Avalanche photodiode is basically a PN junction diode which operates in the avalanche breakdown region. Its structure is similar to the PIN photodiode. It is a highly sensitive semiconductor electronic device that utilizes the photo electric effect to convert light to electricity. The electron hole pairs that are generated by the incident light are accelerated by the electric field to produce more such carriers thereby achieving a photo multiplication of 50 or so. The avalanche action enables the gain of the diode to be increased many times, providing a much greater level of sensitivity. The avalanche photodiode is ultrafast like PIN diode and can be operated at a modulation frequency of a few GHZ with an excellent signal to noise ratio.