A novel approach by employing a Cu filled via hole under device active area to improve heat dissipation of AlGaN/GaN high electron mobility transistors (HEMTs) grown on silicon substrate is proposed. In this study, finite element analysis was employed to simulate the temperature distributions of reference HEMT and proposed model. By introducing a Cu filled via underneath the active area and removing a thermal resistive layer resulted from the nucleation layer generated during the HEMT structure grwoth, the maximum junction temperature could be reduced from 147 to 123ºC at a power density of 5W/mm.