Surface influence on the conductance DLTS spectra of GaAs MESFET´s

The observation of hole traps in small-signal GaAs MESFET\´s has been extensively reported in the literature. Previously these have been attributed to trapping at the active layer-substrate interface. Evidence is presented here, based on conductance DLTS and low-field low-frequency transconductance dispersion measurements on MESFET\´s of various geometries, to suggest that the main contribution to the "hole trap-like" spectrum in conductance DLTS is not bulk hole traps. Instead we believe that this phenomenon arises from changes in the population of surface states in the ungated access regions of the device, resulting in modulation of the surface depletion layer in series with the gate depletion region.