Ambipolar Transport of Electrons and Holes in Anisotropic Crystals

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Abstract
The problem of ambipolar diffusion and drift of electrons and holes, in which the diffusivity and mobility tensors for the two carrier species may be of different forms, is examined. Problems of this type arise, for example, in studying the diffusion and drift of excess carrier distributions in uniaxially stressed germanium and silicon as well as in certain naturally anisotropic substances. General ambipolar transport equations are obtained in situations where the quasineutrality approximation is justified. Solutions to these equations are quite easily obtained in certain cases where particular simplifying assumptions can be made. These solutions are explicitly obtained and the range of conditions under which they are applicable is outlined in detail. Certain other procedures have been employed to solve the general problem in cases when these conditions are not satisfied. Such methods are usually applicable only in cases involving rather special and restricted sample geometries. The transformation properties of the various terms in the ambipolar transport equation are discussed in various situations of physical interest and importance.