View source for Physics:Avalanche photodiode

{{Short description|Highly sensitive semiconductor electronic device}}
An '''avalanche photodiode''' ('''APD''') is a highly sensitive [[Physics:Semiconductor|semiconductor]] photodiode [[Physics:Semiconductor detector|detector]] that exploits the [[Physics:Photoelectric effect|photoelectric effect]] to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of [[Physics:Photomultiplier tube|photomultiplier tube]]s. The avalanche [[Physics:Photodiode|photodiode]] was invented by Japanese engineer Jun-ichi Nishizawa in 1952.<ref>{{Cite web |url=http://jqrmag.com/en/jqr-interview-eng/jun-ichi-nishizawa-engineer-sophia-university-special-professor/ |title=Jun-ichi Nishizawa – Engineer, Sophia University Special Professor – JAPAN QUALITY REVIEW |access-date=2017-05-15 |archive-url=https://web.archive.org/web/20180721043727/http://jqrmag.com/en/jqr-interview-eng/jun-ichi-nishizawa-engineer-sophia-university-special-professor/ |archive-date=2018-07-21 |url-status=dead }}</ref> However, study of avalanche breakdown, microplasma defects in silicon and germanium and the investigation of optical detection using p-n junctions predate this patent. Typical applications for APDs are [[Physics:Laser rangefinder|laser rangefinder]]s, long-range [[Engineering:Optical fiber|fiber-optic]] [[Engineering:Telecommunication|telecommunication]], and quantum sensing for control algorithms. New applications include [[Physics:Positron emission tomography|positron emission tomography]] and [[Physics:Particle physics|particle physics]].

==Principle of operation==
By applying a high reverse bias voltage (typically 100–200 V in [[Chemistry:Silicon|silicon]]), APDs show an internal current [[Engineering:Gain (electronics)|gain]] effect (around 100) due to [[Physics:Impact ionization|impact ionization]] (avalanche effect). However, some silicon APDs employ alternative [[Physics:Doping (semiconductor)|doping]] and beveling techniques compared to traditional APDs that allow greater voltage to be applied (> 1500 V) before breakdown is reached and hence a greater operating gain (> 1000). In general, the higher the reverse voltage, the higher the gain. Among the various expressions for the APD multiplication factor (''M''), an instructive expression is given by the formula

:<math>M = \frac{1}{1 - \int_0^L\alpha(x)\, dx},</math>

where ''L'' is the space-charge boundary for electrons, and <math>\alpha</math> is the multiplication coefficient for electrons (and holes). This coefficient has a strong dependence on the applied electric field strength, temperature, and doping profile. Since APD gain varies strongly with the applied reverse bias and temperature, it is necessary to control the reverse voltage to keep a stable gain. Avalanche photodiodes therefore are more sensitive compared to other semiconductor [[Physics:Photodiode|photodiode]]s.

If very high gain is needed (10<sup>5</sup> to 10<sup>6</sup>), detectors related to APDs called SPADs ([[Physics:Single-photon avalanche diode|single-photon avalanche diode]]s) can be used and operated with a reverse voltage above a typical APD's [[Physics:Breakdown voltage|breakdown voltage]]. In this case, the photodetector needs to have its signal current limited and quickly diminished. Active and passive current-quenching techniques have been used for this purpose. SPADs that operate in this high-gain regime are sometimes referred to being in Geiger mode. This mode is particularly useful for single-photon detection, provided that the dark count event rate and afterpulsing probability are sufficiently low.

