Drain-current transients of GaAs MESFET’s with deep donors “EL2" in the semi-insulating substrate are simulated in the range t = 10-13 to 102 s. It is shown that in the drain step responses, there exists a “quasi-steady state" where the deep donors do not respond to the voltage change and the drain currents become constant temporarily. The drain currents begin to decrease or increase gradually when the deep donors begin to capture or emit electrons, reaching real steady-state values. I-V curves are quite different between the “quasi-steady state" and the steady state. Therefore, the deep donors in the semi-insulating substrate can be causes of drain-current drifts and hysteresis in I-V curves. Effects of introducing a p-buffer layer are also studied. It is concluded that the use of a low acceptor density semi-insulating substrate combined with introducing a p-buffer layer is effective to minimize the unfavorable phenomena and to utilize high performances of GaAs MESFET’s.
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