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A cutoff frequency (f/sub T/) of 11 GHz is realized in the hydrogen-terminated surface channel diamond metal-insulator-semiconductor field-effect transistor (MISFET) with 0.7 /spl mu/m gate length. This value is five times higher than that of 2 /spl mu/m gate metal-semiconductor (MES) FETs and the maximum value in diamond FETs at present. Utilizing CaF/sub 2/ as an insulator in the MIS structure, the gate-source capacitance is reduced to half that of the diamond MESFET because of the gate insulator capacitance being in series to the surface-channel capacitance. This FET also exhibits the highest f/sub max/ of 18 GHz and 15 dB of power gain at 2 GHz. The high-frequency equivalent circuits of diamond MISFET are deduced from the S-parameters obtained from RF measurement.