Drift and diffusion in materials with traps

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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 anomalous temperature dependence for some energy distributions of localized states.