We report on a temperature study of flip-chip mounted multi-finger AlGaN/GaN heterostructure field effect transistors (HFETs) using micro-Raman spectroscopy and infrared (IR) thermography. Flip-chip mounting can be used to improve thermal device management, in particular, for devices grown on low thermal conductivity substrates such as sapphire. In this study, we compare two flip-chip mounted HFETs of different flip-chip contact bump layout designs and a non flip-chip mounted HFET. Both temperature measurements and 3D temperature simulations are presented. The results show that minimizing the distance between the bumps and the active area of the HFET is essential for obtaining a low device operating temperature.