The effects of the interfacial oxide layer in Schottky barrier junctions on submicron GaAs metal-semiconductor field-effect transistors (MESFETs) have been investigated. These effects include a shift in threshold voltage and compression in transconductance which can be explained by the current transport theory for the Schottky barrier with an interfacial layer. MESFETs were characterized by direct current measurements and oxide-related degradation was evaluated by measuring the gate leakage current. It has been shown that a gate-length dependent study of threshold voltage and transconductance is only possible if all the devices under consideration have identical Schottky responses. The current through a Schottky barrier appears to flow in a voltage-dependent resistor and the voltage drop across this resistor cannot be neglected when the density of states at the metal-semiconductor interface becomes prominent because it will effectively increase the threshold voltage and reduce the device gain.