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Normally-OFF AlGaN/GaN-based HEMTs with decreasingly graded AlGaN cap layer

In this work, an enhancement-mode (E-mode) AlGaN/GaN-based high-electron-mobility transistor (HEMT) with a graded AlGaN cap layer (GACL) is proposed and numerically studied by Silvaco technology computer-aided design. The GACL is designed with a decreasingly graded Al composition x along [0001] direction and the initial x is smaller than the Al composition of the Al0.2Ga0.8N barrier layer (BL). This GACL scheme can simultaneously produce high-concentration polarization-induced holes and negative net polarization charges at the GACL/BL interface. This can facilitate the separation of the conduction band (E C) and Fermi level (E F) at the 2DEG channel and therefore benefit the normally-OFF operation of the device. The optimized graded-AlGaN-gated metal-semiconductor HEMT can achieve a large threshold voltage of 4 V. Furthermore, we demonstrated that shortening the gate length on the GACL and inserting an oxide layer between the gate and GACL can be both effective to suppress gate leakage current, enhance gate voltage swing, and improve on-state drain current of the device. These numerical investigations can provide insights into the physical mechanisms and structural innovations of the E-mode GaN-based HEMTs in the future.