Excess-Carrier Transport in Anisotropic Semiconductors: The Photovoltaic Effect

The problem of ambipolar diffusion and drift of electrons and holes in anisotropic semiconductors is examined. A continuity equation for excess carriers is obtained in a form suitable for a discussion of both the electrical pinch effect and the anisotropic photovoltaic effect. The photovoltaic effect is discussed in detail and expressions for the short-circuit current and open-circuit field that are to be expected for an illuminated anisotropic semiconductor are presented. It is pointed out that under certain specified conditions, measurements of the open-circuit field may be used to obtain values for surface recombination velocities and effective diffusion lengths. The photovoltaic effect should be observable in naturally anisotropic semiconductors and in cubic semiconductors, such as germanium and silicon, that display directionally dependent piezoresistance. For the latter case, a method is discussed for obtaining suitable specimens, which, when subjected to uniaxial stress, will display the proper anisotropy. Finally, curves are included which depict the magnitude of the photovoltaic effect that is expected for uniaxially stressed germanium.