CHAPTER III
MOBILITY AND SCATTERING RELAXATION TIME
3.1 Introduction
The study of transport phenomena in semiconductors requires an
accurate knowledge of variations in the conductivity mobility and the resistivity with changes in temperature and dopant density. The resistivity is an easily-measured parameter, but the conductivity mobility, is a more difficult parameter to evaluate. In general, four different kinds of mobility enter into common discussion [42]. The microscopic mobility is the actual-velocity per unit electric field of a free carrier in a crystal. This cannot be measured directly. The conductivity mobility is the mobility associated with the conductivity expression, cr eppC. This mobility involves an average relaxation time dependent on the nature of the scattering process, and in the case of nonspherical equal energy surfaces, this mobility also involves a combined effective mass. The Hall mobility is the product of the measured conductivity and the measured Hall coefficient. In general, the Hall mobility differs from the conductivity mobility by a factor called-the Hall factor. The drift mobility is the velocity or drift per unit field for'a carrier moving in an-electric field. If trapping centers are present, so that the actual drift process is not simply motion through the conduction band, but involves a series of trapping and unwrapping processes, the drift mobility can be much less" than the conductivity mobility. The four mobilities are all equal only when the 29