An avalanche diode is a special type of semiconductor device designed to operate in the reverse breakdown mode. Avalanche diodes are used as over pressure valves (a type of valve to control the pressure in a system) to protect electrical systems from over voltages.
When the reverse bias voltage applied to the avalanche diode exceeds the breakdown voltage, junction breakdown occurs. This connection disorder is called avalanche disorder.
When a forward bias voltage is applied to the avalanche diode, it functions like a normal pn junction diode by passing electrical current through it.
The main difference of the avalanche photodiode to other types of photodiodes is that it operates under a high reverse bias voltage. This enables avalanche multiplication of the holes and electrons generated by the photon / light impact.
When a photon enters the depletion region and creates a pair of hole electrons, these charge carriers are pulled away from each other by the very high electric field. Their speed increases so much that they form in a collision with the grid more pairs of hole electron and repeat the process.