  An expression is derived within the framework of the Debye-H\"uckel
approximation for the screening length of a field in a p-type semiconductor
taking into account the energy spread of immobile acceptor levels and the
density of states tail of the valence band. It is shown that the screening
length depends additively on the product of the carrier density and their drift
mobility to diffusion coefficient ratio (for free holes in valence band and
holes hopping via acceptors).
