An ultra-thin double barrier enhancement mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMT) with p-type buffer layer and Si 3 N 4 /graded p-AlGaN gate is proposed and investigated by Silvaco TCAD. The simulation results show that the designed HEMT can obtain a high threshold voltage over 5.0 V and large gate swing. The maximum gate leakage current is 3.11×10 -4 A/mm at 30 V gate voltage, which decreases four orders of magnitude compared to the conventional double barrier HEMTs. Due to the p-type buffer layer, the cut-off frequency for the proposed HEMT is raised over three-times compared to the conventional double barrier structure HEMT with n-type buffer layer. Meanwhile the designed HEMT exhibits high breakdown voltage and large current-gain. Moreover, the impacts of Si 3 N 4 layer thickness under gate and GaN channel layer thickness are analyzed. Both layers play significantly roles in obtaining high threshold voltage for the device by adjusting the conduction band energy of AlGaN/GaN interface quantum well.