a semiconductor-based photodetector ( photodiode ) which is operated with a relatively high reverse voltage (typically tens or even hundreds of volts), sometimes just below breakdown

a silicon-based semiconductor containing a pn junction consisting of a positively doped p region and a negatively doped n region sandwiching an area of neutral charge termed the depletion region

A photodiode operated in the avalanche breakdown region to achieve internal photocurrent multiplication, thereby providing rapid light-controlled switching operation. A photodiode is a semiconductor diode in which the reverse current varies with illumination.

(APD): A photodiode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region. APD's usually require relatively high bias voltages and are very temperature sensitive. They offer much better sensitivity than PIN photodiodes. They are limited to digital applications.

(APD) A photodiode that shows gain in its output power that it receives through avalanche multiplication of photo current. Note: As the reverse-bias voltage approaches the breakdown voltage, hole-electron pairs created by absorbed photons acquire sufficient energy to create additional hole-electron pairs when they collide with ions; thus, a multiplication (signal gain) is achieved. See also: Photon; PIN Photodiode.

Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog to photomultipliers. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain effect (around 100) due to impact ionization (avalanche effect). However, some silicon APDs employ alternative doping and beveling techniques compared to traditional APDs that allow greater voltage to be applied (> 1500 V) before breakdown is reached and hence a greater operating gain (> 1000).