SIMULATION OF SURFACE-STATE DYNAMICS ON GAAS-MESFETS

This paper describes a model for surface states on GaAs MESFETs which is used to characterise the dynamic behaviour of these traps under pulsed operating conditions. The surface charge due to trap filling is shown to be highly non-uniform between the gate and the drain, and the form of the surface charge distribution is found to depend on the current-voltage characteristics of the gate Schottky contact. The trap filling is calculated in a self-consistent manner within the framework of a two-dimensional unipolar physical simulation. The resulting model is used to investigate some anomalous features of the behaviour of the MESFET. In particular, time dependence of the gate-drain breakdown voltage under transient conditions is explained in terms of a reduction on the gate edge electric fields as surface states near the drain end of the gate fill with excess electrons and thus become more negatively charged at high values of the drain-gate voltage. This excess surface charge modifies the electric fields near the drain end of the gate. These calculations are supported by experimental measurements on a real device.