Drift and diffusion in materials with traps

Abstract

A concept of mobile and localized states is used for a description of carrier transport in disordered materials. The localized states are assumed to be distributed over some energy region in the gap. Two regimes of transport are possible: quasi-equilibrium and non-equilibrium. In the present work we concentrate on the quasi-equilibrium regime. Quasi-equilibrium transport takes place under conditions of thermal equilibrium between mobile and localized carrier fractions. The quasi-equilibrium transport equation obtained has the form of a conventional Fokker-Planck equation with drift and diffusion terms. The drift term is characterized by the equilibrium trap-controlled mobility. The diffusion term is determined by a coefficient which in its turn consists of two parts. The first describes conventional trap-controlled diffusion, while the second represents the so-called ‘field diffusion coefficient’ proportional to the second power of the electric field. The field diffusion is shown to have an anoma...