| Summary: | Surface passivation is critically important to improve the current collapse and the overall device performance in metal-oxide semiconductor high-electron mobility transistors (MOS-HEMTs) and, thus, their reliability. In this paper, we demonstrate the surface passivation effects in AlGaN/AlN/GaN-based MOS-HEMTs using ultraviolet-ozone (UV/O<sub>3</sub>) plasma treatment prior to SiO<sub>2</sub> -gate dielectric deposition. X-ray photoelectron spectroscopy (XPS) was used to verify the improved passivation of the GaN surface. The threshold voltage (V<sub>TH</sub>) of the MOS-HEMT was shifted towards positive due to the band bending at the SiO<sub>2</sub>/GaN interface by UV/O<sub>3</sub> surface treatment. In addition, the device performance, especially the current collapse, hysteresis, and 1/f characteristics, was further significantly improved with an additional 15 nm thick hafnium silicate (HfSiO<sub>X</sub>) passivation layer after the gate metallization. Due to combined effects of the UV/O<sub>3</sub> plasma treatment and HfSiO<sub>X</sub> surface passivation, the magnitude of the interface trap density was effectively reduced, which further improved the current collapse significantly in SiO<sub>2</sub>-MOS-HEMT to 0.6% from 10%. The UV/O<sub>3</sub>-surface-modified, HfSiO<sub>X</sub>-passivated MOS-HEMT exhibited a decent performance, with I<sub>DMAX</sub> of 655 mA/mm, G<sub>MMAX</sub> of 116 mS/mm, higher I<sub>ON</sub>/I<sub>OFF</sub> ratio of approximately <inline-formula><math display="inline"><semantics><mrow><msup><mrow><mn>10</mn></mrow><mn>7</mn></msup></mrow></semantics></math></inline-formula>, and subthreshold swing of 85 mV/dec with significantly reduced gate leakage current (I<sub>G</sub>) of <inline-formula><math display="inline"><semantics><mrow><mn>9.1</mn><mo> </mo><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>10</mn></mrow></msup></mrow></semantics></math></inline-formula> mA/mm.
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