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[Display omitted] • A high channel hole mobility of 200 cm2V−1s−1 was achieved in C-Si interface (1 1 1) diamond MOSFETs. • The C-Si interface provides the MOSFETs with an excellent normally-off operation. • The advantage of boron doping in (1 1 1) diamond provides a large maximum current density. • The anatomically flat and strain-free interface between the (1 1 1) diamond and SiO 2 film was confirmed by HRTEM. • The existence of C-Si bonds at the interface was proved by EELS and XPS. In this paper, a diamond-silicon (C-Si) interface was constructed on a (1 1 1) diamond substrate by annealing the SiO 2 gate insulator in a reductive atmosphere. Corresponding metal-oxide-semiconductor field effect transistors (MOSFETs) with a C-Si conductive channel were fabricated. The MOSFETs demonstrate excellent normally-off operation with a high threshold voltage (V th) of −16 V and a high current density of −167 mA/mm, with a gate length (L G) of 4 μm. The channel hole mobility (μ FE) reaches 200 cm2V−1s−1 with a L G of 10 μm, and the interface state density (D it) is as low as 3.8 × 1011 cm−2 eV−1. The high-resolution transmission electron microscopy (HRTEM) image displays a coherent and strain-free interface between the SiO 2 film and (1 1 1) diamond, which ensures a high μ FE and low D it in the MOSFETs. The interface is dominated by C-Si bonds, which are confirmed by atomic-scale electron energy loss (EELS) quantification, spectroscopic characterization, and X-ray photoelectron spectroscopy (XPS). These results demonstrate that diamond, directly combined with SiO 2 , is ideal for implementation in power devices. [ABSTRACT FROM AUTHOR]