.onlinelibrary.wiley.com/doi/abs/10.1002/sdtp.10406 | doi=10.1002/sdtp.10406 | title=62.4: PFS, K<sub>2</sub>SiF<sub>6</sub>:Mn<sup>4+</sup>: The Red-line Emitting LED Phosphor behind GE's TriGain Technology™ Platform | date=2015 | last1=Murphy | first1=James E. | last2=Garcia-Santamaria | first2=Florencio | last3=Setlur | first3=Anant A. | last4=Sista | first4=Srinivas | journal=Sid Symposium Digest of Technical Papers | volume=46 | pages=927–930 }}</ref><ref>{{Cite journal|doi = 10.1149/2.0251801jss|title = Full Spectrum White LEDs of Any Color Temperature with Color Rendering Index Higher Than 90 Using a Single Broad-Band Phosphor|year = 2018|last1 = Dutta|first1 = Partha S.|last2 = Liotta|first2 = Kathryn M.|journal = ECS Journal of Solid State Science and Technology|volume = 7|pages = R3194–R3198| s2cid=103600941 |doi-access = free}}</ref>

The 'whiteness' of the light produced is engineered to suit the human eye. Because of metamerism, it is possible to have quite different spectra that appear white. The appearance of objects illuminated by that light may vary as the spectrum varies. This is the issue of color rendition, quite separate from color temperature. An orange or cyan object could appear with the wrong color and much darker as the LED or phosphor does not emit the wavelength it reflects. The best color rendition LEDs use a mix of phosphors, resulting in less efficiency and better color rendering.{{citation needed|date=October 2020}}

The first white light-emitting diodes (LEDs) were offered for sale in the autumn of 1996.<ref>{{Cite journal | doi = 10.1002/lpor.201600147 | last1 = Cho | first1 = Jaehee | last2 = Park | first2 = Jun Hyuk | last3 = Kim | first3 = Jong Kyu | last4 = Schubert | first4 = E. Fred | title = White light-emitting diodes: History, progress, and future | journal = Laser & Photonics Reviews | volume = 11 | issue = 2 | pages = 1600147 | year = 2017| bibcode = 2017LPRv...1100147C | s2cid = 53645208 | issn = 1863-8880 | url=https://onlinelibrary.wiley.com/doi/10.1002/lpor.201600147 }}</ref>

=== RGB systems ===
[[File:RGB_LED_Spectrum.svg|thumb|right|Combined spectral curves for blue, yellow-green, and high-brightness red solid-state semiconductor LEDs. [[Full width at half maximum|FWHM]] spectral bandwidth is approximately 24–27 nm for all three colors.]]

[[File:RGB-Led-projection.jpg|thumb|An RGB LED projecting red, green, and blue onto a surface]]

Mixing red, green, and blue sources to produce white light needs electronic circuits to control the blending of the colors.  Since LEDs have slightly different emission patterns, the color balance may change depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors,<ref>{{Cite journal | doi = 10.1364/OE.15.003607 | last1 = Moreno | first1 = I. | last2 = Contreras | first2 = U. | title = Color distribution from multicolor LED arrays | journal = Optics Express | volume = 15 | issue = 6 | pages = 3607–3618 | year = 2007 | pmid = 19532605| bibcode = 2007OExpr..15.3607M | s2cid = 35468615 | doi-access = free }}</ref> and in principle, this mechanism also has higher quantum efficiency in producing white light.<ref>{{Cite web|url=http://spie.org/newsroom/1069-making-white-light-emitting-diodes-without-phosphors?SSO=1|title=Making white-light-emitting diodes without phosphors {{!}} SPIE Homepage: SPIE|last1=Yeh|first1=Dong-Ming|last2=Huang|first2=Chi-Feng|website=spie.org|access-date=2019-04-07|last3=Lu|first3=Chih-Feng|last4=Yang|first4=Chih-Chung}}</ref>

There are several types of multicolor white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods include color stability, [[Earth:Color rendering index|color rendering]] capability, and luminous efficacy. Often, higher efficiency means lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. Although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.<ref>{{cite book |last1=Cabrera |first1=Rowan |title=Electronic Devices and Circuits |date=2019 |publisher=EDTECH |isbn=978-1839473838}}</ref>

One of the challenges is the development of more efficient green LEDs. The theoretical maximum for green LEDs is 683 lumens per watt but as of 2010 few green LEDs exceed even 100 lumens per watt. The blue and red LEDs approach their theoretical limits.{{citation needed|date=October 2020}}

Multicolor LEDs offer a means to form light of different colors. Most [[Color#Perception|perceivable colors]] can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. Their emission power [[Exponential decay|decays exponentially]] with rising temperature,<ref>{{Cite journal |last1=Schubert |first1=E. Fred |last2=Kim |first2=Jong Kyu |journal=Science |volume=308 |issue=5726 |doi=10.1126/science.1108712 |pmid=15919985 |pages=1274–1278 |year=2005 |title=Solid-State Light Sources Getting Smart |bibcode=2005Sci...308.1274S |s2cid=6354382 |url=https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf|archive-url=https://web.archive.org/web/20160205165109/https://www.ecse.rpi.edu/~schubert/Reprints/2005%20Schubert%20and%20Kim%20(Science)%20Solid-state%20light%20sources%20getting%20smart.pdf |archive-date=February 5, 2016 }}</ref>
resulting in a substantial change in color stability. Such problems inhibit industrial use. Multicolor LEDs without phosphors cannot provide good color rendering because each LED is a narrowband source. LEDs without phosphor, while a poorer solution for general lighting, are the best solution for displays, either backlight of LCD, or direct LED based pixels.

Dimming a multicolor LED source to match the characteristics of incandescent lamps is difficult because manufacturing variations, age, and temperature change the actual color value output. To emulate the appearance of dimming incandescent lamps may require a feedback system with color sensor to actively monitor and control the color.<ref>{{cite journal | title = Sensors and Feedback Control of Multicolor LED Systems | format = PDF | first1 = Thomas | last1 = Nimz | first2 = Fredrik | last2 = Hailer | first3 = Kevin | last3 = Jensen | journal = Led Professional Review: Trends & Technologie for Future Lighting Solutions | publisher = LED Professional | date = November 2012 | issue = 34 | issn = 1993-890X | pages = 2–5 | url = http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | archive-url = https://web.archive.org/web/20140429162806/http://www.mazet.de/en/english-documents/english/featured-articles/sensors-and-feedback-control-of-multi-color-led-systems-1/download#.UX7VXYIcUZI | url-status = dead | archive-date = 2014-04-29 }}</ref>

=== Phosphor-based LEDs ===
[[File:White LED.png|thumb|right|upright=1.6|Spectrum of a white LED showing blue light directly emitted by the GaN-based LED (peak at about 465 nm) and the more broadband [[Physics:Stokes shift|Stokes-shifted]] light emitted by the Ce<sup>3+</sup>:YAG phosphor, which emits at roughly 500–700 nm]]

This method involves [[Physics:Coating|coating]] LEDs of one color (mostly blue LEDs made of InGaN) with [[Physics:Phosphor|phosphor]]s of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs).<ref>{{cite journal|title=YAG glass-ceramic phosphor for white LED (II): luminescence characteristics|author1=Tanabe, S. |author2=Fujita, S. |author3=Yoshihara, S. |author4=Sakamoto, A. |author5=Yamamoto, S.|journal=Proceedings of SPIE|url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5941/594112.pdf|archive-url=https://web.archive.org/web/20110511182527/http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5941/594112.pdf|archive-date=2011-05-11|volume=5941|doi=10.1117/12.614681|page=594112|year=2005|series=Fifth International Conference on Solid State Lighting|bibcode=2005SPIE.5941..193T |s2cid=38290951 |editor1-last=Ferguson |editor1-first=Ian T |editor2-last=Carrano |editor2-first=John C |editor3-last=Taguchi |editor3-first=Tsunemasa |editor4-last=Ashdown |editor4-first=Ian E }}</ref>  A fraction of the blue light undergoes the Stokes shift, which transforms it from shorter wavelengths to longer.  Depending on the original LED's color, various color phosphors are used.  Using several phosphor layers of distinct colors broadens the emitted spectrum, effectively raising the color rendering index (CRI).<ref>{{Cite journal|title=Color rendering and luminous efficacy of white LED spectra|author=Ohno, Y.|journal=Proc. SPIE|doi=10.1117/12.565757|url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5530/5530-88.pdf|archive-url=https://web.archive.org/web/20110511182632/http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/SPIE/vol5530/5530-88.pdf|archive-date=2011-05-11|volume=5530|page=89|year=2004|series=Fourth International Conference on Solid State Lighting|bibcode=2004SPIE.5530...88O|s2cid=122777225|editor1-last=Ferguson|editor1-first=Ian T|editor2-last=Narendran|editor2-first=Nadarajah|editor3-last=Denbaars|editor3-first=Steven P|editor4-last=Carrano|editor4-first=John C}}</ref>

Phosphor-based LEDs have efficiency losses due to heat loss from the [[Physics:Stokes shift|Stokes shift]] and also other phosphor-related issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the [[Physics:Luminosity function|luminosity function]]). Due to the simplicity of manufacturing, the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.{{citation needed|date=October 2020}}

Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stokes shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.{{citation needed|date=October 2020}}

Some phosphor-based white LEDs encapsulate InGaN blue LEDs inside phosphor-coated epoxy. Alternatively, the LED might be paired with a remote phosphor, a preformed polycarbonate piece coated with the phosphor material. Remote phosphors provide more diffuse light, which is desirable for many applications. Remote phosphor designs are also more tolerant of variations in the LED emissions spectrum. A common yellow phosphor material is [[Chemistry:Cerium|cerium]]-doped [[Chemistry:Yttrium aluminium garnet|yttrium aluminium garnet]] (Ce<sup>3+</sup>:YAG).{{citation needed|date=October 2020}}

White LEDs can also be made by [[Physics:Coating|coating]] near-ultraviolet (NUV) LEDs with a mixture of high-efficiency [[Chemistry:Europium|europium]]-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way [[Physics:Fluorescent lamp|fluorescent lamp]]s work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.{{citation needed|date=August 2023}}

A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly [[Chemistry:Sapphire|sapphire]] [[Physics:Substrate (materials science)|substrate]] in relatively small 100- or 150-mm wafer sizes.<ref name="electronicdesign.com">[http://electronicdesign.com/europe-news/next-generation-gan-si-white-leds-suppress-costs Next-Generation GaN-on-Si White LEDs Suppress Costs], Electronic Design, 19 November 2013</ref> The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It was predicted that since 2020, 40% of all GaN LEDs are made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.<ref>[http://www.isuppli.com/Semiconductor-Value-Chain/News/Pages/GaN-on-Silicon-LEDs-Forecast-to-Increase-Market-Share-to-40Percent-by-2020.aspx GaN-on-Silicon LEDs Forecast to Increase Market Share to 40 Percent by 2020], iSuppli, 4 December 2013</ref>

=== Mixed white LEDs ===
[[File:Led Lights Panel.jpg|thumb|Tunable white LED array in a floodlight]]
There are RGBW LEDs that combine RGB units with a phosphor white LED on the market. Doing so retains the extremely tunable color of RGB LED, but allows color rendering and efficiency to be optimized when a color close to white is selected.<ref>{{cite web |title=All You Want to Know about RGBW LED Light |url=https://www.agcled.com/blog/all-you-want-to-know-about-rgbw-led-light.html |website=AGC Lighting}}</ref>

