A numerical approach to modeling the ultrashort-gate MESFET

A simple numerical model compatible to small computers is developed for an ultrashort-gate GaAs MESFET that takes into account the transient electron dynamics leading to velocity overshoot of electrons. It is assumed that because of the velocity overshoot phenomenon, the carriers move with constant high mobility at fields greater than the threshold field necessary for intervalley scattering for a certain time before relaxing to the equilibrated velocity of the low-mobility satellite valley. These time constants are taken from results of Monte Carlo calculations. The model also takes into account nonuniform channel doping as well as the nonabrupt depletion boundary. It is shown that changing the gate length from 1.0 to 0.5 µm does result in improved gain bandwidth although not in perfect proportion to the gate length reduction. This is due to excess charge dipole buildup in saturation as well as capacitance from fringing effects. The effects of profiles on the peak electric fields are also pointed out.