Two-dimensional simulations of GaAs MESFET’s are made in which impact ionization of carriers and impurity compensation by deep levels in semi-insulating substrates are considered. It is shown that in cases with high acceptor densities in the semi-insulating substrates, a steep increase in output conductance with the drain voltage ("kink") arises because holes that are generated by impact ionization flow into the substrates and are captured by the deep levels to modulate the space-charge distributions much. In cases with low acceptor densities in the substrates, a sudden increase in drain currents due to conductivity modulation in the substrates is observed. It is concluded that carrier trapping and current transport in the semi-insulating substrate should be taken into account when considering kink effects and/or breakdown phenomena in GaAs MESFET’s.
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