Some phosphor white LED units are "tunable white", blending two extremes of color temperatures (commonly 2700K and 6500K) to produce intermediate values. This feature allows users to change the lighting to suit the current use of a multifunction room.<ref>{{cite web |title=Tunable White Application Note |url=https://support.enlightedinc.com/hc/en-us/articles/360031886233-Tunable-White-Application-Note |website=enlightedinc.com}}</ref> As illustrated by a straight line on the chromaticity diagram, simple two-white blends will have a pink bias, becoming most severe in the middle. A small amount of green light, provided by another LED, could correct the problem.<ref>{{cite web |title=2021 How Green Light Can Maximize the Quality of Tunable White – LEDucation |url=https://leducation.org/green-light-can-maximize/}}</ref> Some products are RGBWW, i.e. RGBW with tunable white.<ref name=EG.COL.T>{{cite web |title=Understanding LED Color-Tunable Products |url=https://www.energy.gov/eere/ssl/understanding-led-color-tunable-products |website=Energy.gov |language=en}}</ref>

A final class of white LED with mixed light is dim-to-warm. These are ordinary 2700K white LED bulbs with a small red LED that turns on when the bulb is dimmed. Doing so makes the color warmer, emulating an incandescent light bulb.<ref name=EG.COL.T/>

=== Other white LEDs ===
Another method used to produce experimental white light LEDs used no phosphors at all and was based on [[Physics:Epitaxy|homoepitaxially]] grown [[Chemistry:Zinc selenide|zinc selenide]] (ZnSe) on a ZnSe substrate that simultaneously emitted blue light from its active region and yellow light from the substrate.<ref>{{cite web| title = Joint venture to make ZnSe white LEDs| date = December 6, 2002| author = Whitaker, Tim| url = http://optics.org/cws/article/research/16534| access-date = January 3, 2009}}</ref>

==Organic light-emitting diodes (OLEDs)==
{{Main|Chemistry:OLED}}

In an organic light-emitting diode ([[Chemistry:OLED|OLED]]), the [[Physics:Electroluminescence|electroluminescent]] material composing the emissive layer of the diode is an [[Chemistry:Organic compound|organic compound]]. The organic material is electrically conductive due to the [[Physics:Delocalized electron|delocalization]] of [[Chemistry:Pi bond|pi electrons]] caused by [[Chemistry:Conjugated system|conjugation]] over all or part of the molecule, and the material therefore functions as an [[Chemistry:Organic semiconductor|organic semiconductor]].<ref>{{Cite journal |last1 = Burroughes |first1 = J. H. |last2 = Bradley | first2 = D. D. C. |last3 = Brown |first3 = A. R. |last4 = Marks |first4 = R. N. |last5 = MacKay |first5 = K. |last6 = Friend |first6 = R. H. |last7 = Burns |first7 = P. L. |last8 = Holmes |first8 = A. B. |doi = 10.1038/347539a0 |title = Light-emitting diodes based on conjugated polymers |journal = Nature |volume = 347 |issue = 6293 |pages = 539–541 |year = 1990 |bibcode=1990Natur.347..539B|s2cid = 43158308 }}</ref> The organic materials can be small organic [[Physics:Molecule|molecule]]s in a [[Earth:Crystal|crystal]]line [[Physics:Phase (matter)|phase]], or [[Chemistry:Polymer|polymer]]s.<ref name="OLEDSolidState">{{Cite conference |last1=Kho |first1=Mu-Jeong |last2=Javed |first2=T. |last3=Mark |first3=R. |last4=Maier |first4=E. |last5=David |first5=C |title=Final Report: OLED Solid State Lighting |conference=Kodak European Research |date=March 4, 2008 |location=Cambridge Science Park, Cambridge, UK}}</ref>

The potential advantages of OLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, and high contrast and color [[Gamut|gamut]].<ref name="bardsley">{{Cite journal |last1 = Bardsley |first1 = J. N. |doi = 10.1109/JSTQE.2004.824077 |title = International OLED Technology Roadmap |journal = IEEE Journal of Selected Topics in Quantum Electronics |volume = 10 | issue = 1 |pages = 3–4 |year = 2004 |bibcode = 2004IJSTQ..10....3B |s2cid = 30084021 |url = https://zenodo.org/record/1232213 }}</ref> Polymer LEDs have the added benefit of printable and [[Chemistry:Flexible organic light-emitting diode|flexible]] displays.<ref>{{Cite journal |last1 = Hebner | first1 = T. R. |last2 = Wu |first2 = C. C. |last3 = Marcy |first3 = D. |last4 = Lu |first4 = M. H. |last5 = Sturm |first5 = J. C. |title = Ink-jet printing of doped polymers for organic light emitting devices |doi = 10.1063/1.120807 |journal = Applied Physics Letters |volume = 72 |issue = 5 | page = 519 |year = 1998 |bibcode = 1998ApPhL..72..519H | s2cid = 119648364 }}</ref><ref>{{Cite journal |last1 = Bharathan |first1 = J. |last2 = Yang |first2 = Y. |doi = 10.1063/1.121090 |title = Polymer electroluminescent devices processed by inkjet printing: I. Polymer light-emitting logo |journal = Applied Physics Letters |volume = 72 |issue = 21 |page = 2660 |year = 1998 |bibcode = 1998ApPhL..72.2660B |s2cid = 44128025 }}</ref><ref>{{Cite journal |last1 = Gustafsson |first1 = G. |last2 = Cao |first2 = Y. |last3 = Treacy |first3 = G. M. |last4 = Klavetter |first4 = F. |last5 = Colaneri |first5 = N. |last6 = Heeger |first6 = A. J. |doi = 10.1038/357477a0 |title = Flexible light-emitting diodes made from soluble conducting polymers |journal = Nature |volume = 357 |issue = 6378 |pages = 477–479 |year = 1992 |bibcode=1992Natur.357..477G|s2cid = 4366944 }}</ref> OLEDs have been used to make visual displays for portable electronic devices such as cellphones, digital cameras, lighting and televisions.<ref name="OLEDSolidState" /><ref name="bardsley" />

== Perovskite Light-emitting diodes (PeLEDs) ==
[[Physics:Perovskite (structure)|Perovskite]] light-emitting diodes (PeLEDs) have emerged as promising candidates for next-generation display and lighting technologies. In recent years, researchers have shown a growing interest in perovskite light-emitting diodes (PeLEDs) owing to their capacity for emitting light with narrow bandwidth, adjustable [[Spectrum|spectrum]], ability to deliver high color purity, and cost-effective solution fabrication.<ref name=":5">{{Cite journal |last1=Jiang |first1=Ji |last2=Chu |first2=Zema |last3=Yin |first3=Zhigang |last4=Li |first4=Jingzhen |last5=Yang |first5=Yingguo |last6=Chen |first6=Jingren |last7=Wu |first7=Jinliang |last8=You |first8=Jingbi |last9=Zhang |first9=Xingwang |date=September 2022 |title=Red Perovskite Light-Emitting Diodes with Efficiency Exceeding 25% Realized by Co-Spacer Cations |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.202204460 |journal=Advanced Materials |language=en |volume=34 |issue=36 |pages=e2204460 |doi=10.1002/adma.202204460 |pmid=35855612 |bibcode=2022AdM....3404460J |s2cid=250697931 |issn=0935-9648}}</ref><ref name=":6">{{Cite journal |last1=Bai |first1=Wenhao |last2=Xuan |first2=Tongtong |last3=Zhao |first3=Haiyan |last4=Dong |first4=Haorui |last5=Cheng |first5=Xinru |last6=Wang |first6=Le |last7=Xie |first7=Rong-Jun |date=September 2023 |title=Perovskite Light-Emitting Diodes with an External Quantum Efficiency Exceeding 30% |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.202302283 |journal=Advanced Materials |language=en |volume=35 |issue=39 |pages=e2302283 |doi=10.1002/adma.202302283 |pmid=37246938 |bibcode=2023AdM....3502283B |s2cid=258959858 |issn=0935-9648}}</ref>

=== Green PeLEDs ===
When it comes to efficiency, PeLEDs have not surpassed commercial organic light-emitting diodes ([[Chemistry:OLED|OLED]]s) because specific critical parameters, such as [[Physics:Charge carrier|charge carrier]] transport and optical output coupling efficiency, have not been thoroughly optimized.<ref name=":6" />

In response to this challenge, the development of ultra-efficient green PeLEDs with a external quantum efficiency (EQE) exceeding the remarkable 30% milestone was reported by Bai and his colleagues on May 29, 2023.<ref name=":6" /> This achievement was made by strategic adjustments in charge carrier transport and the distribution of near-field light. These optimizations effectively reduced electron leakage and resulted in an exceptional light output coupling efficiency of 41.82%. A Ni<sub>0.9</sub>Mg<sub>0.1</sub>O<sub>x</sub> film with high [[Physics:Refractive index|refractive index]] and increased hole carrier mobility was used as a hole injection layer to balance charge carrier injection, and a polyethylene glycol layer was inserted between the hole transport layer and the perovskite emission layer to prevent electron leakage and minimize photon loss.<ref name=":6" />

The modified structure of green PeLED enabled it to achieve a world-record external quantum efficiency of 30.84% (with a mean of 29.05 ± 0.77%) at a brightness level of 6514 cd/m<sup>2</sup>. This pioneering work introduces a compelling approach to building ultra-efficient PeLEDs by effectively balancing electron-hole recombination and enhancing light outcoupling.<ref name=":6" />

However, expanding the effective area of perovskite LEDs can lead to a significant drop in their performance. To address this issue, Sun et.al<ref name=":7">{{Cite journal |last1=Sun |first1=Changjiu |last2=Jiang |first2=Yuanzhi |last3=Cui |first3=Minghuan |last4=Qiao |first4=Lu |last5=Wei |first5=Junli |last6=Huang |first6=Yanmin |last7=Zhang |first7=Li |last8=He |first8=Tingwei |last9=Li |first9=Saisai |last10=Hsu |first10=Hsien-Yi |last11=Qin |first11=Chaochao |last12=Long |first12=Run |last13=Yuan |first13=Mingjian |date=2021-04-13 |title=High-performance large-area quasi-2D perovskite light-emitting diodes |journal=Nature Communications |language=en |volume=12 |issue=1 |pages=2207 |doi=10.1038/s41467-021-22529-x |pmid=33850141 |pmc=8044177 |bibcode=2021NatCo..12.2207S |issn=2041-1723}}</ref> introduced L-methionine (NVAL) to construct an intermediate phase with low formation enthalpy and COO<sup>-</sup> coordination. This new intermediate phase altered the crystallization pathway, effectively inhibiting phase segregation. Consequently, high-quality large-area quasi-2D perovskite films were achieved. They further fine-tuned the film's composite dynamics, leading to high-efficiency quasi-2D perovskite green LEDs with an effective area of 9.0 cm<sup>2</sup>. An external quantum efficiency (EQE) of 16.4% was attained at <n> = 3, making it the most efficient large-area perovskite LED. Moreover, a [[Physics:Luminance|luminance]] of 9.1×104 cd/m<sup>2</sup> was achieved in the <n> = 10 films.<ref name=":7" />

=== Blue PeLEDs ===
On March 16, 2023, Zhou et al.<ref name=":8">{{Cite journal |last1=Zhou |first1=Wei |last2=Shen |first2=Yang |last3=Cao |first3=Long-Xue |last4=Lu |first4=Yu |last5=Tang |first5=Ying-Yi |last6=Zhang |first6=Kai |last7=Ren |first7=Hao |last8=Xie |first8=Feng-Ming |last9=Li |first9=Yan-Qing |last10=Tang |first10=Jian-Xin |date=July 2023 |title=Manipulating Ionic Behavior with Bifunctional Additives for Efficient Sky-Blue Perovskite Light-Emitting Diodes |url=https://onlinelibrary.wiley.com/doi/10.1002/adfm.202301425 |journal=Advanced Functional Materials |language=en |volume=33 |issue=27 |doi=10.1002/adfm.202301425 |s2cid=257609652 |issn=1616-301X}}</ref> published a study demonstrating their successful control of ion behavior to create highly efficient sky-blue perovskite light-emitting diodes. They achieved this by utilizing a [[Engineering:Passivation|bifunctional passivator]], which consisted of Lewis base benzoic acid anions and alkali metal cations. This passivator had a dual role: it effectively passivated the deficient lead atom while inhibited the migration of halide ions. The outcome of this innovative approach was the realization of an efficient perovskite LED that emitted light at a stable wavelength of 483 nm. The LED exhibited a commendable external quantum efficiency (EQE) of 16.58%, with a peak EQE reaching 18.65%. Through optical coupling enhancement, the EQE was further boosted to 28.82%.<ref name=":8" />

=== Red PeLEDs ===
One of the most crucial aspects of lighting and display technology is the efficient generation of red emission. [[Physics:Perovskite solar cell|Quasi-2D perovskites]] have demonstrated potential for high emission efficiency due to robust carrier confinement. However, the external quantum efficiencies (EQE) of most red quasi-2D PeLEDs are not optimal due to different n-value phases within complex quasi-2D perovskite films.

