The saturated velocity model of a short channel MESFET has been extended by incorporating the effects of interface states and interfacial layer at the gate contact of the device. Simple analytical forms for the channel current, drain conductance and transconductance of the device are derived and they are found to be much sensitive to interface state density. The proposed model has been applied to compare the drain characteristics of Si, SiC and GaN MESFETs. For all three materials, the channel current has been found to decrease with drain voltage as the field across the channel exceeds the critical field required for velocity saturation. The decrease in the drain current yields a negative drain conductance and transconductance with respect to drain and gate voltage, respectively. The experimental results reported by the earlier researchers on GaN and SiC MESFETs are considered to compare the theory with experiment over the domain of velocity saturation. It has been found that the experimental results can be best explained on the basis of an energy distribution of interface states at the gate contact of both the devices.