The goal of this research is to look at the effects of temperature on the forward transmission coefficient (S 21 ) of high electron-mobility transistors (HEMTs), employing various technologies. The analysis is carried out on six different devices under different bias conditions with the ambient temperature varied from -40°C to 150°C. The low-frequency magnitude of S 21 is significantly reduced in GaN-based HEMTs when the temperature is increased. On the other hand, an operating bias point where this magnitude is temperature insensitive has been identified in GaAs-based HEMTs.