A major limitation in the performances of AlGaN/GaN high-electron mobility transistors (HEMTs) is due to self heating effects, connected to the efficiency of heat removal from the device. In this paper, we present a new experimental investigation on the thermal handling capabilities of AlGaN/GaN HEMTs, with conventional and flip-chip bonding. Efficient photocurrent measurements were performed in order to extract directly the channel temperature for all the device configurations. We were able to measure devices realized on sapphire substrate both with conventional and flip-chip bonding, and to compare them with devices on SiC substrate with conventional bonding, demonstrating that flip-chip bonding allows to achieve almost the same results that SiC substrate. Measured results are in good agreement with the presented simulation data.