Editing Single-photon avalanche diode (section)

==History, development and early pioneers==
The history and development of SPADs and APDs shares a number of important points with the development of solid-state technologies such as diodes and early p–n junction transistors (particularly war-efforts at Bell Labs). [[John Sealy Townsend|John Townsend]] in 1901 and 1903 investigated the ionisation of trace gases within vacuum tubes, finding that as the electric potential increased, gaseous atoms and molecules could become ionised by the kinetic energy of free electrons accelerated though the electric field. The new liberated electrons were then themselves accelerated by the field, producing new ionisations once their kinetic energy has reached sufficient levels. This theory was later instrumental in the development of the [[thyratron]] and the [[Geiger–Müller tube|Geiger-Mueller tube]]. The [[Townsend discharge]] was also instrumental as a base theory for electron multiplication phenomena, (both DC and AC), within both silicon and germanium.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}

However, the major advances in early discovery and utilisation of the avalanche gain mechanism were a product of the study of [[Zener breakdown]], related ([[avalanche breakdown|avalanche) breakdown]] mechanisms and structural defects in early silicon and germanium transistor and p–n junction devices.<ref>{{cite journal|last1=McIntyre|first1=RJ|year=1961|title=Theory of microplasma instability in silicon|journal=Journal of Applied Physics|publisher=American Institute of Physics|volume=32|issue=6|pages=983–995|bibcode=1961JAP....32..983M|doi=10.1063/1.1736199}}</ref> These defects were called '[[microplasma]]s' and are critical in the history of APDs and SPADs. Likewise investigation of the light detection properties of p–n junctions is crucial, especially the early 1940s findings of [[Russell Ohl|Russel Ohl]]. Light detection in semiconductors and solids through the internal photoelectric effect is older with Foster Nix <ref>{{cite journal|last1=Nix|first1=Foster C.|year=1932|title=Photo-conductivity|journal=Reviews of Modern Physics|volume=4|issue=4|pages=723–766|bibcode=1932RvMP....4..723N|doi=10.1103/RevModPhys.4.723}}</ref> pointing to the work of Gudden and Pohl in the 1920s,{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}} who use the phrase primary and secondary to distinguish the internal and external photoelectric effects respectively. In the 1950s and 1960s, significant effort was made to reduce the number of microplasma breakdown and noise sources, with artificial microplasmas being fabricated for study. It became clear that the avalanche mechanism could be useful for signal amplification within the diode itself, as both light and alpha particles were used for the study of these devices and breakdown mechanisms.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}

In the early 2000s, SPADs have been implemented within [[CMOS]] processes. This has radically increased their performance (dark count rate, jitter, array [[Dot pitch|pixel pitch]], etc.), and has leveraged analog and digital circuits that can be implemented alongside these devices. Notable circuits include photon counting using fast digital counters, photon timing using both [[time-to-digital converter]]s (TDCs) and time-to-analog converters (TACs), passive quenching circuits using either NMOS or PMOS transistors in place of poly-silicon resistors, active quenching and reset circuits for high counting rates, and many on-chip digital signal processing blocks. Such devices are now reaching optical [[fill factor (image sensor)|fill factors]] of >70%, with >1024 SPADs, with DCRs < 10&nbsp;Hz, jitter values in the 50ps region, and dead times of 1-2ns.{{Citation needed|date=October 2018}} Recent devices leverage 3D-IC technologies such as [[Through-silicon via|through-silicon-vias]] (TSVs) to present a high-fill-factor SPAD optimised top CMOS layer (90&nbsp;nm or 65&nbsp;nm node) with a dedicated signal processing and readout CMOS layer (45&nbsp;nm node). Significant advancements in the noise terms for SPADs have been obtained by silicon process modelling tools such as TCAD, where [[Driven guard|guard rings]], junction depths and device structures and shapes can be optimised prior to validation by experimental SPAD structures.