Technical Note: Avalanche Photodiode Performance Metrics

By Voxtel, Inc.

Avalanche photodiodes (APDs) are photodetectors that can be regarded as the semiconductor analog of photomultiplier tubes (PMTs). One important difference is that APDs don't have a photocathode that is physically separate from their current gain medium, and so they typically use primary photocarriers more efficiently than PMTs. For the same reason, the quantum efficiency of an APD does not degrade over the lifetime of the detector. A second difference is that the multiplication process in an APD is normally bi-directional, so it has different statistics than a PMT in which the gain process is uni-directional.

All avalanche photodiodes generate excess noise due to the statistical nature of the avalanche process. The 'excess noise factor' is generally denoted by the symbol 'F', and is the ratio of the mean square gain to the square of the mean gain – it is also the ratio by which the spectral intensity of shot noise on an APD's current exceeds that which would be expected from a noiseless multiplier on the basis of Poisson statistics alone.