Thermo-mechanical aspects of gamma irradiation effects on GaN HEMTs

We report thermal and mechanical responses accompanying electrical characteristics of depletion mode GaN high electron mobility transistors exposed to gamma radiation up to 107 rads. Changes in the lattice strain and temperature were simultaneously characterized by changes in the phonon frequency of E2 (high) and A1 (LO) from the on-state and unpowered/pinched off reference states. Lower doses of radiation improved electrical properties; however, degradation initiated at about 106 rads. We observed about 16% decrease in the saturation current and 6% decrease in the transconductance at the highest dose. However, a leakage current increase by three orders of magnitude was the most notable radiation effect. We observed temperature increase by 40% and mechanical stress increase by a factor of three at a dose of 107 rads compared to the pristine devices. Spatial mapping of mechanical stress along the channel identifies the gate region as a mechanically affected area, whereas the thermal degradation was mostly uniform. Transmission electron microscopy showed contrast changes reflecting a high vacancy concentration in the gate region. These findings suggest that localized stress (mechanical hotspots) may increase vulnerability to radiation damage by accommodating higher concentration of defects that promote the leakage current.