Editing Single-photon avalanche diode (section)

===Biasing regions and current-voltage characteristic===
[[File:Spad-apd-characteristic-comparison.svg|thumb|right|Current-voltage characteristic of a SPAD showing the off- and on-branch]]
A semiconductor p-n junction can be biased at several operating regions depending on the applied voltage. For normal uni-directional [[diode]] operation, the forward biasing region and the forward voltage are used during conduction, while the reverse bias region prevents conduction. When operated with a low reverse bias voltage, the p-n junction can operate as a unity gain [[photodiode]]. As the reverse bias increases, some internal gain through carrier multiplication can occur allowing the photodiode to operate as an [[Avalanche photodiode|avalanche photodiode (APD)]] with a stable gain and a linear response to the optical input signal. However, as the bias voltage continues to increase, the p-n junction breaks down when the electric field strength across the p-n junction reaches a critical level. As this electric field is induced by the bias voltage over the junction it is denoted as the breakdown voltage, VBD. A SPAD is reverse biased with an excess bias voltage, V<sub>ex</sub>, above the breakdown voltage, but below a second, higher breakdown voltage associated with the SPAD's guard ring. The total bias (VBD+V<sub>ex</sub>) therefore exceeds the breakdown voltage to such a degree that "At this bias, the [[electric field]] is so high [higher than 3×10<sup>5</sup> V/cm] that a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly [sub-nanosecond rise-time] to a macroscopic steady level in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks [with picosecond time jitter] the arrival time of the detected [[photon]]".<ref name="Cova96" />

As the current vs voltage (I-V) characteristic of a p-n junction gives information about the conduction behaviour of the diode, this is often measured using an analogue curve-tracer. This sweeps the bias voltage in fine steps under tightly controlled laboratory conditions. For a SPAD, without photon arrivals or thermally generated carriers, the I-V characteristic is similar to the reverse characteristic of a standard semi-conductor diode, i.e. an almost total blockage of charge flow (current) over the junction other than a small leakage current (nano-amperes). This condition can be described as an "off-branch" of the characteristic.

However, when this experiment is conducted, a "flickering" effect and a second I-V characteristic can be observed beyond breakdown. This occurs when the SPAD has experienced a triggering event (photon arrival or thermally generated carrier) during the voltage sweeps that are applied to the device. The SPAD, during these sweeps, sustains an avalanche current which is described as the "on-branch" of the I-V characteristic. As the curve tracer increases the magnitude of the bias voltage over time, there are times that the SPAD is triggered during the voltage sweep above breakdown. In this case a transition occurs from the off-branch to the on-branch, with an appreciable current starting to flow. This leads to the flickering of the I-V characteristic that is observed and was denoted by early researchers in the field as "bifurcation"<ref name=":1" /> (def: the division of something into two branches or parts). To detect single-photons successfully, the p-n junction must have very low levels of the internal generation and recombination processes. To reduce thermal generation, devices are often cooled, while phenomena such as tunnelling across the p-n junctions also need to be reduced through careful design of semi-conductor dopants and implant steps. Finally, to reduce noise mechanisms being exacerbated by trapping centres within the p-n junction's band gap structure the diode needs to have a "clean" process free of erroneous dopants.