Simulation of a GaAs MESFET Including Velocity Overshoot: An Extended Drift-Diffusion Formalism

This letter describes numerical simulations of GaAs MESDET's based on an extended drift-diffusion equation formalism which allows for the inclusion of velocity overshoot effect [1]. The overshoot correction term which augments the drift-diffusion current equation is proportional to the gradient of the local electric field. The proportionality constant referred to as Thornber's gradient or phenomenological length coefficient has been calculated by Monte Carlo methods as a function of the electric field. The model developed here solves the extended drift-diffusion, current continuity equation and Poisson's equation over a two dimensional grid with the method of finite differences. Simulations performed for normally-on and normally-off devices, with gate lengths of 0.2, 0.5 and 1.0 microns, are compared with already published Monte Carlo calculations. The results obtained with both methods for the calculated drain currents and average electron velocities in the device channel are in excellent agreement. It is concluded that the extended drift-diffusion formalism represents an approach which is useful and easy to implement in modeling field-effect transistors with submicron feature size.