| Summary: | High-k HfO<sub>2</sub> has been widely adopted in Si based MOSFETs as gate dielectric for the superior control over gate leakage and channel electrostatics. However, in AlGaN/GaN HEMTs, the additional interface issue as well as high oxygen transparency of HfO<sub>2</sub> has hindered its practical applications. In this work, high-k Y<sub>2</sub>O<sub>3</sub> with ultra-low oxygen permeability and high thermodynamic robustness has been introduced as the interfacial layer between HfO<sub>2</sub>/GaN for the interface engineering. It has been demonstrated that, the HfO<sub>2</sub>/Y<sub>2</sub>O<sub>3</sub> gate dielectric stacks have obtained the GaN MIS-HEMT an ultra-small subthreshold swing of ~70 mV/decade, an extremely low gate leakage of ~10<sup>-12</sup> A/mm, and a desirable dielectric/semiconductor interface quality with interface state density in level of ~10<sup>12</sup> cm<sup>-2</sup>eV<sup>-1</sup>. Meanwhile, a maximum drain current of 600mA/mm has been achieved together with an on-state resistance (R<sub>on</sub>) of 10.7 Ω·mm and a specific R<sub>on</sub> of 2.62 mΩ·cm<sup>2</sup>.
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