Effect of electron-hole scattering on ambipolar diffusion in semiconductors

Abstract

The ambipolar diffusion coefficient of a semiconductor in the absence of an external electric field is calculated from the Boltzmann transport equation using Kohler's variational principle. For the regime where electron-hole scattering is important, it is found that the ambipolar Einstein relation is not valid unless a newly defined free-carrier "diffusion mobility" is employed rather than the usual "conductivity mobility." The distinction between the two mobilities is due to the different relative velocities of the electron and hole systems which characterize the two cases.