To address this challenge, Jiang et.al<ref name=":5" /> published their findings in Advanced Materials on July 20, 2022. Their research focused on strategically incorporating large cations to enhance the efficiency of red light perovskite LEDs. By introducing phenethylammonium iodide (PEAI)/3-fluorophenylethylammonium iodide (m-F-PEA) and 1-naphthylmethylammonium iodide (NMAI), they achieved precise control over the phase distribution of quasi-2D perovskite materials. This approach effectively reduced the prevalence of smaller n-index phases and concurrently addressed lead and halide defects in the perovskite films. The outcome of this research was the development of perovskite LEDs capable of achieving an EQE of 25.8% at 680 nm, accompanied by a peak brightness of 1300 cd/m<sup>2</sup>.<ref name=":5" />

=== White PeLEDs ===
High-performance white perovskite LED with high light extraction efficiency can be constructed through near-field optical coupling.<ref name=":9">{{Cite journal |last1=Chen |first1=Ziming |last2=Li |first2=Zhenchao |last3=Chen |first3=Zhen |last4=Xia |first4=Ruoxi |last5=Zou |first5=Guangruixing |last6=Chu |first6=Linghao |last7=Su |first7=Shi-Jian |last8=Peng |first8=Junbiao |last9=Yip |first9=Hin-Lap |last10=Cao |first10=Yong |date=February 2021 |title=Utilization of Trapped Optical Modes for White Perovskite Light-Emitting Diodes with Efficiency over 12% |journal=Joule |volume=5 |issue=2 |pages=456–466 |doi=10.1016/j.joule.2020.12.008 |s2cid=233896421 |issn=2542-4351|doi-access=free }}</ref> The near-field optical coupling between blue perovskite diode and red perovskite nanocrystal was achieved by a reasonably designed multi-layer translucent electrode (LiF/Al/Ag/LiF). The red perovskite nano-crystalline layer allows the waveguide mode and surface plasmon polarization mode captured in the blue perovskite diode to be extracted and converted into red light emission, increasing the light extraction efficiency by 50%. At the same time, the complementary emission spectra of blue photons and down-converted red photons contribute to the formation of white LEDs. Finally, the off-device quantum efficiency exceeds 12%, and the brightness exceeds 2000cd/m<sup>2</sup>, which are both the highest in white PeLEDs.<ref name=":9" />

=== Lifetime ===
Preparing high-quality all-inorganic perovskite films through solution-based methods remains a formidable challenge, primarily attributed to the rapid and uncontrollable crystallization of such materials. The key innovation involved controlling the crystal orientation of the all-inorganic perovskite along the (110) plane through a low-temperature [[Physics:Annealing (materials science)|annealing]] process (35-40°C). This precise control led to the orderly stacking of crystals, which significantly increased surface coverage and reduced defects within the material. After thorough optimization, the well-oriented CsPbBr<sub>3</sub> perovskite LED achieved an external quantum efficiency (EQE) of up to 16.45%, a remarkable brightness of 79,932 cd/m<sup>2</sup>, and a lifespan of 136 hours when initially operated at a brightness level of 100 cd/m<sup>2</sup>.<ref>{{Cite journal |last1=Feng |first1=Wenjing |last2=Lin |first2=Kebin |last3=Li |first3=Wenqiang |last4=Xiao |first4=Xiangtian |last5=Lu |first5=Jianxun |last6=Yan |first6=Chuanzhong |last7=Liu |first7=Xinyi |last8=Xie |first8=Liqiang |last9=Tian |first9=Chengbo |last10=Wu |first10=Dan |last11=Wang |first11=Kai |last12=Wei |first12=Zhanhua |date=2021-05-04 |title=Efficient all-inorganic perovskite light-emitting diodes enabled by manipulating the crystal orientation |url=https://pubs.rsc.org/en/content/articlelanding/2021/ta/d1ta00093d |journal=Journal of Materials Chemistry A |language=en |volume=9 |issue=17 |pages=11064–11072 |doi=10.1039/D1TA00093D |s2cid=234226202 |issn=2050-7496}}</ref>

On September 20, 2021, the team led by Sargent et.al<ref name=":10">{{Cite journal |last1=Liu |first1=Yuan |last2=Dong |first2=Yitong |last3=Zhu |first3=Tong |last4=Ma |first4=Dongxin |last5=Proppe |first5=Andrew |last6=Chen |first6=Bin |last7=Zheng |first7=Chao |last8=Hou |first8=Yi |last9=Lee |first9=Seungjin |last10=Sun |first10=Bin |last11=Jung |first11=Eui Hyuk |last12=Yuan |first12=Fanglong |last13=Wang |first13=Ya-kun |last14=Sagar |first14=Laxmi Kishore |last15=Hoogland |first15=Sjoerd |date=2021-09-29 |title=Bright and Stable Light-Emitting Diodes Based on Perovskite Quantum Dots in Perovskite Matrix |url=https://pubs.acs.org/doi/10.1021/jacs.1c02148 |journal=Journal of the American Chemical Society |language=en |volume=143 |issue=38 |pages=15606–15615 |doi=10.1021/jacs.1c02148 |pmid=34542273 |s2cid=237574321 |issn=0002-7863}}</ref> from the University of Toronto published their research findings in the Journal of the American Chemical Society (JACS) on bright and stable light-emitting diodes (LEDs) based on perovskite quantum dots within a perovskite matrix. The research reported that perovskite quantum dots remain stable in a precursor solution thin film of perovskite and drive the uniform [[Physics:Crystallization|crystallization]] of the perovskite matrix using strain quantum dots as nucleation centers. The type I band alignment ensures that quantum dots act as charge acceptors and radiative emitters.<ref name=":10" />

The new material exhibits suppressed biexciton Auger recombination and bright luminescence even at high excitation (600 W/cm<sup>2</sup>). The red LEDs based on the new material demonstrate an [[Engineering:Quantum efficiency|external quantum efficiency]] of 18% and maintain high performance at a brightness exceeding 4700 cd/m<sup>2</sup>. The new material extends the LED's operating half-life to 2400 hours at an initial brightness of 100 cd/m<sup>2</sup>.<ref name=":10" />

==Types==
[[File:Verschiedene LEDs.jpg|thumb|LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern high-power LEDs such as those used for lighting and backlighting are generally found in [[Engineering:Surface-mount technology|surface-mount technology]] (SMT) packages (not shown).]]

LEDs are made in different packages for different applications. A single or a few LED junctions may be packed in one miniature device for use as an indicator or pilot lamp. An LED array may include controlling circuits within the same package, which may range from a simple resistor, blinking or color changing control, or an addressable controller for RGB devices.  Higher-powered white-emitting devices will be mounted on heat sinks and will be used for illumination. Alphanumeric displays in dot matrix or bar formats are widely available. Special packages permit connection of LEDs to optical fibers for high-speed data communication links.

===Miniature===
[[File:Single and multicolor surface mount miniature LEDs in most common sizes.jpg|thumb|Image of miniature [[Engineering:SMD LED|surface mount LED]]s in most common sizes. They can be much smaller than a traditional 5{{nbsp}}mm lamp type LED, shown on the upper left corner.]]
[[File:Very small 1.6x1.6x0.35 mm RGB Surface Mount LED EAST1616RGBA2.jpg|thumb|Very small (1.6×1.6×0.35{{nbsp}}mm) red, green, and blue surface mount miniature LED package with gold [[Engineering:Wire bonding|wire bonding]] details.]]

These are mostly single-die LEDs used as indicators, and they come in various sizes from 2&nbsp;mm to 8&nbsp;mm, through-hole and surface mount packages.<ref>[http://www.elektor.com/magazines/2008/february/power-to-the-leds.350167.lynkx LED-design]. Elektor.com. Retrieved on March 16, 2012. {{webarchive |url=https://web.archive.org/web/20120831112624/http://www.elektor.com/magazines/2008/february/power-to-the-leds.350167.lynkx |date=August 31, 2012 }}</ref> Typical current ratings range from around 1&nbsp;mA to above 20&nbsp;mA.  Multiple LED dies attached to a flexible backing tape form an LED strip light.{{citation needed|date=October 2020}}

Common package shapes include round, with a domed or flat top, rectangular with a flat top (as used in bar-graph displays), and triangular or square with a flat top. The encapsulation may also be clear or tinted to improve contrast and viewing angle. Infrared devices may have a black tint to block visible light while passing infrared radiation.{{citation needed|date=October 2020}}

Ultra-high-output LEDs are designed for viewing in direct sunlight.{{citation needed|date=October 2020}}

5&nbsp;V and 12&nbsp;V LEDs are ordinary miniature LEDs that have a series resistor for direct connection to a 5{{nbsp}}V or 12{{nbsp}}V supply.{{citation needed|date=October 2020}}

===High-power===
[[File:2007-07-24 High-power light emitting diodes (Luxeon, Lumiled).jpg|thumb|High-power light-emitting diodes attached to an LED star base (Luxeon, Lumileds)]]

High-power LEDs (HP-LEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens.<ref>{{cite web|url=http://www.luminus.com/content1044|archive-url=https://web.archive.org/web/20080725033952/http://www.luminus.com/content1044 |archive-date=2008-07-25|title=Luminus Products |publisher=Luminus Devices |access-date=October 21, 2009}}</ref><ref>{{cite web|url=http://www.luminus.com/stuff/contentmgr/files/0/7c8547b3575bcecc577525b80d210ac7/misc/pds_001314_rev_03__cst_90_w_product_datasheet_illumination.pdf|archive-url=https://web.archive.org/web/20100331100545/http://www.luminus.com/stuff/contentmgr/files/0/7c8547b3575bcecc577525b80d210ac7/misc/pds_001314_rev_03__cst_90_w_product_datasheet_illumination.pdf |archive-date=2010-03-31|title=Luminus Products CST-90 Series Datasheet |publisher=Luminus Devices |access-date=October 25, 2009}}</ref> LED [[Physics:Power density|power densities]] up to 300 W/cm<sup>2</sup> have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from an HP-LED is not removed, the device fails in seconds. One HP-LED can often replace an incandescent bulb in a [[Engineering:Flashlight|flashlight]], or be set in an array to form a powerful [[Engineering:LED lamp|LED lamp]].

