Diamond metal-oxide-semiconductor field-effect transistors (FETs) have been fabricated on IIa-type large-grain diamond substrates with a (110) preferential surface. The drain current and cutoff frequency are −790mA∕mm and 45GHz, respectively, which are higher than those of single-crystal diamond FETs fabricated on (001) homoepitaxial diamond films. The hole carrier density of the hole accumulation layer depends on the orientation of the hydrogen-terminated diamond surface, for which (110) preferentially oriented films show 50%–70% lower sheet resistance than a (001) substrate. We propose that the hole density of the surface accumulation layer is proportional to the C–H bond density on the surface.

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RF diamond transistors have been developed on a hydrogen-terminated surface conductive layer. f T and f max of 23 and 25 GHz, respectively, have been achieved in a diamond MISFET with a 0.2 µm gate length. Utilizing de-embedding and small-signal equivalent circuit analysis, parasitic components are extracted. The intrinsic f T and f max of the 0.2-µm-gate diamond MISFET are estimated to be 26 and 36 GHz, respectively. In this report, some of the challenging steps in device fabrication processes such as the development of a low-resistivity ohmic layer, a high-quality gate insulator and acceptor density control technology, toward high-power and high-frequency diamond transistors with high reliability, are introduced.

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