Avalanche photodiode
An avalanche photodiode (APD) is a highly sensitive type of photodiode, which in general are semiconductor diodes that convert light into electricity via the photovoltaic effect. APDs use materials and a structure optimised for operating with high reverse bias voltage, approaching the reverse breakdown voltage, such that charge carriers generated by the photovoltaic effect are multiplied by an avalanche breakdown; thus they can be used to detect relatively small amounts of light.