Some well-known HP-LEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamp. As of September 2009, some HP-LEDs manufactured by Cree now exceed 105&nbsp;lm/W.<ref name="Xlamp Xp-G Led">{{cite web|url=http://www.cree.com/products/xlamp_xpg.asp |title=Xlamp Xp-G Led |website=Cree.com |publisher=Cree, Inc. |access-date=March 16, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120313082324/http://www.cree.com/products/xlamp_xpg.asp |archive-date=March 13, 2012 }}</ref>

Examples for [[Haitz's law]]—which predicts an exponential rise in light output and efficacy of LEDs over time—are the CREE XP-G series LED, which achieved 105{{nbsp}}lm/W in 2009<ref name="Xlamp Xp-G Led" /> and the Nichia 19 series with a typical efficacy of 140{{nbsp}}lm/W, released in 2010.<ref>[http://www.nichia.co.jp/en/about_nichia/2010/2010_110201.html High Power Point Source White Led NVSx219A]. Nichia.co.jp, November 2, 2010.</ref>

===AC-driven===
LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficiency of this type of HP-LED is typically 40{{nbsp}}lm/W.<ref>{{cite web|url=http://www.ledsmagazine.com/news/3/11/14|title=Seoul Semiconductor launches AC LED lighting source Acrich |publisher=LEDS Magazine |access-date=February 17, 2008|date=November 17, 2006}}</ref> A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, named 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.<ref name="IDA" />

===Strip===
{{excerpt|LED strip light}}

=== Application-specific ===
[[File:RGB-SMD-LED.jpg|thumb|RGB-SMD-LED]]
[[File:Macro photo of LED matrix.jpg|thumb|Composite image of an {{nowrap|11 × 44}} LED matrix lapel [[Engineering:Name tag|name tag]] display using 1608/0603-type SMD LEDs. Top: A little over half of the {{nowrap|21 × 86 mm}} display. Center: Close-up of LEDs in ambient light. Bottom: LEDs in their own red light.]]
; Flashing: Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated [[Physics:Voltage regulator|voltage regulator]] and a [[Engineering:Multivibrator|multivibrator]] circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs, this circuit is visible as a small black dot. Most flashing LEDs emit light of one color, but more sophisticated devices can flash between multiple colors and even fade through a color sequence using RGB color mixing. Flashing SMD LEDs in the 0805 and other size formats have been available since early 2019.
; Flickering: Integrated electronics Simple electronic circuits integrated into the LED package have been around since at least 2011 which produce a random LED intensity pattern reminiscent of a flickering candle.<ref>{{Cite web |last=Oskay |first=Windell |date=2011-06-22 |title=Does this LED sound funny to you? |url=https://www.evilmadscientist.com/2011/does-this-led-sound-funny-to-you/ |url-status=live |archive-url=https://web.archive.org/web/20230924100327/https://www.evilmadscientist.com/2011/does-this-led-sound-funny-to-you/ |archive-date=2023-09-24 |access-date=2024-01-30 |website=Evil Mad Scientist Laboratories |language=en-US}}</ref> [[Reverse engineering]] in 2024 has suggested that some flickering LEDs with automatic sleep and wake modes might be using an integrated [[8-bit computing|8-bit]] [[Engineering:Microcontroller|microcontroller]] for such functionally.<ref>{{Cite web |last=Tim's Blog |date=2024-01-14 |title=Revisiting Candle Flicker-LEDs: Now with integrated Timer |url=https://cpldcpu.wordpress.com/2024/01/14/revisiting-candle-flicker-leds-now-with-integrated-timer/ |url-status=live |archive-url=https://web.archive.org/web/20240129164612/https://cpldcpu.wordpress.com/2024/01/14/revisiting-candle-flicker-leds-now-with-integrated-timer/ |archive-date=2024-01-29 |access-date=2024-01-30 |website=cpldcpu.wordpress.com |language=en}}</ref>
; Bi-color: Bi-color LEDs contain two different LED emitters in one case. There are two types of these. One type consists of two dies connected to the same two leads [[Engineering:Antiparallel (electronics)|antiparallel]] to each other. Current flow in one direction emits one color, and current in the opposite direction emits the other color. The other type consists of two dies with separate leads for both dies and another lead for common anode or cathode so that they can be controlled independently.  The most common bi-color combination is [[RG color space|red/traditional green]]. Others include amber/traditional green, red/pure green, red/blue, and blue/pure green.
; RGB tri-color: Tri-color LEDs contain three different LED emitters in one case. Each emitter is connected to a separate lead so they can be controlled independently. A four-lead arrangement is typical with one common lead (anode or cathode) and an additional lead for each color. Others have only two leads (positive and negative) and have a built-in electronic controller. [[RGB color model|RGB]] LEDs consist of one red, one green, and one blue LED.<ref>{{Cite book |url=https://books.google.com/books?id=qk1hmpEQVxIC&pg=PA349 |title=5th Kuala Lumpur International Conference on Biomedical Engineering 2011: BIOMED 2011, 20–23 June 2011, Kuala Lumpur, Malaysia |last=Ting |first=Hua-Nong |date=2011-06-17|publisher=Springer Science & Business Media |isbn=9783642217296}}</ref> By independently [[Pulse-width modulation|adjusting]] each of the three, RGB LEDs are capable of producing a wide color gamut. Unlike dedicated-color LEDs, these do not produce pure wavelengths. Modules may not be optimized for smooth color mixing.
; Decorative-multicolor: Decorative-multicolor LEDs incorporate several emitters of different colors supplied by only two lead-out wires. Colors are switched internally by varying the supply voltage.
; Alphanumeric: Alphanumeric LEDs are available in [[Engineering:Seven-segment display|seven-segment]], starburst, and [[Dot-matrix display|dot-matrix]] format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5×7 pixels per character. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.
; Digital RGB: Digital RGB addressable LEDs contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, clock and sometimes a strobe signal. These are connected in a [[Daisy chain (electrical engineering)|daisy chain]], which allows individual LEDs in a long LED strip light to be easily controlled by a microcontroller. Data sent to the first LED of the chain can control the brightness and color of each LED independently of the others. They are used where a combination of maximum control and minimum visible electronics are needed such as strings for Christmas and LED matrices. Some even have refresh rates in the kHz range, allowing for basic video applications. These devices are known by their part number ([https://cdn-shop.adafruit.com/datasheets/WS2812.pdf WS2812] being common) or a brand name such as [[Company:Adafruit Industries#NeoPixel|NeoPixel]].
; Filament: An LED filament consists of multiple LED chips connected in series on a common longitudinal substrate that forms a thin rod reminiscent of a traditional incandescent filament.<ref>{{cite web|url=http://www.ledinside.com/knowledge/2015/2/the_next_generation_of_led_filament_bulbs|title=The Next Generation of LED Filament Bulbs|website=LEDInside.com|publisher=Trendforce|access-date=October 26, 2015}}</ref> These are being used as a low-cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments use a rather high voltage, allowing them to work efficiently with mains voltages. Often a simple rectifier and capacitive current limiting are employed to create a low-cost replacement for a traditional light bulb without the complexity of the low voltage, high current converter that single die LEDs need.<ref>Archived at [https://ghostarchive.org/varchive/youtube/20211211/H_XiunR-cAQ Ghostarchive]{{cbignore}} and the [https://web.archive.org/web/20151122213511/https://www.youtube.com/watch?v=H_XiunR-cAQ Wayback Machine]{{cbignore}}: {{cite web|url=https://www.youtube.com/watch?v=H_XiunR-cAQ|title=LED Filaments|website=[[Company:YouTube|YouTube]]|access-date=October 26, 2015}}{{cbignore}}</ref> Usually, they are packaged in bulb similar to the lamps they were designed to replace, and filled with inert gas at slightly lower than ambient pressure to remove heat efficiently and prevent corrosion.
; Chip-on-board arrays: Surface-mounted LEDs are frequently produced in [[Engineering:Chip on board|chip on board]] (COB) arrays,  allowing better heat dissipation than with a single LED of comparable luminous output.<ref>{{cite book|title=Handbook on the Physics and Chemistry of Rare Earths: Including Actinides|url=https://books.google.com/books?id=lO_lCgAAQBAJ&pg=PA89|date=1 August 2016|publisher=Elsevier Science|isbn=978-0-444-63705-5|page=89}}</ref>  The LEDs can be arranged around a cylinder, and are called "corn cob lights" because of the rows of yellow LEDs.<ref>{{cite web |title=Corn Lamps: What Are They & Where Can I Use Them? |date=September 1, 2016 |publisher=Shine Retrofits |url=https://www.shineretrofits.com/lighting-center/corn-lamps |access-date=December 30, 2018}}</ref>

== Considerations for use ==
{{pro and con list|section|date=October 2023}}

=== Power sources ===
[[File:LED circuit.svg|thumb|upright|Simple LED circuit with resistor for current limiting]]

The current in an LED or other diodes rises exponentially with the applied voltage (see [[Shockley diode equation]]), so a small change in voltage can cause a large change in current. Current through the LED must be regulated by an external circuit such as a [[Physics:Constant current|constant current]] source to prevent damage. Since most common power supplies are (nearly) constant-voltage sources, LED fixtures must include a power converter, or at least a current-limiting resistor. In some applications, the internal resistance of small batteries is sufficient to keep current within the LED rating.{{citation needed|date=October 2020}}

=== Electrical polarity ===

Unlike a traditional incandescent lamp, an LED will light only when voltage is applied in the forward direction of the diode. No current flows and no light is emitted if  voltage is applied in the reverse direction. If the reverse voltage exceeds the [[Physics:Breakdown voltage|breakdown voltage]], which is typically about five volts, a large current flows and the LED will be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful [[Hardware random number generator|noise diode]].{{citation needed|date=October 2020}}

By definition, the energy band gap of any diode is higher when reverse-biased than when forward-biased.  Because the band gap energy determines the wavelength of the light emitted, the color cannot be the same when reverse-biased. The reverse breakdown voltage is sufficiently high that the emitted wavelength cannot be similar enough to still be visible.  Though dual-LED packages exist that contain a different color LED in each direction, it is not expected that any single LED element can emit visible light when reverse-biased.{{citation needed|date=December 2022}}

It is not known if any zener diode could exist that emits light only in reverse-bias mode.  Uniquely, this type of LED would conduct when connected backwards.

