The Avalanche PhotoDiode(APD) is a highly sensitive light sensor.
It has a shorter switching time than the phototransistor.
It is basicaly a PIN diode with a high backwards
bias. This accelerates charge carriers in the depletion
area of the diode, the high energy of these carriers causes
an impact ionization effect, that causes further release off
carriers. Because of this avalanche effect the diodes
have high amplification, even higher than a phototransistor.
The gain can be changed by altering the bias voltage.
The APD is most used where high speed and high amplification
is needed, and are more expensive than normal photodiodes.