AlGaN/GaN metal oxide semiconductor high electron mobility transistors (MOSHEMTs) are very attractive for high power and high frequency and high temperature applications, with low gate leakage current.  In this work   we investigated the simulation of Hf₂O₂/AlGaN/GaN, Al₂O₃/AlGaN/GaN and TiO₂/AlGaN/GaN MOS-HEMTs, on sapphire substrates and the trapping effects of the devices using TCAD device simulation. Good simulation I_D-V_DS, I_D-V_GS, and g_m-V_GS plots of the passivated and unpassivated devices were obtained. Simulations have revealed in passivation case for Al₂O₃ as gate oxide , a very high drain current of 2.728A, peak Gm of 0.390 S, and a threshold potential V_T of - 5.022 V. The peak values of f_T and f_max, extracted from S-parameters, were found to be 299.91GHz and 194.53 GHz, respectively for highly scaled device. These results demonstrate the potential of AlGaN/GaN underlap MOS-HEMT for high power and high-frequency applications.