=== Safety and health ===

Certain blue LEDs and cool-white LEDs can exceed safe limits of the so-called blue-light hazard as defined in eye safety specifications such as "ANSI/IESNA RP-27.1–05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems".<ref name="BlueLEDAhealthHazard">{{cite news | url=http://texyt.com/bright+blue+leds+annoyance+health+risks | title=Blue LEDs: A health hazard? | publisher=texyt.com | date=January 15, 2007 | access-date=September 3, 2007}}</ref> One study showed no evidence of a risk in normal use at domestic illuminance,<ref>[https://www.radioprotection.org/articles/radiopro/abs/2017/04/radiopro170025/radiopro170025.html Some evidences that white LEDs are toxic for human at domestic radiance?]. Radioprotection (2017-09-12). Retrieved on 2018-07-31.</ref> and that caution is only needed for particular occupational situations or for specific populations.<ref>Point, S. and Barlier-Salsi, A. (2018) [https://web.archive.org/web/20180614144321/http://www.sfrp.asso.fr/medias/sfrp/documents/Divers/Fiche%20lampes%20%C3%A0%20Led%20SFRP%20-%20Anglais%20_%2006-2018%20(2).pdf LEDs lighting and retinal damage], technical information sheets, SFRP</ref> In 2006, the [[Organization:International Electrotechnical Commission|International Electrotechnical Commission]] published ''IEC 62471 Photobiological safety of lamps and lamp systems'', replacing the application of early laser-oriented standards for classification of LED sources.<ref>{{cite web |url=https://www.ledsmagazine.com/articles/2012/11/led-based-products-must-meet-photobiological-safety-standards-part-2-magazine.html |title=LED Based Products Must Meet Photobilogical Safety Standards: Part 2 |website=ledsmagazine.com |date=29 November 2011 |access-date=9 January 2022 }}</ref>

While LEDs have the advantage over [[Physics:Fluorescent lamp|fluorescent lamp]]s, in that they do not contain [[Chemistry:Mercury (element)|mercury]], they may contain other hazardous metals such as [[Chemistry:Lead|lead]] and [[Chemistry:Arsenic|arsenic]].<ref name=Limetal2011>{{Cite journal | last1 = Lim | first1 = S. R. | last2 = Kang | first2 = D. | last3 = Ogunseitan | first3 = O. A. | last4 = Schoenung | first4 = J. M. | title = Potential Environmental Impacts of Light-Emitting Diodes (LEDs): Metallic Resources, Toxicity, and Hazardous Waste Classification | doi = 10.1021/es101052q | journal = Environmental Science & Technology | volume = 45 | issue = 1 | pages = 320–327 | year = 2011 | pmid = 21138290 | bibcode = 2011EnST...45..320L}}</ref>

In 2016 the American Medical Association (AMA) issued a statement concerning the possible adverse influence of blueish [[Engineering:Street light|street light]]ing on the sleep-wake cycle of city-dwellers. Industry critics claim exposure levels are not high enough to have a noticeable effect.<ref>{{cite web |url=https://www.ledroadwaylighting.com/fr/nouvelles/612-response-to-the-american-medical-association-statement-on-high-intensity-street-lighting.html |title=Response to the AMA Statement on High Intensity Street Lighting |website=ledroadwaylighting.com |access-date=17 January 2019 }}</ref>

=== Advantages ===

* Efficiency: LEDs emit more lumens per watt than incandescent light bulbs.<ref>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/comparing.html|archive-url=https://web.archive.org/web/20090505080533/http://www1.eere.energy.gov/buildings/ssl/comparing.html |archive-date=2009-05-05|title=Solid-State Lighting: Comparing LEDs to Traditional Light Sources|website=eere.energy.gov}}</ref> The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
* Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
* Size: LEDs can be very small (smaller than 2&nbsp;mm<sup>2</sup><ref>{{cite web|url=http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/MDEI5980603.pdf|archive-url=https://web.archive.org/web/20090205040334/http://www.dialight.com/Assets/Brochures_And_Catalogs/Indication/MDEI5980603.pdf |archive-date=2009-02-05|title=Dialight Micro LED SMD LED "598 SERIES" Datasheet|website=Dialight.com}}
</ref>) and are easily attached to printed circuit boards.
* Switch on time: LEDs light up extremely quickly. A typical red indicator LED achieves full brightness in under a [[Microsecond|microsecond]].<ref>{{cite web|url=http://www.avagotech.com/docs/AV02-1555EN|title=Data Sheet&nbsp;— HLMP-1301, T-1 (3 mm) Diffused LED Lamps |publisher=Avago Technologies |access-date=May 30, 2010}}</ref> LEDs used in communications devices can have even faster response times.
* Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike incandescent and fluorescent lamps that fail faster when cycled often, or [[Physics:High-intensity discharge lamp|high-intensity discharge lamp]]s (HID lamps) that require a long time to warm up to full output and to cool down before they can be lighted again if they are being restarted.
* Dimming: LEDs can very easily be [[Earth:Dimmer|dimmed]] either by [[Pulse-width modulation|pulse-width modulation]] or lowering the forward current.<ref>{{Cite book |last1=Narra |first1=Prathyusha |last2=Zinger |first2=D.S. |title=Conference Record of the 2004 IEEE Industry Applications Conference, 2004. 39th IAS Annual Meeting |chapter=An effective LED dimming approach |year=2004|volume=3 |pages= 1671–1676 |doi=10.1109/IAS.2004.1348695 |isbn=978-0-7803-8486-6 |s2cid=16372401 }}</ref> This pulse-width modulation is why LED lights, particularly headlights on cars,  when viewed on camera or by some people, seem to flash or flicker. This is a type of [[Physics:Stroboscopic effect|stroboscopic effect]].
* Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.
* Slow failure: LEDs mainly fail by dimming over time, rather than the abrupt failure of incandescent bulbs.<ref name=eere>{{cite web|url=http://www1.eere.energy.gov/buildings/ssl/lifetime.html |title=Lifetime of White LEDs |access-date=2009-04-10 |url-status=dead |archive-url=https://web.archive.org/web/20090410145015/http://www1.eere.energy.gov/buildings/ssl/lifetime.html |archive-date=April 10, 2009 |df=mdy }}, US Department of Energy</ref>
* Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be shorter or longer.<ref>[http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lifetime_white_leds_aug16_r1.pdf Lifetime of White LEDs] {{Webarchive|url=https://web.archive.org/web/20160528075610/http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/lifetime_white_leds_aug16_r1.pdf |date=May 28, 2016 }}. US Department of Energy. (PDF). Retrieved on March 16, 2012.</ref> Fluorescent tubes typically are rated at about 10,000 to 25,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 to 2,000 hours. Several DOE demonstrations have shown that reduced maintenance costs from this extended lifetime, rather than energy savings, is the primary factor in determining the payback period for an LED product.<ref>{{cite web|url=http://energy.ltgovernors.com/in-depth-advantages-of-led-lighting.html|title=In depth: Advantages of LED Lighting|website=energy.ltgovernors.com}}</ref>
* Shock resistance: LEDs, being solid-state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs, which are fragile.<ref>{{cite web|url=https://www.larsonelectronics.com/a-5-led-light-bars-for-off-road-illumination.aspx|title=LED Light Bars For Off Road Illumination|website=Larson Electronics}}</ref>
* Focus: The solid package of the LED can be designed to [[Physics:Focus (optics)|focus]] its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner. For larger LED packages [[Total internal reflection|total internal reflection]] (TIR) lenses are often used to the same effect. When large quantities of light are needed, many light sources are usually deployed, which are difficult to focus or collimate on the same target.

=== Disadvantages ===

* Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment&nbsp;– or thermal management properties. Overdriving an LED in high ambient temperatures may result in overheating the LED package, eventually leading to device failure. An adequate [[Physics:Heat sink|heat sink]] is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures, and require low failure rates. 
* Voltage sensitivity: LEDs must be supplied with a voltage above their [[Physics:P–n junction#Forward bias|threshold voltage]] and a current below their rating. Current and lifetime change greatly with a small change in applied voltage. They thus require a current-regulated supply (usually just a series resistor for indicator LEDs).<ref>[http://www.ledmuseum.org/ The LED Museum]. Retrieved on March 16, 2012.</ref>
* Color rendition: Most cool-white LEDs have spectra that differ significantly from a [[Astronomy:Black body|black body]] radiator like the sun or an incandescent light. The spike at 460&nbsp;nm and dip at 500&nbsp;nm can make the color of objects [[Color vision|appear differently]] under cool-white LED illumination than sunlight or incandescent sources, due to metamerism,<ref>{{cite web|url = http://www.jimworthey.com/jimtalk2006feb.html|title = How White Light Works|author = Worthey, James A. |website = LRO Lighting Research Symposium, Light and Color|access-date = October 6, 2007}}</ref> red surfaces being rendered particularly poorly by typical phosphor-based cool-white LEDs. The same is true with green surfaces. The quality of color rendition of an LED is measured by the [[Earth:Color rendering index|Color Rendering Index (CRI)]]. 
* Area light source: Single LEDs do not approximate a [[Physics:Point source|point source]] of light giving a spherical light distribution, but rather a [[Lambert's cosine law|lambertian]] distribution. So, LEDs are difficult to apply to uses needing a spherical light field. Different fields of light can be manipulated by the application of different optics or "lenses". LEDs cannot provide divergence below a few degrees.<ref>{{Cite book|author=Hecht, E. |title=Optics|url=https://archive.org/details/optics00ehec |url-access=limited |edition=4|page=[https://archive.org/details/optics00ehec/page/n596 591]|publisher=Addison Wesley|year= 2002|isbn=978-0-19-510818-7}}</ref>
* [[Astronomy:Light pollution|Light pollution]]: Because white LEDs emit more short wavelength light than sources such as high-pressure sodium vapor lamps, the increased blue and green sensitivity of [[Biology:Scotopic vision|scotopic vision]] means that white LEDs used in outdoor lighting cause substantially more sky glow.<ref name="IDA">{{Cite book|title=Visibility, Environmental, and Astronomical Issues Associated with Blue-Rich White Outdoor Lighting |publisher=International Dark-Sky Association |date=May 4, 2010 |url=http://www.darksky.org/assets/documents/Reports/IDA-Blue-Rich-Light-White-Paper.pdf |url-status=dead |archive-url=https://web.archive.org/web/20130116003035/http://darksky.org/assets/documents/Reports/IDA-Blue-Rich-Light-White-Paper.pdf |archive-date=January 16, 2013 }}</ref>
* Efficiency droop: The efficiency of LEDs decreases as the [[Physics:Electric current|electric current]] increases. Heating also increases with higher currents, which compromises LED lifetime. These effects put practical limits on the current through an LED in high power applications.<ref name="stevenson">Stevenson, Richard (August 2009), "[https://web.archive.org/web/20090805082614/http://www.spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret The LED's Dark Secret: Solid-state lighting will not supplant the lightbulb until it can overcome the mysterious malady known as droop]". ''IEEE Spectrum''.</ref>
* Impact on wildlife: LEDs are much more attractive to insects than sodium-vapor lights, so much so that there has been speculative concern about the possibility of disruption to [[Biology:Food web|food web]]s.<ref>{{cite web |title=LEDs: Good for prizes, bad for insects |url=https://www.science.org/content/article/leds-good-prizes-bad-insects |website=Science |first1=Erik |last1=Stokstad |date=7 October 2014 |access-date=7 October 2014 }}</ref><ref>{{Cite journal|title=LED Lighting Increases the Ecological Impact of Light Pollution Irrespective of Color Temperature |bibcode-access=free |journal=Ecological Applications|volume=24|issue=7|pages=1561–1568|doi=10.1890/14-0468.1|pmid=29210222|year=2014|last1=Pawson|first1=S. M.|last2=Bader|first2=M. K.-F.|bibcode=2014EcoAp..24.1561P |doi-access=free}}</ref> LED lighting near beaches, particularly intense blue and white colors, can disorient turtle hatchlings and make them wander inland instead.<ref>{{Cite web|url=https://news.usc.edu/144389/usc-scientist-database-reduce-effects-of-led-light-on-animals/ |first1=Gary |last1=Polakovic |title=Scientist's new database can help protect wildlife from harmful hues of LED lights|date=2018-06-12|website=USC News|language=en-US|access-date=2019-12-16 |url-status=live |archive-url=https://web.archive.org/web/20200519125811/https://news.usc.edu/144389/usc-scientist-database-reduce-effects-of-led-light-on-animals/ |archive-date= May 19, 2020 }}</ref> The use of "turtle-safe lighting" LEDs that emit only at narrow portions of the visible spectrum is encouraged by conservancy groups in order to reduce harm.<ref>{{Cite web|url=https://conserveturtles.org/information-sea-turtles-threats-artificial-lighting/|title=Information About Sea Turtles: Threats from Artificial Lighting |website=Sea Turtle Conservancy|language=en-US|access-date=2019-12-16}}</ref>
* Use in winter conditions: Since they do not give off much heat in comparison to incandescent lights, LED lights used for traffic control can have snow obscuring them, leading to accidents.<ref>{{cite web|url=https://abcnews.go.com/GMA/ConsumerNews/led-traffic-lights-unusual-potentially-deadly-winter-problem/story?id=9506449|title=Stoplights' Unusual, Potentially Deadly Winter Problem |date=January 8, 2010|publisher=ABC News |url-status=live |archive-url=https://web.archive.org/web/20231212132630/https://abcnews.go.com/GMA/ConsumerNews/led-traffic-lights-unusual-potentially-deadly-winter-problem/story?id=9506449 |archive-date= Dec 12, 2023 }}</ref><ref>{{cite web|url=https://www.cars.com/articles/2009/12/led-traffic-lights-cant-melt-snow-ice/|title=LED Traffic Lights Can't Melt Snow, Ice |website=Cars.com |date= December 17, 2009 |first1=Stephen |last1=Markley |url-status=live |archive-url=https://web.archive.org/web/20190606011840/https://www.cars.com/articles/2009/12/led-traffic-lights-cant-melt-snow-ice/ |archive-date= Jun 6, 2019 }}</ref>
* Thermal runaway: Parallel strings of LEDs will not share current evenly due to the manufacturing tolerances in their forward voltage. Running two or more strings from a single current source may result in LED failure as the devices warm up. If forward voltage binning is not possible, a circuit is required to ensure even distribution of current between parallel strands.<ref>{{cite web |url=https://www.ledsmagazine.com/articles/print/volume-6/issue-2/features/led-design-forum-avoiding-thermal-runaway-when-driving-multiple-led-strings-magazine.html |title=LED Design Forum: Avoiding thermal runaway when driving multiple LED strings |work=LEDs Magazine |date=20 April 2009 |access-date=17 January 2019 }}</ref>

