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

• Grading Al composition of barrier layer alleviates the impact of piezoelectric polarization on 2-DEG.
• Adjusting Al-composition of AlyGa1−yN cap-layer and p-doping concentration on GaN buffer-layer enhances HEMT's drain-current.

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 AlxGa1−xN, 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 AlyGa1−yN 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.

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