The Photovoltaic Characteristics of p-n Ge-Si and Ge-GaAs Heterojunctions†

Abstract

A model for minority photocarrier collection in p-n heterojunctions consistent with experimental results on Ge-GaAs and Ge-Si junctions is developed. Due to the discontinuities in hand edges which exist at a heterojunction interface, barriers usually exist which impede the collection of the minority photocarriers generated in the narrow gap material. Considering reasonable values for the number and capture cross sections of interface states, these carriers, once impeded, will recombine via these states. The photovoltaic response of these junctions is expected, from this model, to depend on barrier conditions and therefore to be a function of the relative impurity concentrations. With light incident on the wide gap material perpendicular to the junction, the photovoltaic response can be dominated by either the response due to generation of carriers in the narrow gap material or that due to generation of carriers in the wide gap material. Both types of characteristics have been observed experimentally by varying the impurity concentrations.