Current-voltage characteristics of GaAs MESFETs (with p-butter layers) on semi-insulating substrates compensated by deep levels are simulated by two-carrier and one-carrier models. For a thicker p-buffer layer or for higher acceptor density in the substrate, the drain current becomes lower because the substrate current is reduced. The one-carrier model also gives reasonable results for a case with a hole-trap substrate. Small-signal parameters of GaAs MESFETs on various types of substrates are also simulated. For a thicker p-buffer layer or for higher acceptor densities in the semi-insulating substrates, the substrate current is reduced, and both transconductance and cutoff frequency become higher. It is concluded that, to utilize the high-speed and high-frequency performance of GaAs MESFETs, acceptor densities in the substrate should be made high.>