Editing Single-photon avalanche diode (section)

==Comparison with APDs==
While both [[Avalanche photodiode|APDs]] and SPADs are semiconductor p-n junctions that are heavily reverse biased, the principle difference in their properties is derived from their different biasing points upon the reverse I-V characteristic, i.e. the reverse voltage applied to their junction.<ref name="Cova96" /> An [[Avalanche photodiode|APD]], in comparison to a SPAD, is not biased above its breakdown voltage. This is because the multiplication of charge carriers is known to occur prior to the breakdown of the device with this being utilised to achieve a stable gain that varies with the applied voltage.<ref name=":0">{{Cite journal|last=McIntyre|first=R.J.|date=1972|title=The Distribution of Gains in Uniformly Multiplying Avalanche Photodiodes: Theory|journal=IEEE Transactions on Electron Devices |volume=19|issue=6|pages=703–713|doi=10.1109/T-ED.1972.17485|bibcode=1972ITED...19..703M}}</ref><ref>{{Cite journal|last=E. Fisher|date=2018|title=Principles and Early Historical Development of Silicon Avalanche and GeigerMode Photodiodes|journal=In Book: Photon Counting - Fundamentals and Applications. Edited by: N. Britun and A. Nikiforov}}</ref> For optical detection applications, the resulting avalanche and subsequent current in its biasing circuit is linearly related to the optical signal intensity.<ref name=":4">{{Cite book|last=Sze|first=S.M.|title=Semiconductor Devices: Physics and Technology, 2nd Edition|publisher=John Wiley and Sons, Inc|year=2001}}</ref> The APD is therefore useful to achieve moderate up-front amplification of low-intensity optical signals but is often combined with a [[Transimpedance amplifier|trans-impedance amplifier]] (TIA) as the APD's output is a current rather than the voltage of a typical amplifier. The resultant signal is a non-distorted, amplified version of the input, allowing for the measurement of complex processes that modulate the amplitude of the incident light. The internal multiplication gain factors for APDs vary by application, however typical values are of the order of a few hundred. The avalanche of carriers is not divergent in this operating region, while the avalanche present in SPADs quickly builds into a run-away (divergent) condition.<ref name=":1">{{Cite journal|last=F. Zappa, S. Tisa, A. Tosi, and S. Cova|date=2007|title=Principles and Features of Single-Photon Avalanche Diode Arrays|url=https://www.sciencedirect.com/science/article/abs/pii/S0924424707004967|journal=Sensors and Actuators A: Physical|volume=140|issue=1|pages=103–112|doi=10.1016/j.sna.2007.06.021|bibcode=2007SeAcA.140..103Z |url-access=subscription}}</ref>

In comparison, SPADs operate at a bias voltage above the breakdown voltage. This is such a highly unstable above-breakdown regime that a single photon or a single dark-current electron can trigger a significant avalanche of carriers.<ref name="Cova96" /> The semiconductor p-n junction breaks down completely, and a significant current is developed. A single photon can trigger a current spike equivalent to billions of billions of electrons per second (with this being dependent on the physical size of the device and its bias voltage). This allows subsequent electronic circuits to easily count such trigger events.<ref>{{Cite book|last=Fishburn|first=Matthew|url=https://repository.tudelft.nl/islandora/object/uuid%3A7ed6e57d-404e-4372-8053-6b0b5c7fa0fe|title=Fundamentals of CMOS Single-Photon Avalanche Diodes|publisher=Delft University of Technology: Doctoral Thesis|year=2012|isbn=978-94-91030-29-1|location=Delft, The Netherlands}}</ref> As the device produces a trigger event, the concept of gain is not strictly compatible. However, as the photon detection efficiency (PDE) of SPADs varies with the reverse bias voltage,<ref name=":1" /><ref>{{Cite journal|last=C. Kimura and J. Nishizawa|date=1968|title=Turn-on Mechanism of a Microplasma|journal=Japanese Journal of Applied Physics|volume=7|issue=12|pages=1453–1463|doi=10.1143/JJAP.7.1453|bibcode=1968JaJAP...7.1453K|s2cid=98529637 }}</ref> gain, in a general conceptual sense can be used to distinguish devices that are heavily biased and therefore highly sensitive in comparison to lightly biased and therefore of lower sensitivity. While APDs can amplify an input signal preserving any changes in amplitude, SPADs distort the signal into a series of trigger or pulse events. The output can still be treated as proportional to the input signal intensity, however it is now transformed into the frequency of trigger events, i.e. [[Pulse-frequency modulation|pulse frequency modulation]] (PFM). Pulses can be counted<ref name="Eisele" /> giving an indication of the input signal's optical intensity, while pulses can trigger timing circuits to provide accurate time-of-arrival measurements.<ref name="Cova96" /><ref name=":1" />

One crucial issue present in [[Avalanche photodiode|APDs]] is multiplication noise induced by the statistical variation of the avalanche multiplication process.<ref name=":0" /><ref name=":1" /> This leads to a corresponding noise factor on the output amplified photo current. Statistical variation in the avalanche is also present in SPAD devices, however due to the runaway process it is often manifest as timing jitter on the detection event.<ref name=":1" />

Along with their bias region, there are also structural differences between APDs and SPADs, principally due to the increased reverse bias voltages required and the need for SPADs to have a long quiescent period between noise trigger events to be suitable for the single-photon level signals to be measured.