HOPPING IN EXPONENTIAL BANDTAILS IN HIGH ELECTRIC-FIELDS AND TRANSPORT MODELS IN AMORPHOUS-SILICON

A calculation of electrical transport during the thermalization of excess carrier hopping between states of an exponential bandtail is presented. The calculation is based on a typical rate approximation. For small electric fields the calculation reproduces multiple-trapping dispersion, in agreement with prior research. For high electric fields electrical transport becomes nonlinear, primarily due to an electric-field-induced increase in the dispersion parameter. The calculation should be contrasted to recent work of Esipov which extended bandtail multiple-trapping to incorporate tunnelling to a transport edge; in his calculation nonlinear transport set in without changes in the dispersion. We suggest that these results discriminate between hopping and mobility-edge models for electron thermalization measurements in a-Si: H.