This research aims to conduct a comparative investigation of the heat effects on the microwave performance of high electron-mobility transistors (HEMTs) using GaAs and GaN technology. In order to achieve this difficult objective, the impact of changes in the surrounding temperature on the microwave performance is assessed by using scattering parameter data and the related equivalent-circuit models. The devices under investigation are two High Electron Mobility Transistors (HEMTs) with an identical gate width of 200 μm. However, they are manufactured utilizing distinct semiconductor materials: GaAs and GaN technologies. The examination is conducted under controlled settings of both low and high temperatures, with the temperature range being adjusted from - 40 ◦C to 150 ◦C. The temperature's influence varies significantly based on the chosen operating state. However, the bias point is picked to provide a fair comparison between the two distinct technologies to the greatest extent feasible. Both technologies exhibit similar tendencies, although the temperature has a greater influence on the GaN device.