Measurement of Temperature in GaN HEMTs by Gate End-to-End Resistance

We have used the gate end-to-end (GEE) resistance method to measure channel temperatures in GaN HEMTs. This method is appealing for its simplicity and sensitivity to temperature immediately adjacent to the base of the gate, where several important degradation mechanisms occur. This region is not normally accessible with optical measurement techniques, due to shadowing by the gate and field plate overhangs; yet, it is considerably hotter than the regions that can be reached with optics. We found agreement with a finite-difference model, with reasonable inputs for the thermal barrier resistance at the SiC-GaN interface and temperature coefficients. We repeated this successfully for a second GaN HEMT technology with a very different gate geometry. Finally, we conducted micro-Raman measurements on FETs from the first technology and found excellent agreement with the GEE measurement, once it had been adapted to the more-distant location of the micro-Raman laser spot, by means of our finite-difference model. We conclude that GEE is a valuable tool, complementary to micro-Raman, for calibrating thermal models.