==Materials==
In principle, any semiconductor material can be used as a multiplication region:
* Silicon will detect in the visible and near infrared, with low multiplication noise (excess noise).
* [[Chemistry:Germanium|Germanium]] (Ge) will detect [[Physics:Infrared|infrared]] out to a wavelength of 1.7&nbsp;µm, but has high multiplication noise.
* InGaAs will detect out to longer than 1.6&nbsp;µm and has less multiplication noise than Ge. It is normally used as the absorption region of a heterostructure diode, most typically involving [[Chemistry:Indium phosphide|InP]] as a substrate and as a multiplication layer.<ref name=tsang>{{cite book|title=Semiconductors and Semimetals|volume=22, Part D "Photodetectors"|editor=Tsang, W. T.|publisher=Academic Press|year=1985}}</ref> This material system is compatible with an absorption window of roughly 0.9–1.7&nbsp;µm. [[Chemistry:Indium gallium arsenide|InGaAs]] exhibits a high [[Physics:Absorption coefficient|absorption coefficient]] at the wavelengths appropriate to high-speed telecommunications using [[Engineering:Optical fiber|optical fiber]]s, so only a few micrometres of InGaAs are required for nearly 100% light absorption.<ref name=tsang/> The excess noise factor is low enough to permit a gain-bandwidth product in excess of 100&nbsp;GHz for a simple InP/InGaAs system,<ref>{{cite journal|author=Tarof, L. E.|title=Planar InP/GaAs Avalanche Photodetector with Gain-Bandwidth Product in Excess of 100 GHz|journal=Electronics Letters|volume= 27|pages=34–36|year=1991|issue=1 |doi=10.1049/el:19910023|bibcode=1991ElL....27...34T }}</ref> and up to 400&nbsp;GHz for InGaAs on silicon.<ref>{{cite journal|author1=Wu, W. |author2=Hawkins, A. R. |author3=Bowers, J. E. |editor2-first=Ramu V |editor2-last=Ramaswamy |editor1-first=Yoon-Soo |editor1-last=Park |title=Design of InGaAs/Si avalanche photodetectors for 400-GHz gain-bandwidth product|journal=Proceedings of SPIE|volume=3006|pages=36–47|year=1997|doi=10.1117/12.264251|series=Optoelectronic Integrated Circuits |bibcode=1997SPIE.3006...38W |s2cid=109777495 }}</ref> Therefore, high-speed operation is possible: commercial devices are available to speeds of at least 10 Gbit/s.<ref>{{cite journal|author=Campbell, J. C.|title=Recent advances in Telecommunications Avalanche Photodiodes|journal= Journal of Lightwave Technology|volume=25|issue=1|pages=109–121|doi=10.1109/JLT.2006.888481|year=2007|bibcode = 2007JLwT...25..109C |s2cid=1398387|url=https://zenodo.org/record/896867}}</ref>
* [[Chemistry:Gallium nitride|Gallium-nitride]]–based diodes have been used for operation with [[Physics:Ultraviolet|ultraviolet]] light.
* HgCdTe-based diodes operate in the infrared, typically at wavelengths up to about 14&nbsp;µm, but require cooling to reduce dark currents. Very low excess noise can be achieved in this material system.

==Performance limits==
APD applicability and usefulness depends on many parameters. Two of the larger factors are: [[Engineering:Quantum efficiency|quantum efficiency]], which indicates how well incident optical photons are absorbed and then used to generate primary charge carriers; and total leakage current, which is the sum of the dark current, photocurrent and noise. Electronic dark-noise components are series and parallel noise. Series noise, which is the effect of [[Shot noise|shot noise]], is basically proportional to the APD capacitance, while the parallel noise is associated with the fluctuations of the APD bulk and surface dark currents.

===Gain noise, excess noise factor{{anchor|Excess noise}}===
Another noise source is the excess noise factor, ENF. It is a multiplicative correction applied to the noise that describes the increase in the statistical noise, specifically Poisson noise, due to the multiplication process. The ENF is defined for any device, such as photomultiplier tubes, silicon solid-state photomultipliers, and APDs, that multiplies a signal, and is sometimes referred to as "gain noise".   At a gain ''M'', it is denoted by ENF(''M'') and can often be expressed as

:<math>\text{ENF} = \kappa M + \left(2 - \frac{1}{M}\right)(1 - \kappa),</math>

where <math>\kappa</math> is the ratio of the hole impact ionization rate to that of electrons. For an electron multiplication device it is given by the hole impact ionization rate divided by the electron impact ionization rate. It is desirable to have a large asymmetry between these rates to minimize ENF(''M''), since ENF(''M'') is one of the main factors that limit, among other things, the best possible energy resolution obtainable.

