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This paper presents high performance hydrogen-terminated diamond MOSFETs fabricated on a (111)-oriented single-crystal diamond substrate. The diamond surface was passivated by a high-quality Al2O3 grown by ALD at 350&#x00B0;C as well as a secondary passivation layer Si3N4 deposited by PECVD. After passivation, a low ohmic contact resistance <inline-formula> <tex-math notation="LaTeX">$R_{c}$ </tex-math></inline-formula> of <inline-formula> <tex-math notation="LaTeX">$0.5 \Omega \cdot $ </tex-math></inline-formula>mm was obtained and the 2DHG sheet density was as high as <inline-formula> <tex-math notation="LaTeX">$1.0\times 10\,\,^{\mathrm{ 13}}\,\,{\mathrm{ cm}}^{-2}$ </tex-math></inline-formula> with a corresponding mobility of <inline-formula> <tex-math notation="LaTeX">$104 {\mathrm{ cm}}^{2} /\text{V}\cdot \text{s}$ </tex-math></inline-formula>. The fabricated diamond MOSFET with gate length of <inline-formula> <tex-math notation="LaTeX">$0.5 ~\mu \text{m}$ </tex-math></inline-formula> showcased a high current density of 750 mA/mm, a low on-resistance of <inline-formula> <tex-math notation="LaTeX">$24 \Omega \cdot $ </tex-math></inline-formula>mm, and a high off-state breakdown voltage of 117 V. Thanks to the high current density and low on-resistance, a record high output power density of 2.1 W/mm was achieved at 10 GHz with drain biased at a low voltage of &#x2212;30 V. These results demonstrate that the output current and output power can be improved by using a (111)-oriented diamond, which is benefit for high-frequency and high-power RF devices.