Photonic NetworkBy Dr. M H ZaidiResponsivity of the photodectectors we have discussed thus far has been limited by the fact that one photon can generate only one electron when it is absorbed.However, if the generated electron is subjected to a very high electric field, it can acquire sufficient energy to knock off more electrons from the valence band to the conduction band.These secondary electron-hole pairs can generate even further electron-hole pairs when they are accelerated to sufficient levels.This process is called avalanche multiplication.Such a photodiode is called an avalanche photodiode, or simple an APD.Avalanche Photodiodes
Photonic NetworkBy Dr. M H ZaidiDepletion region with a reverse biasVoltage, Va.A pn-junction photodiode.Depletion region with no biasedvoltage applied.A reverse biased pn-junction used as a photodetectorBuilt-in electric field on reverse bias.
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Photonic NetworkBy Dr. M H ZaidiThe number of secondary electron-hole pairs generated by the avalanche multiplication process by a single (primary) electron is random, and the mean value of this number is termed the multiplicative gainand denoted by Gm.Thus there is a trade-off between the multiplicative gain and the noise factor.APDs are usually designed to have a moderate value of Gmthat optimized their performance.Avalanche Photodiodes