Impact ionization characteristics in avalanche photodiodes

Abstract

Avalanche photodiodes are a classification of semiconductor devices known for their high sensitivity and high speed. They convert light to electricity using the photo-electric effect. Like normal photodiodes, absorption of incident photons generate electron-hole pairs (EHPs), which cause conductivity. Gain is produced when impact ionization results in avalanche multiplication of electrons or holes (can be electron-initiated or hole-initiated multiplication). The difference here is the reverse bias voltage applied to the device, which can be as high as 100-200 V in Silicon.

Most common APDs are fabricated from Silicon. APDs are used in many applications where conventional unity gain photodiodes cannot provide enough sensitivity and the extra amplification provided by the impact ionization process gives it an advantage. For instance, they are used as laser rangefinders and in fiber optic communications.

Avalanche photodiode gain is typically in the range of x30 to x100 in most commercial devices, although it can go as high as x1000 in specialist manufactures APDs.