Editing Single-photon avalanche diode (section)

=== Structures ===
SPADs are [[semiconductor]] devices that are based on a [[p–n junction]] that is reverse-biased at an operating voltage that exceeds the junction's breakdown voltage ([[:File:SPAD Cross-section.gif|Figure 1]]).<ref name=Cova96>{{cite journal |doi= 10.1364/AO.35.001956 |pmid= 21085320|bibcode= 1996ApOpt..35.1956C|title= Avalanche photodiodes and quenching circuits for single-photon detection|journal= Applied Optics|volume= 35|issue= 12|pages= 1956–76|year= 1996|last1= Cova|first1= S.|last2= Ghioni|first2= M.|last3= Lacaita|first3= A.|last4= Samori|first4= C.|last5= Zappa|first5= F.|s2cid= 12315693}}</ref> "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/> The current continues until the avalanche is quenched by lowering the [[bias voltage]] down to or below the breakdown voltage:<ref name=Cova96/> the lower electric field is no longer able to accelerate carriers to impact-ionize with [[crystal structure|lattice]] atoms, therefore current ceases. In order to be able to detect another photon, the bias voltage must be raised again above breakdown.<ref name=Cova96/>

"This operation requires a suitable circuit, which has to:
# Sense the leading edge of the avalanche current.
# Generate a standard output pulse synchronous with the avalanche build-up.
# Quench the avalanche by lowering the bias down to the breakdown voltage.
# Restore the [[photodiode]] to the operative level.

This circuit is usually referred to as a quenching circuit."<ref name=Cova96/>