In the framework of the Top Amplifier Research Groups in a European Team (TARGET) project, we developed a new electrical method for the temperature measurement of HEMTs and performed several unique studies on the self-heating effects in AlGaN/GaN HEMTs. This method, in combination with transient interferometric mapping (TIM), provides a fundamental understanding of the heat propagation in a transient state of HEMTs. The AlGaN/GaN/Si HEMT thermal resistance was determined to be ~70 K/W after 400 ns from the start of a pulse, and the heating time constant was ~200 ns. Our experimental methods were further applied on multifinger high-power AlGaN/GaN/sapphire HEMTs. The TIM method indicates that the airbridge structure serves as a cooler, removing approximately 10% of the heat energy. In the next study we used TIM and the micro-Raman technique to quantify thermal boundary resistance (TBR) between different wafer materials and GaN epi-structure. We found TBR to be ~7 × 10−8 m2K/W for GaN/Si and ~1.2 × 10−7 m2K/W for GaN/SiC interfaces. The role of TBR at the GaN/sapphire interface was found to be less important.