Steady-state, low-field transport in bulk semiconductors is described by the drift diffusion equation,

where _n and D _n are material-dependent transport parameters related by the Einstein relation. The parameters are independent of the field because the lowfield perturbation in the distribution function is directly proportional to the electric field (recall homework problem 5.11). Under high fields, however, the distribution function becomes a nonlinear function of the field, so _n and D _n become field-dependent parameters. In devices, the situation is even more complex because the distribution function has a nonlocal dependence on the electric field. In this chapter, our focus is on steady-state, high-field transport in homogenous semiconductors. In the following chapter, we discuss the interesting and important effects that can occur in devices when the applied fields vary rapidly in time and space.

The chapter begins in Section 7.1 with a qualitative description of the general features of high-field transport in bulk semiconductors. The electron temperature approach for solving the momentum and energy balance equations to obtain the field-dependent mobility and diffusion coefficient is introduced in Section 7.2. Although limited in accuracy, this approach is useful for the insight it provides. The application of Monte Carlo simulation to accurately evaluate the fielddependent transport coefficients is then described. Some experimental results for the high-field transport of electrons in bulk Si and GaAs are examined in Section 7.4, and high-field transport of confined carriers is examined in Section 7.5.