Editing Single-photon avalanche diode (section)

{{short description|Solid-state photodetector}}

[[File:Single-Photon-Counting-Module Excelitas.jpg|thumb|right|upright=1.25|Commercial single-photon avalanche diode module for optical photons]]
A '''single-photon avalanche diode''' ('''SPAD'''), also called '''Geiger-mode avalanche photodiode'''<ref name="acerbi2019">{{cite journal |vauthors=Acerbi F, Gundacker S|date=2019|title=Understanding and simulating SiPMs |journal=[[Nucl. Instrum. Methods Phys. Res. A]]|issn=0168-9002|eissn=1872-9576|volume=926|issue=|pages=16–35|doi=10.1016/j.nima.2018.11.118|doi-access=free|bibcode=2019NIMPA.926...16A }}</ref> ('''G-APD''' or '''GM-APD'''<ref>{{cite journal|vauthors=Gatt P, Johnson S, Nichols T|title=Geiger-mode avalanche photodiode ladar receiver performance characteristics and detection statistics|doi=10.1364/AO.48.003261|journal=Applied Optics|issn=2155-3165|volume=48|issue=17|year=2009|pages=3261–3276|pmid=19516383 |bibcode=2009ApOpt..48.3261G }}</ref>) is a solid-state [[photodetector]] within the same family as [[photodiode]]s and [[avalanche photodiode]]s (APDs), while also being fundamentally linked with basic [[diode]] behaviours. As with photodiodes and APDs, a SPAD is based around a semi-conductor [[P–n junction|p-n junction]] that can be illuminated with [[ionizing radiation]] such as gamma, x-rays, beta and alpha particles along with a wide portion of the [[electromagnetic spectrum]] from ultraviolet (UV) through the visible wavelengths and into the infrared (IR).

In a photodiode, with a low [[Reverse bias|reverse bias voltage]], the leakage current changes linearly with absorption of photons, i.e. the liberation of current carriers (electrons and/or holes) due to the internal [[photoelectric effect]]. However, in a SPAD,<ref name="Cova96" /><ref name=":1" /> the reverse bias is so high that a phenomenon called [[Impact ionization|impact ionisation]] occurs which is able to cause an avalanche current to develop. Simply, a photo-generated carrier is accelerated by the [[electric field]] in the device to a [[kinetic energy]] which is enough to overcome the [[Ionization energy|ionisation energy]] of the bulk material, knocking electrons out of an atom. A large avalanche of current carriers grows exponentially and can be triggered from as few as a single photon-initiated carrier. A SPAD is able to detect single photons providing short duration trigger pulses that can be counted. However, they can also be used to obtain the time of arrival of the incident photon due to the high speed that the avalanche builds up and the device's low timing [[jitter]].

The fundamental difference between SPADs and [[Avalanche photodiode|APDs]] or photodiodes, is that a SPAD is biased well above its [[Breakdown voltage|reverse-bias breakdown voltage]] and has a structure that allows operation without damage or undue noise. While an APD is able to act as a linear amplifier, the level of impact ionisation and avalanche within the SPAD has prompted researchers to liken the device to a [[Geiger counter|Geiger-counter]] in which output pulses indicate a trigger or "click" event. The diode bias region that gives rise to this "click" type behaviour is therefore called the "''Geiger-mode''" region.

As with photodiodes the wavelength region in which it is most sensitive is a product of its material properties, in particular the [[Band gap|energy bandgap]] within the [[semiconductor]]. Many materials including [[silicon]], [[germanium]],<ref>A. Tosi, A.D. Mora, F. Zappa and S. Cova, "Germanium and InGaAs/InP SPADs for single-photon detection in the near-infrared" Proc. SPIE 6771, 67710P-1 (2007) - DOI: https://doi.org/10.1117/12.734961 </ref> [[germanium]] on [[silicon]]<ref>P. Vines, K. Kuzmenko, J. Kirdoda, D.C.S. Dumas, M.M. Mirza, R.W. Millar, D.J. Paul and G.S. Buller "High performance planar germanium-on-silicon single-photon avalanche diode detectors" Nature Communications 10, 1086 (2019) - DOI: https://doi.org/10.1038/s41467-019-08830-w</ref> and [[III-V]] elements such as [[InGaAs]]/[[InP]]<ref>S. Pellegrini, R.E. Warburton, L.J.J. Tan, J.S. Ng, A.B. Krysa, K. Groom, J.P.R. David, S. Cova, M.J. Robertson and G. S. Buller, "Design and performance of an InGaAs-InP single-photon avalanche diode detector" IEEE J. Quantum Elec. 42(4), pp 397 - 403 (2006) - DOI: https://doi.org/10.1109/JQE.2006.871067 </ref> have been used to fabricate SPADs for the large variety of applications that now utilise the run-away avalanche process. There is much research in this topic with activity implementing SPAD-based systems in [[CMOS]] fabrication technologies,<ref name=":5" /> and investigation and use of III-V material combinations<ref>{{Cite journal|last=J. Zhang, M. Itzler, H. Zbinden and J. Pan|s2cid=6865451|date=2015|title=Advances in InGaAs/InP single-photon detector systems for quantum communication|url=https://www.nature.com/articles/lsa201559|journal=Light: Science & Applications|volume=4|issue=5|pages=e286|doi=10.1038/lsa.2015.59|arxiv=1501.06261|bibcode=2015LSA.....4E.286Z}}</ref> and Ge on Si <ref>L. Ferre Llin, J. Kirdoda, F. Thorburn, L.L. Huddleston, Z.M. Greener, K. Kuzmenko, P. Vines, D.C.S. Dumas, R.W. Millar, G.S. Buller and D.J. Paul, "High sensitivity Ge-on-Si single-photon avalanche diode detectors" Optics Letters 45(23), pp. 6406 - 6409 (2020) - DOI: https://doi.org/10.1364/OL.396756</ref> for single-photon detection at short-wave infrared wavelengths suitable for telecommunications applications.