Optical absorption in semiconductors with high impurity concentrations in the presence of an electric field

The calculation of the absorption coefficient of a direct gap semiconductor in the presence of a high density of impurities is considered, in the absence and presence of an electric field. A two-band model is used and the absorption coefficient is calculated directly from the Kubo equation in the one-electron approximation. The method uses essentially a semiclassical approximation and calculates the two-particle propagator. Typical results are given for p-type GaAs and are found to agree reasonably with experiments.