==Applications==
[[File:LED DaytimeRunningLights.jpg|thumb|Daytime running light LEDs of an automobile]]

LED uses fall into five major categories:

* Visual signals where light goes more or less directly from the source to the human eye, to convey a message or meaning
* [[Earth:Lighting|Illumination]] where light is reflected from objects to give visual response of these objects
* Measuring and interacting with processes involving no human vision<ref>[[Organization:European Photonics Industry Consortium|European Photonics Industry Consortium]] (EPIC). This includes use in data communications over [[Engineering:Optical fiber|fiber optics]] as well as "broadcast" data or signaling.</ref>
* Narrow band light sensors where LEDs operate in a reverse-bias mode and respond to incident light, instead of emitting light<ref>
Mims, Forrest M. III. [http://www.instesre.org/papers/Snowmass/MimsSnowmass.htm "An Inexpensive and Accurate Student Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors"].</ref><ref>[https://www.cs.drexel.edu/~dbrooks/globe/special_measurements/water_vapor.htm "Water Vapor Measurements with LED Detectors"]. cs.drexel.edu (2002).</ref><ref>Dziekan, Mike (February 6, 2009) [http://www.soamsci.org/tcs/weeklyIssues_2009/2009-02-06/feature1/index.html "Using Light-Emitting Diodes as Sensors"]. soamsci.or. {{webarchive |url=https://web.archive.org/web/20130531090631/http://www.soamsci.org/tcs/weeklyIssues_2009/2009-02-06/feature1/index.html |date=May 31, 2013 }}</ref><ref>{{Cite book|doi=10.1109/CVPR.2008.4587766|chapter=An LED-only BRDF measurement device|title=2008 IEEE Conference on Computer Vision and Pattern Recognition|pages=1–8|year=2008|last1=Ben-Ezra|first1=Moshe|last2=Wang|first2=Jiaping|last3=Wilburn|first3=Bennett|last4=Xiaoyang Li|last5=Le Ma|isbn=978-1-4244-2242-5|citeseerx=10.1.1.165.484|s2cid=206591080}}</ref>
* Indoor cultivation, including cannabis.<ref>Bantis, Filippos, Sonia Smirnakou, Theoharis Ouzounis, Athanasios Koukounaras, Nikolaos Ntagkas, and Kalliopi Radoglou. "[https://www.plantgrower.org/uploads/6/5/5/4/65545169/1-s2.0-s0304423818301420-main.pdf Current status and recent achievements in the field of horticulture with the use of light-emitting diodes (LEDs).]" Scientia horticulturae 235 (2018): 437-451.</ref>

===Indicators and signs===
The [[Physics:Energy conservation|low energy consumption]], low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area [[Engineering:LED display|LED display]]s are used as stadium displays, dynamic decorative displays, and dynamic message signs on freeways. Thin, lightweight message displays are used at airports and railway stations, and as [[Engineering:Destination sign|destination displays]] for trains, buses, trams, and ferries.

[[File:Red and green traffic signals, Stamford Road, Singapore - 20111210.jpg|thumb|upright|Red and green LED traffic signals]]

One-color light is well suited for traffic lights and signals, [[Engineering:Exit sign|exit sign]]s, [[Engineering:Emergency vehicle lighting|emergency vehicle lighting]], ships' navigation lights, and LED-based Christmas lights

Because of their long life, fast switching times, and visibility in broad daylight due to their high output and focus, LEDs have been used in automotive brake lights and turn signals. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, about 0.1 second faster{{citation needed|date=April 2016}} than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED appear if the eyes quickly scan across the array. White LED headlamps are beginning to appear. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with [[Parabolic reflector|parabolic reflector]]s.

Due to the relative cheapness of low output LEDs, they are also used in many temporary uses such as glowsticks, throwies, and the photonic textile Lumalive. Artists have also used LEDs for [[Engineering:LED art|LED art]].

===Lighting===
{{main|Engineering:LED lamp}}
With the development of high-efficiency and high-power LEDs, it has become possible to use LEDs in lighting and illumination. To encourage the shift to [[Engineering:LED lamp|LED lamp]]s and other high-efficiency lighting, in 2008 the US Department of Energy created the L Prize competition. The [[Company:Philips|Philips]] Lighting North America LED bulb won the first competition on August 3, 2011, after successfully completing 18 months of intensive field, lab, and product testing.<ref>[http://www.lightingprize.org/ "L-Prize U.S. Department of Energy"], L-Prize Website, August 3, 2011</ref>

Efficient lighting is needed for [[Social:Sustainable architecture|sustainable architecture]]. As of 2011, some LED  bulbs provide up to 150&nbsp;lm/W and even inexpensive low-end models typically exceed 50&nbsp;lm/W, so that a 6-watt LED could achieve the same results as a standard 40-watt incandescent bulb. The lower heat output of LEDs also reduces demand on [[Physics:Air conditioning|air conditioning]] systems. Worldwide, LEDs are rapidly adopted to displace less effective sources such as incandescent lamps and [[Physics:Compact fluorescent lamp|CFLs]] and reduce electrical energy consumption and its associated emissions. Solar powered LEDs are used as [[Engineering:Street light|street light]]s and in [[Earth:Architectural lighting design|architectural lighting]].

The mechanical robustness and long lifetime are used in [[Engineering:Automotive lighting|automotive lighting]] on cars, motorcycles, and [[Earth:Bicycle lighting#LEDs|bicycle lights]]. LED street lights are employed on poles and in parking garages. In 2007, the Italian village of Torraca was the first place to convert its street lighting to LEDs.<ref>[http://www.scientificamerican.com/article.cfm?id=led-there-be-light LED There Be Light], Scientific American, March 18, 2009</ref>

Cabin lighting on recent{{when|date=October 2022}} [[Company:Airbus|Airbus]] and [[Company:Boeing|Boeing]] jetliners uses LED lighting.  LEDs are also being used  in airport and heliport lighting. LED airport fixtures currently include medium-intensity runway lights, runway centerline lights, taxiway centerline and edge lights, guidance signs, and obstruction lighting.

LEDs are also used as a light source for [[Engineering:Digital Light Processing|DLP]] projectors, and to [[Physics:Backlight|backlight]] newer LCD television (referred to as LED TV), computer monitor (including [[Engineering:Laptop|laptop]]) and handheld device LCDs, succeeding older CCFL-backlit LCDs although being superseded by [[Chemistry:OLED|OLED]] screens. RGB LEDs raise the color gamut by as much as 45%. Screens for TV and computer displays can be made thinner using LEDs for backlighting.<ref>{{cite news|url=https://www.nytimes.com/2007/06/24/business/yourmoney/24novel.html |url-access=subscription |newspaper=New York Times|title=In Pursuit of Perfect TV Color, With L.E.D.'s and Lasers|date=June 24, 2007|first=Anne|last=Eisenberg|access-date=April 4, 2010}}</ref>

LEDs are small, durable and need little power, so they are used in handheld devices such as [[Engineering:Flashlight|flashlight]]s. LED [[Physics:Strobe light|strobe light]]s or camera flashes operate at a safe, low voltage, instead of the 250+ volts commonly found in [[Chemistry:Xenon|xenon]] flashlamp-based lighting. This is especially useful in cameras on [[Engineering:Mobile phone|mobile phone]]s, where space is at a premium and bulky voltage-raising circuitry is undesirable.

LEDs are used for infrared illumination in [[Physics:Night vision|night vision]] uses including security cameras. A ring of LEDs around a [[Physics:Video camera|video camera]], aimed forward into a retroreflective [[Engineering:Projection screen|background]], allows chroma keying in [[Engineering:Video production|video production]]s.

[[File:LED for mines.jpg|thumb|LED for miners, to increase visibility inside mines]]

[[File:Los Angeles Bridge.jpg|thumb|right| Los Angeles Vincent Thomas Bridge illuminated with blue LEDs]]

LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury to the miners.<ref>{{Cite journal | url = https://www.cdc.gov/niosh/docs/2011-192/ | title = CDC – NIOSH Publications and Products – Impact: NIOSH Light-Emitting Diode (LED) Cap Lamp Improves Illumination and Decreases Injury Risk for Underground Miners | publisher = cdc.gov | access-date=May 3, 2013| doi = 10.26616/NIOSHPUB2011192 | year = 2011 | doi-access = free }}</ref>

LEDs are increasingly finding uses in medical and educational applications, for example as mood enhancement.<ref>{{cite news |last=Janeway |first=Kimberly |url=https://www.consumerreports.org/cro/news/2014/12/led-lightbulbs-that-promise-to-help-you-sleep/index.htm |title=LED lightbulbs that promise to help you sleep |work=Consumer Reports |date=2014-12-12 |access-date=2018-05-10}}</ref> [[Organization:NASA|NASA]] has even sponsored research for the use of LEDs to promote health for astronauts.<ref>{{cite press release | url=http://www.sti.nasa.gov/tto/Spinoff2008/hm_3.html | archive-url=https://web.archive.org/web/20081013083802/http://www.sti.nasa.gov/tto/Spinoff2008/hm_3.html | url-status=dead | archive-date=October 13, 2008 | title=LED Device Illuminates New Path to Healing | publisher=nasa.gov | access-date=January 30, 2012}}</ref>

===Data communication and other signalling===
Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.<ref>{{cite journal|url=http://ntv.ifmo.ru/en/article/11192/chastotnye_harakteristiki_sovremennyh_svetodiodnyh_lyuminofornyh_materialov.htm |title=Frequency characteristics of modern LED phosphor materials |author1=Fudin, M. S. |author2=Mynbaev, K. D. |author3=Aifantis, K. E. |author4=Lipsanen H. |author5=Bougrov, V. E. |author6=Romanov, A. E. |journal=Scientific and Technical Journal of Information Technologies, Mechanics and Optics|volume=14|issue=6|year=2014}}</ref>

[[Engineering:Assistive listening device|Assistive listening device]]s in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of [[Engineering:Optical fiber|fiber optic]] cable, from digital audio over [[TOSLINK]] cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.

