Frequency-dependent transients in GaAs MESFETs: process, geometry and material effects

Transient aberrations have long been observed in GaAs MESFETs. The authors report on the magnitude of current transients that occur when a voltage step is applied to the drain of a device. A simple measurement technique that was applied to both enhancement and depletion mode devices is described. The overshoot as a function of bias condition (from subthreshold to saturation), drain pulse input frequency, substrate temperature, and device contact spacing were measured. The results show that the current overshoot grows with increasing drain voltage step magnitude and temperature, lighter channel doping, lower standing current, smaller gate-drain spacing, and larger gate-source spacing. Beryllium p-well implants also increase the overshoot significantly. The magnitude of the aberrations can grow to ten times the quasi-static pulse amplitude at frequencies in the 10 Hz range. Surface preparation and dielectrics are shown to have minimal effects on the problem. It is indicated that charge trapping in the channel-substrate region is the chief cause of the current overshoot.<>