The physics of avalanche photodiodes.

This article represents a review of the physics of avalanche photodiodes. In the first part a unified approach to the theory of impact ionization is given, based on microscopic quantities such as threshold energies and scattering rates. The second part deals with the avalanche rate equations and the measurement of ionization rates. In the last section of the paper novel APD structures with reduced avalanche excess noise are discussed (superlattice, channeling, graded gap and staircase solid state photomultipliers).