Screening of the electric field in covalent crystals containing point   defects
  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). 
N.A. Poklonski
cond-mat.mtrl-sci