GaN and GaAs materials are the preferred materials of choice of worldwide researchers for High Electron Mobility Transistor (HEMT) due to suitable material properties. In this paper temperature dependence scattering effects are discussed. The degradation of mobility from different scattering mechanisms and its effect on drain current is also shown. The total mobility in bulk GaN and GaAs semiconductors are compared. The variations in electronic concentration with temperature are also presented in these types of HEMTs. The mobility degradation and its effect on drain current are also portrayed for AlGaAs/AlGaN HEMT’s.