Estimates of Impact Ionization Coefficients in Superlattice-Based Mid-Wavelength Infrared Avalanche Photodiodes

We describe band engineering strategies to either enhance or suppress electron-initiated impact ionization relative to hole-initiated impact ionization in type II superlattice mid-wavelength infrared avalanche photodiodes. The strategy to enhance electron-initiated impact ionization involves placing a high density of states at approximately one energy gap above the bottom of the conduction band and simultaneously removing valence band states from the vicinity of one energy gap below the top of the valence band. This gives the electrons a low threshold energy and the holes a high one. The opposite strategy enhances hole-initiated impact ionization. Estimates of the electron (α) and hole (β) impact ionization coefficients predict that α/β>>1 in the first type of superlattice and α/β<<1 in the second type.