Manipulation of polarization effect to engineer III-nitride HEMTs for normally-off operation

We propose a novel, normally-off AlGaN/GaN high-electron-mobility transistor (HEMT) governed by polarization engineering. The fundamental concept is to grade the Al composition of the barrier layer from GaN to Al_xGa_{1- x}N, thereby alleviating the impact of piezoelectric polarization on the two-dimensional electron gas (2-DEG) and establishing a conduction-band profile well above the Fermi energy. These effects lead to a positive shift in the threshold voltage of the device and benefit the normally-off operation. It is observed that the device's DC transfer characteristics can be further modulated simply by adjusting the Al composition of the Al_yGa_{1- y}N cap layer and the p-type doping concentration at the top of the GaN buffer layer. These findings, based on a physical simulation of the proposed device, provide a guideline for the implementation of highly efficient, normally-off AlGaN/GaN HEMTs.