Review of the theory of amorphous semiconductors

The existing simple models of the electronic structure of disordered materials are reviewed. The focus is on the universal features of these models and their consequences for amorphous semiconductors. Simple plausibility arguments are given showing that continuous bands of extended states with tails of localized states associated with fluctuations within the disordered material can always be expected. Models for the mobility in which shoulders occur at the energies of transition from localized to extended states are reviewed. It is a mobility gap rather than a gap in the density of states which is responsible for the activated temperature dependence of the conductivity in amorphous semiconductors. Hopping conduction and the nature of the electronic motion in extended states near the mobility edges is discussed. The latter is likened to Brownian motion within certain limitations. Finally, the Anderson transition is discussed within the present models.