Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. By applying a high reverse bias voltage, APDs show an internal gain effect due to impact ionization. The higher the reverse voltage the higher the gain. Avalanche photodiodes therefore are more sensitive compared to other semiconductor photodiodes.
If very high gain is needed (105 to 106), a reverse voltage above the APD's breakdown voltage can be applied if the current is limited to below the APD's latching current (Geiger mode). This is particularly useful for single photon detection.
A typical application for APDs is laser range finders.
Also, APD arrays are commercially available.
See also