The impact of the channel temperature on the electrical device performance is important. For instance it has a strong influence on device reliability and efficiency. Therefore the determination of device temperature is a key issue for device topology optimisation. In this work the temperature variation of AlGaN/GaN high-electron-mobility transistors grown either on silicon or sapphire substrate under DC bias operation was measured by two methods, namely micro Raman scattering spectroscopy and pulsed I-V measurements. Thermal impedance was calculated for different structures and different topologies, the results were discussed to optimise the component design. The impedance cartography of the structure along the axis defined by the thermal length diffusion is determined. Good agreement between the Raman scattering and I-V pulsed techniques was verified.