We investigated the heat dissipation in heterostructure field-effect transistors (HFETs) using microRaman measurement of the temperature in active AIGaN/GaN. By varying the gate structure, the heat dissipation through the gate was clearly revealed. The temperature increased to 120 °C at the flat gate device although the inserted gate increased to only 37 °C. Our results showed that the inserted gate structure reduced the self-heating effect by three times compared to the flat gate structure. Temperature mapping using micro-Raman measurement confirmed that the temperature of the near gate area was lower than that of the near drain area. This indicated that the inserted gate electrode structure effectively prohibited self-heating effects.