Because LEDs can [[Physics:Frequency|cycle on and off]] millions of times per second, very high data bandwidth can be achieved.<ref>{{Cite news|first=Hank |last=Green |url=http://www.ecogeek.org/content/view/2194/74/ |title=Transmitting Data Through LED Light Bulbs |publisher=EcoGeek |date=October 9, 2008 |access-date=February 15, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20081212050729/http://www.ecogeek.org/content/view/2194/74/ |archive-date=December 12, 2008 }}</ref> For that reason, [[Physics:Visible light communication|Visible Light Communication]] (VLC) has been proposed as an alternative to the increasingly competitive radio bandwidth.<ref name=":4">{{Cite book|last1=Dimitrov|first1=Svilen|url=https://www.cambridge.org/core/books/principles-of-led-light-communications/0528063BAA6863F6B6D61F6FF69F37CB|title=Principles of LED Light Communications: Towards Networked Li-Fi|last2=Haas|first2=Harald|date=2015|publisher=Cambridge University Press|isbn=978-1-107-04942-0|location=Cambridge|doi=10.1017/cbo9781107278929}}</ref> By operating in the visible part of the electromagnetic spectrum, data can be transmitted without occupying the frequencies of radio communications.

The main characteristic of VLC, lies on the incapacity of light to surpass physical opaque barriers. This characteristic can be considered a weak point of VLC, due to the susceptibility of interference from physical objects, but is also one of its many strengths: unlike radio waves, light waves are confined in the enclosed spaces they are transmitted, which enforces a physical safety barrier that requires a receptor of that signal to have physical access to the place where the transmission is occurring.<ref name=":4" />

A promising application of VLC lies on the [[Indoor positioning system|Indoor Positioning System]] (IPS), an analogous to the GPS built to operate in enclosed spaces where the satellite transmissions that allow the GPS operation are hard to reach. For instance, commercial buildings, shopping malls, parking garages, as well as subways and tunnel systems are all possible applications for VLC-based indoor positioning systems. Additionally, once the VLC lamps are able to perform lighting at the same time as data transmission, it can simply occupy the installation of traditional single-function lamps.

Other applications for VLC involve communication between appliances of a smart home or office. With increasing [[Internet of things|IoT]]-capable devices, connectivity through traditional radio waves might be subjected to interference.<ref>{{Cite web|title=Cisco Annual Internet Report - Cisco Annual Internet Report (2018–2023) White Paper|url=https://www.cisco.com/c/en/us/solutions/collateral/executive-perspectives/annual-internet-report/white-paper-c11-741490.html|access-date=2020-10-21|website=Cisco|language=en}}</ref> Light bulbs with VLC capabilities can transmit data and commands for such devices.

=== Machine vision systems ===
{{main|Machine vision}}
[[Machine vision]] systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used.

Barcode scanners are the most common example of machine vision applications, and many of those scanners use red LEDs instead of lasers. Optical computer mice use LEDs as a light source for the miniature camera within the mouse.

LEDs are useful for machine vision  because they provide a compact, reliable source of light.  LED lamps can be turned on and off to suit the needs of the vision system, and the shape of the beam produced can be tailored to match the system's requirements.

=== Biological detection ===
The discovery of radiative recombination in aluminum gallium nitride (AlGaN) alloys by U.S. Army Research Laboratory (ARL) led to the conceptualization of UV light-emitting diodes (LEDs) to be incorporated in light-induced [[Biology:Fluorescence|fluorescence]] sensors used for biological agent detection.<ref>{{Citation |last1=Sampath |first1=A. V. |title=THE EFFECTS OF INCREASING AlN MOLE FRACTION ON THE PERFORMANCE OF AlGaN ACTIVE REGIONS CONTAINING NANOMETER SCALE COMPOSITIONALLY INHOMOGENEITIES |date=2009-12-01 |work=Advanced High Speed Devices |volume=51 |pages=69–76 |series=Selected Topics in Electronics and Systems |publisher=WORLD SCIENTIFIC |doi=10.1142/9789814287876_0007 |isbn=9789814287869 |last2=Reed |first2=M. L. |last3=Moe |first3=C. |last4=Garrett |first4=G. A. |last5=Readinger |first5=E. D. |last6=Sarney |first6=W. L. |last7=Shen |first7=H. |last8=Wraback |first8=M. |last9=Chua |first9=C.}}</ref><ref name=":1">{{Cite journal |last1=Liao |first1=Yitao |last2=Thomidis |first2=Christos |last3=Kao |first3=Chen-kai |last4=Moustakas |first4=Theodore D. |date=2011-02-21 |title=AlGaN based deep ultraviolet light emitting diodes with high internal quantum efficiency grown by molecular beam epitaxy |journal=Applied Physics Letters |volume=98 |issue=8 |pages=081110 |doi=10.1063/1.3559842 |issn=0003-6951 |bibcode=2011ApPhL..98h1110L |doi-access=free}}</ref><ref name=":2">{{Cite journal |last1=Cabalo |first1=Jerry |last2=DeLucia |first2=Marla |last3=Goad |first3=Aime |last4=Lacis |first4=John |last5=Narayanan |first5=Fiona |last6=Sickenberger |first6=David |date=2008-10-02 |title=Overview of the TAC-BIO detector |journal=Optically Based Biological and Chemical Detection for Defence IV |publisher=International Society for Optics and Photonics |volume=7116 |pages=71160D |doi=10.1117/12.799843 |editor1-last=Carrano |editor1-first=John C. |editor2-last=Zukauskas |editor2-first=Arturas |bibcode=2008SPIE.7116E..0DC |s2cid=108562187}}</ref> In 2004, the Edgewood Chemical Biological Center (ECBC) initiated the effort to create a biological detector named TAC-BIO. The program capitalized on semiconductor UV optical sources (SUVOS) developed by the Defense Advanced Research Projects Agency (DARPA).<ref name=":2" />

UV-induced fluorescence is one of the most robust techniques used for rapid real-time detection of biological aerosols.<ref name=":2" /> The first UV sensors were lasers lacking in-field-use practicality. In order to address this, DARPA incorporated SUVOS technology to create a low-cost, small, lightweight, low-power device. The TAC-BIO detector's response time was one minute from when it sensed a biological agent. It was also demonstrated that the detector could be operated unattended indoors and outdoors for weeks at a time.<ref name=":2" />

Aerosolized biological particles fluoresce and scatter light under a UV light beam. Observed fluorescence is dependent on the applied wavelength and the biochemical fluorophores within the biological agent. UV induced fluorescence offers a rapid, accurate, efficient and logistically practical way for biological agent detection. This is because the use of UV fluorescence is reagentless, or a process that does not require an added chemical to produce a reaction, with no consumables, or produces no chemical byproducts.<ref name=":2" />

Additionally, TAC-BIO can reliably discriminate between threat and non-threat aerosols. It was claimed to be sensitive enough to detect low concentrations, but not so sensitive that it would cause false positives. The particle-counting algorithm used in the device converted raw data into information by counting the photon pulses per unit of time from the fluorescence and scattering detectors, and comparing the value to a set threshold.<ref>{{Cite journal |last1=Poldmae |first1=Aime |last2=Cabalo |first2=Jerry |last3=De Lucia |first3=Marla |last4=Narayanan |first4=Fiona |last5=Strauch III |first5=Lester |last6=Sickenberger |first6=David |date=2006-09-28 |title=Biological aerosol detection with the tactical biological (TAC-BIO) detector |journal=Optically Based Biological and Chemical Detection for Defence III |volume=6398 |pages=63980E |publisher=SPIE |doi=10.1117/12.687944 |s2cid=136864366 |editor1-last=Carrano |editor1-first=John C. |editor2-last=Zukauskas |editor2-first=Arturas}}</ref>

The original TAC-BIO was introduced in 2010, while the second-generation TAC-BIO GEN&nbsp;II, was designed in 2015 to be more cost-efficient, as plastic parts were used. Its small, light-weight design allows it to be mounted to vehicles, robots, and unmanned aerial vehicles. The second-generation device could also be utilized as an environmental detector to monitor air quality in hospitals, airplanes, or even in households to detect fungus and mold.<ref>{{Cite web |url=https://www.army.mil/article/141363/army_advances_bio_threat_detector |title=Army advances bio-threat detector |website=www.army.mil |access-date=2019-10-10}}</ref><ref>{{Cite journal |last1=Kesavan |first1=Jana |last2=Kilper |first2=Gary |last3=Williamson |first3=Mike |last4=Alstadt |first4=Valerie |last5=Dimmock |first5=Anne |last6=Bascom |first6=Rebecca |date=2019-02-01 |title=Laboratory validation and initial field testing of an unobtrusive bioaerosol detector for health care settings |journal=Aerosol and Air Quality Research |volume=19 |issue=2 |pages=331–344 |doi=10.4209/aaqr.2017.10.0371 |issn=1680-8584 |doi-access=free}}</ref>

=== Other applications ===
[[File:LED Costume by Beo Beyond.jpg|thumb|LED costume for stage performers]]
[[File:Digitally printed LED wallpaper Dolomites.jpg |thumb|LED wallpaper by Meystyle]]

The light from LEDs can be modulated very quickly so they are used extensively in [[Engineering:Optical fiber|optical fiber]] and free space optics communications. This includes [[Engineering:Remote control|remote control]]s, such as for television sets, where infrared LEDs are often used. [[Engineering:Opto-isolator|Opto-isolator]]s use an LED combined with a [[Physics:Photodiode|photodiode]] or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage [[Engineering:Sensor|sensor]] circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also lets information be transferred between circuits that do not share a common ground potential.

Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors.  The Nintendo [[Engineering:Wii|Wii]]'s sensor bar uses infrared LEDs. Pulse oximeters use them for measuring [[Biology:Oxygen saturation|oxygen saturation]]. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light.

Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a [[Engineering:Touchscreen|touchscreen]] that registers reflected light from a finger or stylus.<ref>{{cite journal |author1=Dietz, P. H. |author2=Yerazunis, W. S. |author3=Leigh, D. L. |title=Very Low-Cost Sensing and Communication Using Bidirectional LEDs |year=2004 |url=http://www.merl.com/publications/TR2003-035/}}</ref> Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase [[Earth:Photosynthesis|photosynthesis]] in [[Biology:Plant|plant]]s,<ref>{{cite journal |author1=Goins, G. D. |author2=Yorio, N. C. |author3=Sanwo, M. M. |author4=Brown, C. S. |title=Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown under red light-emitting diodes (LEDs) with and without supplemental blue lighting |journal=Journal of Experimental Botany |year=1997 |volume=48 |issue=7 |pages=1407–1413 |doi=10.1093/jxb/48.7.1407 |pmid=11541074|doi-access=free }}</ref> and bacteria and viruses can be removed from water and other substances using UV LEDs for [[Biology:Sterilization (microbiology)|sterilization]].<ref name="water sterilization" /> LEDs of certain wavelengths have also been used for light therapy treatment of [[Medicine:Neonatal jaundice|neonatal jaundice]] and [[Engineering:Acne|acne]].<ref>{{cite book |last1=Li |first1=Jinmin |last2=Wang |first2=Junxi |last3=Yi |first3=Xiaoyan |last4=Liu |first4=Zhiqiang |last5=Wei |first5=Tongbo |last6=Yan |first6=Jianchang |last7=Xue |first7=Bin |title=III-Nitrides Light Emitting Diodes: Technology and Applications |date=31 August 2020 |publisher=Springer Nature |isbn=978-981-15-7949-3 |page=248 |url=https://books.google.com/books?id=Smn6DwAAQBAJ&pg=PA248 |language=en}}</ref>

UV LEDs, with spectra range of 220&nbsp;nm to 395&nbsp;nm, have other applications, such as [[Earth:Water purification|water]]/air purification, surface disinfection, glue curing, free-space non-line-of-sight communication, high performance liquid chromatography, UV curing dye printing, phototherapy (295nm [[Chemistry:Vitamin D|Vitamin D]], 308nm [[Physics:Excimer lamp|Excimer lamp]] or laser replacement), medical/ analytical instrumentation, and DNA absorption.<ref name=":1" /><ref>{{Cite book|last1=Gaska|first1=R.|last2=Shur|first2=M. S.|last3=Zhang|first3=J.|title=2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings |chapter=Physics and Applications of Deep UV LEDs |date=October 2006|pages=842–844|doi=10.1109/ICSICT.2006.306525|isbn=1-4244-0160-7|s2cid=17258357}}</ref>

LEDs have also been used as a medium-quality [[Physics:Voltage reference|voltage reference]] in electronic circuits. The forward voltage drop (about 1.7&nbsp;V for a red LED or 1.2V for an infrared) can be used instead of a [[Physics:Zener diode|Zener diode]] in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3&nbsp;V are not widely available.

The progressive miniaturization of low-voltage lighting technology, such as LEDs and OLEDs, suitable to incorporate into low-thickness materials has fostered experimentation in combining light sources and wall covering surfaces for interior walls in the form of [[Engineering:LED wallpaper|LED wallpaper]].

<gallery widths="150px" heights="110px">
File:LED screen behind Tsach Zimroni in Tel Aviv Israel.jpg| A large LED display behind a [[Disc jockey|disc jockey]]
File:LED Digital Display.jpg| [[Engineering:Seven-segment display|Seven-segment display]] that can display four digits and points
File:LED panel and plants.jpg| LED panel light source used in an early experiment on [[Biology:Potato|potato]] growth during Shuttle mission [[Engineering:STS-73|STS-73]] to investigate the potential for growing food on future long duration missions.
</gallery>

== Research and development ==
=== Key challenges ===
LEDs require optimized efficiency to hinge on ongoing improvements such as phosphor materials and [[Physics:Quantum dot|quantum dot]]s.<ref name=":0">{{Cite web|url=https://www.energy.gov/eere/ssl/led-rd-challenges|title=LED R&D Challenges|website=Energy.gov|access-date=2019-03-13}}</ref>

The process of down-conversion (the method by which materials convert more-energetic photons to different, less energetic colors) also needs improvement. For example, the red phosphors that are used today are thermally sensitive and need to be improved in that aspect so that they do not color shift and experience efficiency drop-off with temperature. Red phosphors could also benefit from a narrower spectral width to emit more lumens and becoming more efficient at converting photons.<ref>{{Cite web|url=https://www.energy.gov/eere/ssl/downloads/july-2015-postings|title=JULY 2015 POSTINGS|website=Energy.gov|access-date=2019-03-13}}</ref>

In addition, work remains to be done in the realms of current efficiency droop, color shift, system reliability, light distribution, dimming, thermal management, and power supply performance.<ref name=":0" />

=== Potential technology ===

A new family of LEDs are based on the semiconductors called [[Physics:Perovskite (structure)|perovskites]]. In 2018, less than four years after their discovery, the ability of perovskite LEDs (PLEDs) to produce light from electrons already rivaled those of the best performing [[Chemistry:OLED|OLED]]s.<ref>{{Cite journal|last1=Di|first1=Dawei|last2=Romanov|first2=Alexander S.|last3=Yang|first3=Le|last4=Richter|first4=Johannes M.|last5=Rivett|first5=Jasmine P. H.|last6=Jones|first6=Saul|last7=Thomas|first7=Tudor H.|last8=Abdi Jalebi|first8=Mojtaba|last9=Friend|first9=Richard H.|last10=Linnolahti|first10=Mikko|last11=Bochmann|first11=Manfred|date=2017-04-14|title=High-performance light-emitting diodes based on carbene-metal-amides|journal=Science|language=en|volume=356|issue=6334|pages=159–163|doi=10.1126/science.aah4345|pmid=28360136|issn=0036-8075|url=https://ueaeprints.uea.ac.uk/63288/1/Accepted_manuscript.pdf|bibcode=2017Sci...356..159D|arxiv=1606.08868|s2cid=206651900}}</ref> They have a potential for cost-effectiveness as they can be processed from solution, a low-cost and low-tech method, which might allow perovskite-based devices that have large areas to be made with extremely low cost. Their efficiency is superior by eliminating non-radiative losses, in other words, elimination of [[Physics:Carrier generation and recombination|recombination]] pathways that do not produce photons; or by solving outcoupling problem (prevalent for thin-film LEDs) or balancing charge carrier injection to increase the EQE (external quantum efficiency). The most up-to-date PLED devices have broken the performance barrier by shooting the EQE above 20%.<ref name=":3">{{Cite journal|last1=Armin|first1=Ardalan|last2=Meredith|first2=Paul|date=October 2018|title=LED technology breaks performance barrier|journal=Nature|volume=562|issue=7726|pages=197–198|doi=10.1038/d41586-018-06923-y|pmid=30305755|bibcode=2018Natur.562..197M|doi-access=free}}</ref>

In 2018, Cao et al. and Lin et al. independently published two papers on developing perovskite LEDs with EQE greater than 20%, which made these two papers a mile-stone in PLED development. Their device have similar planar structure, i.e. the active layer (perovskite) is sandwiched between two electrodes. To achieve a high EQE, they not only reduced non-radiative recombination, but also utilized their own, subtly different methods to improve the EQE.<ref name=":3" />

In the work of Cao ''et al.'',<ref name="ReferenceA">{{Cite journal|last1=Cao|first1=Yu|last2=Wang|first2=Nana|last3=Tian|first3=He|last4=Guo|first4=Jingshu|last5=Wei|first5=Yingqiang|last6=Chen|first6=Hong|last7=Miao|first7=Yanfeng|last8=Zou|first8=Wei|last9=Pan|first9=Kang|last10=He|first10=Yarong|last11=Cao|first11=Hui|date=October 2018|title=Perovskite light-emitting diodes based on spontaneously formed submicrometre-scale structures|journal=Nature|language=en|volume=562|issue=7726|pages=249–253|doi=10.1038/s41586-018-0576-2|pmid=30305742|issn=1476-4687|bibcode=2018Natur.562..249C|doi-access=free}}</ref> researchers targeted the outcoupling problem, which is that the optical physics of thin-film LEDs causes the majority of light generated by the semiconductor to be trapped in the device.<ref>{{Cite journal|last1=Cho|first1=Sang-Hwan|last2=Song|first2=Young-Woo|last3=Lee|first3=Joon-gu|last4=Kim|first4=Yoon-Chang|last5=Lee|first5=Jong Hyuk|last6=Ha|first6=Jaeheung|last7=Oh|first7=Jong-Suk|last8=Lee|first8=So Young|last9=Lee|first9=Sun Young|last10=Hwang|first10=Kyu Hwan|last11=Zang|first11=Dong-Sik|date=2008-08-18|title=Weak-microcavity organic light-emitting diodes with improved light out-coupling|journal=Optics Express|language=EN|volume=16|issue=17|pages=12632–12639|doi=10.1364/OE.16.012632|pmid=18711500|issn=1094-4087|bibcode=2008OExpr..1612632C|doi-access=free}}</ref> To achieve this goal, they demonstrated that solution-processed perovskites can spontaneously form submicrometre-scale crystal platelets, which can efficiently extract light from the device. These perovskites are formed via the introduction of [[Chemistry:Amino acid|amino acid]] additives into the perovskite [[Biology:Precursor|precursor]] solutions. In addition, their method is able to passivate perovskite surface [[Chemistry:Crystallographic defects in diamond|defects]] and reduce nonradiative recombination. Therefore, by improving the light outcoupling and reducing nonradiative losses, Cao and his colleagues successfully achieved PLED with EQE up to 20.7%.<ref name="ReferenceA"/>

Lin and his colleague used a different approach to generate high EQE. Instead of modifying the microstructure of perovskite layer, they chose to adopt a new strategy for managing the compositional distribution in the device—an approach that simultaneously provides high [[Physics:Luminescence|luminescence]] and balanced charge injection. In other words, they still used flat emissive layer, but tried to optimize the balance of electrons and holes injected into the perovskite, so as to make the most efficient use of the charge carriers. Moreover, in the perovskite layer, the crystals are perfectly enclosed by MABr additive (where MA is CH<sub>3</sub>NH<sub>3</sub>). The MABr shell passivates the nonradiative defects that would otherwise be present perovskite crystals, resulting in reduction of the nonradiative recombination. Therefore, by balancing charge injection and decreasing nonradiative losses, Lin and his colleagues developed PLED with EQE up to 20.3%.<ref>{{Cite journal|last1=Lin|first1=Kebin|last2=Xing|first2=Jun|last3=Quan|first3=Li Na|last4=de Arquer|first4=F. Pelayo García|last5=Gong|first5=Xiwen|last6=Lu|first6=Jianxun|last7=Xie|first7=Liqiang|last8=Zhao|first8=Weijie|last9=Zhang|first9=Di|last10=Yan|first10=Chuanzhong|last11=Li|first11=Wenqiang|date=October 2018|title=Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent|journal=Nature|language=en|volume=562|issue=7726|pages=245–248|doi=10.1038/s41586-018-0575-3|pmid=30305741|issn=1476-4687|bibcode=2018Natur.562..245L|hdl=10356/141016|s2cid=52958604|hdl-access=free}}</ref>

== See also ==

* LED tattoo
* [[Physics:List of light sources|List of light sources]]
* [[Engineering:Superluminescent diode|Superluminescent diode]]
* [[Engineering:MicroLED|MicroLED]]

== References ==

{{reflist}}

== Further reading ==

* {{Cite book|author1=David L. Heiserman |title=Light -Emitting Diodes|publisher=Electronics World|year=1968|url=https://worldradiohistory.com/Archive-Electronics-World/60s/1968/Electronics-World-1968-01.pdf}}
* {{Cite book|author1=Shuji Nakamura |author2=Gerhard Fasol |author3=Stephen J Pearton |title=The Blue Laser Diode: The Complete Story|publisher=Springer Verlag|year=2000|isbn=978-3-540-66505-2|url=https://books.google.com/books?id=AHyMBJ_LMykC}}

== External links ==

* [https://web.archive.org/web/20121015224322/http://www.dlip.de/?p=99 Building a do-it-yourself LED]
* [http://cdn.sparkfun.com/datasheets/Components/LED/changingLED.pdf Color cycling LED in a  single two pin package],

Please be cautious adding more external links. Wikipedia is not a collection of links and should not be used for advertising. Excessive or inappropriate links will be removed. See Wikipedia:External links and Wikipedia:Spam for details. If there are already suitable links, propose additions or replacements on the talk page, or submit your link to the relevant category at the Open Directory Project (dmoz.org). -->

{{Artificial light sources}}
{{Display technology}}
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[[Category:Optical diodes]]
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