Investigation on the Drift of GaAs MESFET's by High Frequency Parameters

The long term drift of GaAs MESFET's performance was investigated by measuring a change of the equivalent circuit parameters at high frequencies. It was found that the drain conductance and the drain-gate feedback capacitance of the MESFET's decrease remarkably corresponding to the drift of the drain current, and they are not recovered even after the drain current is reset to the original value. These changes of the equivalent circuit parameters are different from the changes observed when a bias is applied on the substrate, and they can be eliminated by an appropriate surface passivation. The change of the equivalent circuit parameters can be explained as the result of the gradual expansion of the surface depletion region from the gate to the drain area.