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In this study, we propose a novel normally-off AlGaN/GaN HFET based on stack AlGaN barrier structure and p-type NiO gate. The residual thin AlGaN barrier (with low Al content) is adopted to alleviate mobility degradation. Besides, p-type conductive NiO formed by thermal oxidation at 500 °C was used as gate electrode, which contribute to the positive shift of threshold voltage. Combining NiO gate and thin barrier structure, normally-off device with a threshold voltage of +1.1 V is realized. Temperature dependent transfer characteristics show that the normally-off device presents good thermally stability within the temperature range from 25 to 150 °C.

In this paper, AlN films grown by magnetron sputtering method have been proposed as the gate insulator layer of AlGaN/GaN high electron mobility transistors (HEMTs) to decrease gate leakage current and suppress the interface trap. The effect of the temperature of substrate on the quality of AlN films have been investigated. By inserting the thin AlN film (35 nm) as a gate insulator layer, the on-state resistance of AlGaN/GaN HEMTs decrease from 11.1 Ω mm to 10.3 Ω mm @V g = 0 V, the current collapse decreases from 16.6% to 3.2%, the gate leakage can be reduced from 1.2 × 10−1A mm−1 to 4.4 × 10−6A mm−1 @V g = 2 V by five orders of magnitude, and the fast interface states disappear and the normal trap density decreases from 0.96–1.3 × 1013 cm−2eV−1 to 1.3–3.4 × 1012 cm−2 eV−1, proving that magnetron-sputtered AlN is an effective way to improve the performance of GaN HEMTs.