In this chapter, a field-plated recessed gate III-nitride High Electron Mobility Transistor (HEMT) grown on β-Ga2O3 substrate is designed. The electrical characteristics of the proposed HEMT is investigated by using the thermal models of ATLAS TCAD simulations. The impact of temperature on the transport properties is studied. Influence of the substrate thickness with temperature changes on drain characteristics are also discussed. A field-plate and gate length of 20 nm each with 30 nm recessed depth is considered for the analysis. Self-heating effect in drain current characteristics are investigated with temperature changes. The maximum drain saturation current observed with 180 nm (230 nm) substrate thickness are 1.1 (1.06) A/mm, 0.708 (0.705) A/mm and 0.502 (0.499) A/mm for 300 K, 550 K and 800 K, respectively. The influence of substrate layer thickness on drain current is less apparent at a higher temperature. The findings demonstrated that scattering processes that emerge when temperature increases above a particular amount cause both the mobility and the carrier concentration of 2DEG to decrease. Furthermore, A kink effect is seen in the drain current–voltage characteristics for gate voltage VGSâ€‰=â€‰− 2 V.