Numerical simulations of GaAs MESFET's are performed in which impurity compensation by deep traps in the semi-insulating substrate is considered. It is found that the higher acceptor density in the substrate results in lower device current due to the formation of a space-charge layer at the channel-substrate interface. It is also found that the drain currents increase continuously with the drain voltage because electrons are injected to fill the traps in the substrate and a current path through the substrate is formed. This substrate current becomes remarkable for shorter gate-length MESFET's on a substrate with lower acceptor and trap densities. It is suggested that, to minimize short-channel effects in GaAs MESFET’s, the acceptor density as well as the trap density in the semi-insulating substrate must be high.
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