High-κ Oxide Charge Engineering on GaN for Normally Off HEMTs

GaN-based enhancement-mode high-electron-mobility transistors (HEMTs) are potential frontrunners for high-frequency power applications due to their simple gate driving circuit and failsafe functioning. A wide range of techniques have been proposed for the fabrication of normally off (E-mode) HEMTs, including recess gate structure, fluorine implantation, P-GaN gate structure, and double/triple gate structure. However, using the above-mentioned techniques, a maximum threshold voltage (V_th) of ~1 V can be achieved, which is not sufficient to prevent faulty operation caused by noise. MIS-HEMTs (metal–insulator–semiconductor HEMT) can facilitate higher positive threshold voltage by capacitance modulation and improve carrier confinement within the two-dimensional electron gas (2DEG), which lowers the gate leakage current. In this paper, different techniques, in particular MIS-HEMT structures, have been studied in detail for normally off operation.