Modeling the effects of surface states on DLTS spectra of GaAs MESFETs

A resistance deep-level transient spectroscopy (DLTS) model which explains the effects of surface states on DLTS spectra of GaAs MESFETs is presented. The model includes both deep traps in the active channel under the gated region and the surface states on the ungated surface between the contacts of gate and source as well as gate and drain. Surface states are shown to result in minority holelike DLTS signals. The model reveals that the surface-state energy levels can be reliably determined from these holelike DLTS signals, although the concentrations cannot be accurately profiled due to the strong dependence of the peak magnitude of the holelike signals on the ungated surface conditions, in particular, the surface leakage current. The peak magnitude of the holelike signals are shown to depend strongly on the filling pulsewidth t_p used in a DLTS measurement. It is also shown that the peak magnitude decreases rapidly as the ratio of the gate length to the gate-source spacing is increased. It is expected that the model can be a useful tool for investigating the passivation effects of the ungated surface on a short-gate GaAs MESFET