Avalanche Photodiode - APD
Avalanche photodiode is a semiconductor diode device that has a barrier region that is very sensitive to photons and the reception of photons causes an avalanche of electrons across p/n junction. This causes a rapid increase in current with the detection of light (photon) energy. APDs are used in optical networks to detect light, typically at the receiver. It is a semiconductor pn junction diode that is heavily biased so that each time a photon is absorbed by the device, the resulting conduction electron has so much energy that it can knock loose many other electrons and each of those electrons can generate others, resulting in an avalanche of electrons and therefore a large current. This causes a rapid increase in current with the detection of light (photon) energy. APDs can detect faint optical signals. However, APDs require higher voltages than other types of semiconductor devices.