===Conversion noise, Fano factor===

The noise term for an APD may also contain a Fano factor, which is a multiplicative correction applied to the Poisson noise associated with the conversion of the energy deposited by a charged particle to the electron-hole pairs, which is the signal before multiplication.  The correction factor describes the decrease in the noise, relative to Poisson statistics, due to the uniformity of conversion process and the absence of, or weak coupling to, bath states in the conversion process.  In other words, an "ideal" semiconductor would convert the energy of the charged particle into an exact and reproducible number of electron hole pairs to conserve energy; in reality, however, the energy deposited by the charged particle is divided into the generation of electron hole pairs, the generation of sound, the generation of heat, and the generation of damage or displacement.  The existence of these other channels introduces a stochastic process, where the amount of energy deposited into any single process varies from event to event, even if the amount of energy deposited is the same.

===Further influences===

The underlying physics associated with the excess noise factor (gain noise) and the Fano factor (conversion noise) is very different.  However, the application of these factors as multiplicative corrections to the expected Poisson noise is similar. In addition to excess noise, there are limits to device performance associated with the capacitance, transit times and avalanche multiplication time.<ref name=tsang/> The capacitance increases with increasing device area and decreasing thickness. The transit times (both electrons and holes) increase with increasing thickness, implying a tradeoff between capacitance and transit time for performance. The avalanche multiplication time times the gain is given to first order by the gain-bandwidth product, which is a function of the device structure and most especially <math>\kappa\,</math>.

==See also==
* [[Physics:Avalanche diode|Avalanche diode]]
* Avalanche breakdown
* [[Physics:Single-photon avalanche diode|Single-photon avalanche diode]]

==References==
{{reflist|30em}}

==Further reading==
* Avalanche photodiode – A User Guide [https://web.archive.org/web/20130626133706/http://www.excelitas.com/downloads/app_apd_a_user_guide.pdf]
* Avalanche Photodiode – Low noise APD receivers [http://www.excelitas.com/downloads/app_lownoiseopticalreceiverusingsiapd.pdf]
* {{cite journal|doi=10.1063/1.92385|title=Fully ion-implanted p+-n germanium avalanche photodiodes|year=1981|last1=Kagawa|first1=S.|journal=Applied Physics Letters|volume=38|pages=429–431|issue=6|bibcode = 1981ApPhL..38..429K }}gh
* {{cite journal|doi=10.1063/1.364225|title=Breakdown characteristics in InP/InGaAs avalanche photodiode with p-i-n multiplication layer structure|year=1997|last1=Hyun|first1=Kyung-Sook|last2=Park|journal=Journal of Applied Physics|volume=81|pages=974|first2=Chan-Yong|issue=2|bibcode = 1997JAP....81..974H }}
* [https://web.archive.org/web/20110713184642/http://www.lasercomponents.com/de/fileadmin/user_upload/home/Datasheets/lc/applikationsreport/avalanche-photodiodes.pdf Selecting the right APD]
* [https://web.archive.org/web/20110713184653/http://www.lasercomponents.com/de/fileadmin/user_upload/home/Datasheets/lc/veroeffentlichung/progress-through-photonics.pdf Pulsed Laserdiodes and Avalanche Photodiodes for Industrial Applications]
* Excelitas Technologies Photonic Detectors [http://www.excelitas.com/Pages/Product/Photonic-Detectors.aspx]

[[Category:Optical devices]]
[[Category:Optical diodes]]
[[Category:Particle detectors]]
[[Category:Photodetectors]]

{{Sourceattribution|Avalanche photodiode}}