Free-electron processes in amorphous semiconductors

Several processes based on the free-electron concept are briefly reviewed in this paper. Using a quantum-tunneling model, a simple theory for determining the effective mass of charge carriers in amorphous semiconductors is presented. It is shown that a charge carrier has different effective masses in the extended and tail states, and that the sign of the effective mass changes as the charge carrier crosses its mobility edge. This is used to explain the anomalous Hall coefficient observed in a-Si : H. The theory is used to calculate Tauc's absorption relation, theoretically. It is found that deviations from the Tauc plot observed in some a-solids may be attributed to the photon-energy-dependent matrix element. Some transport properties of charge carriers are also considered. (c)� 2003 Kluwer Academic Publishers.