The authors describe numerical simulations of GaAs MESFETs (metal-semiconductor field-effect transistors) on a semi-insulating substrate in which impurities are compensated by deep traps. It is shown that 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 shown that drain currents increase continuously because electrons are injected to fill the traps in the substrate and a current path through the substrate is formed. This substrate current becomes pronounced for shorter-gate-length MESFETs on a substrate with lower impurity densities. It is suggested that to minimize short-channel effects in GaAs MESFETs, impurity densities in the semi-insulating substrate must be high.