Design of novel normally-off AlGaN/GaN HEMTs with combined gate recess and floating charge structures

Normally-off operation is strongly desired for safety and efficient power switching in order to make the HEMT devices compatible with the currently used Si based IGBT and MOSFET devices. Combination of partially gate recess etching and gate insulator interface or floating gate charges in MIS structures is proposed and demonstrated for the first time to realize the normally-off mode. Partially gate trench can effectively reduce 2DEG density and shift threshold voltage (V_th) to positive without severely degrading in 2DEG channel conductance, while gate insulator interface or floating gate charges can further increase V_th at a relatively low charge density and thus maintain normally-off mode at a much longer time. Sentaurus TCAD is used to systematically simulate and predict the characteristics of the proposed structures. A positive V_th of larger than 3 V is demonstrated by employment of gate recess with 5~10 nm barrier leftover in combination of gate dielectric charging with a low sheet density of ~10¹² cm^-2. The proposed structures are very promising in future power switching applications due to the large positive V_th and the low gate leakage current density by adjusting the gate insulator thickness.