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°C as well as a secondary passivation layer Si3N4 deposited by PECVD. After passivation, a low ohmic contact resistance $R_{c}$ of $0.5 \Omega \cdot $ mm was obtained and the 2DHG sheet density was as high as $1.0\times 10\,\,^{\mathrm{ 13}}\,\,{\mathrm{ cm}}^{-2}$ with a corresponding mobility of $104 {\mathrm{ cm}}^{2} /\text{V}\cdot \text{s}$ . The fabricated diamond MOSFET with gate length of $0.5 ~\mu \text{m}$ showcased a high current density of 750 mA/mm, a low on-resistance of $24 \Omega \cdot $ 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 −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.
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