Theoretical analysis of heterojunction phototransistors

The optical gain and spectral response of heterojunction phototransistors with wide-gap emitters have been examined theoretically in idealized cases. The optical gain is found to be closely related to the current gain in the common-emitter configuration β of a heterojunction transistor. Because of Kroemer\´s factor, the injection efficiency of the emitter junction is very high, resulting in a high β or a large optical gain. However, β or the optical gain is limited by the base transport efficiency when the injection efficiency is extremely high. From the analyses, the optical gains and spectral responses of an n.GaAs-p. Ge-n.Ge heterojunction phototransistor and n-p-n homojunction phototransistors of GaAs and of Ge are numerically computed. It becomes evident that the heterojunction phototransistor has a higher optical gain and a wider spectral response ( at wavelengths ranging from 0.9 to 1.5 µm) than either of the homojunction phototransistors.