AlGaN/GaN-based heterostructure field effect transistors (HFETs) are excellent candidates for high power and high frequency applications. However, current collapse and large gate leakage currents in these devices limit the output performance. Recently, gate insulation has shown to significantly reduce the leakage currents and enable device operation under high gate biases. Several oxides such as SiO<SUB>2</SUB>, Al<SUB>2</SUB>O<SUB>3</SUB>, MgO and Sc<SUB>2</SUB>O<SUB>3</SUB> have been used as gate insulators, giving rise to metal-oxide-semiconductor HFETs (MOSHFETs) with lower gate leakage currents. Additionally, owing to the complex nature of reliability failure in these devices, it is not clear what the operational stresses are, and it becomes imperative to assess the role of these stresses in long term reliability. In this paper, we focus on the electrical, thermal and mechanical modeling of GaN-based transistors under steady state and pulse mode conditions. Using a coupled electro-thermo-mechanical procedure, we compare self-heating effects and thermal stresses in HFETs and MOSHFETs under various operating conditions, and investigate the influence of device design parameters and boundary conditions on the thermal and mechanical properties. 2-D electrical simulations were performed using the Sentaurus Device simulator, while thermal and mechanical simulations were performed with COMSOL using a one-way coupling procedure. COMSOL was used to first solve the continuum heat transfer equation using the heat generation obtained from Sentaurus Device, and then the thermal expansion strain was determined, followed by the solution of the elasticity equation. Electrical simulations were carrier out near the active regions of the device, while the domain was extended for thermal and mechanical simulations to model realistic heat diffusion. The stress in the device was modeled in two dimensions as in-plane. The active region of the MOSHFET consisted of a 30 nm Al<SUB>0.2</SUB>Ga<SUB>0.8</SUB>N barrier, a 0.45 μm GaN channel region and a 10 nm SiO<SUB>2</SUB> insulator layer under the gate. The gate was 2 μm long and 150 μm wide. The HFET comprised of a 25 nm Al<SUB>0.23</SUB>Ga<SUB>0.77</SUB>N barrier, a 1.2 μm GaN layer with a gate length of 1 μm and gate width of 200 μm. In both cases, the substrate thickness was assumed to be 200 μm. Temperature dependent thermal conductivities were used for the materials in the device. Thermal boundary resistances, thermal effects of metallization, and residual stresses were ignored for simplification purposes. The electrical simulations under steady state indicated that at V<SUB>gs</SUB>=0 V, the AlGaN/GaN MOSHFET structure exhibited about 40% increase in the saturated drain current due to the presence of oxide under the gate. Thermal and mechanical simulations showed that the peak temperature at 40 V drain bias was about 10% higher in MOSHFETs and consequently, the thermal stress was also higher under steady state, with the peaks occurring near the drain end of the gate contact.