In a GaAs field effect transistor there are ungated sections of the channel between the gate and the source/drain. The static characteristics and the transients which occur in various parameters after a change in the bias voltages can be shown to be affected by the surface potential in this region. A model is proposed for these effects which involves a surface region with lower effective energy gap. The gate current leakage characteristics and conductance deep level transient spectroscopy measurements on single and dual gate devices are consistent with this model. ©1996 American Institute of Physics.