The low temperature electrical conductivity of n-type germanium

Abstract

The dominant conductivity mechanism for the samples measured is associated either with the conduction band or with an impurity conduction mechanism according as the temperature is greater or less than ~4°K. The density of impurities is sufficiently low to preclude formation of an impurity band. Rather, the mechanism of impurity conduction fits a model recently proposed by mott wherein compensating acceptors leave ionized donors or defects in the array of neutral donors. Conduction is then possible by migration of these donor defects after they have been thermally activated from the coulomb field of the ionized acceptors. The conductivity of the conduction band is found to deviate from Ohm's law at fields as low as 3 per cent of the threshold field for the breakdown generally ascribed to impact ionization. A heating of the electron distribution with a subsequent variation of both mobility and carrier density is found to be responsible for this deviation. The variation of carrier density is presumed to be caused by a velocity dependence of the probability of capture of a conduction band electron by an ionized donor, whereas the mobility variations are due to the velocity dependence of the cross section for scattering both by ionized impurities and phonons.