Diffusion in Heavily Doped Semiconductors: Local Charge Neutrality and One-Band Approximations

Abstract

Two aspects of diffusion in heavily doped semiconductors are discussed in reference to the suitability of the commonly assumed local charge neutrality approximation and the one-band approximation for the conduction-band density of states. The local electron concentration and internal field have been obtained by solving the Poisson-Boltzmann equation, and are compared to those obtained under the assumption of local charge neutrality. When the impurity diffusion length is several times the Debye length, the discrepancy is small except at the surface. A two-band density-of-states approximation is used to calculate the Fermi level in heavily doped germanium. It was found that the Fermi level thus obtained can be more than 0.1 eV lower than that obtained in the one-bank model.