High-Resolution Thermoreflectance Imaging Investigation of Self-Heating in AlGaN/GaN HEMTs on Si, SiC, and Diamond Substrates

Gallium nitride (GaN) high electron-mobility transistors (HEMTs) offer considerable high-power operation but suffer in reliability due to potentially damaging self-heating. In this study, self-heating in AlGaN/GaN HEMTs on high conductivity substrates is assessed using a high-resolution thermoreflectance (TR) imaging technique, to compare the thermal response between GaN-on-Si, GaNon-Diamond, and GaN-on-4H-SiC. The TR method accuracy at high-power density is verified using a nonlinear coefficient of TR (CTR) as a function of temperature. The acquired steady-state thermal maps give a thermal resistance of 11.5 mm · K/W for GaN-on-Si (based on peak channel temperature), compared to 2.7 and 3.3 mm · K/W for GaN-on-SiC and GaN-on-diamond substrates, respectively. The tested GaN-on-diamond HEMT exhibits similar heating rates to those seen on a SiC substrate, with a slightly higher peak temperature, which indicates a higher thermal boundary resistance that could offset the benefits of using a higher conductivity substrate and lead to faster thermally enhanced degradation. The analysis reveals the importance using high-resolution imaging to detect hotspots and areas of peak temperature that largely affect failure initiation and device reliability and which may not be otherwise observable.