This paper describes the polarization effect of the substrate on the electric characteristics of the GaAs Metal Semiconductor Field Effect Transistor (GaAs MESFET). An analysis based on the existence of a double space charge at the interface active layer – semi-insulating substrate is applied to determine the active layer and interface parameters. The properties of the drain current and the output admittance characteristics as well as the physical phenomena inherent to this interface are assigned to the dynamic response of the double space charge region.