Over 20-GHz cutoff frequency submicrometer-gate diamond MISFETs

Submicrometer-gate (0.2-0.5-μm) diamond metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated on an H-terminated diamond surface. The maximum transconductance in dc mode reaches 165 mS/mm, while the average transconductance is 70 mS/mm in submicrometer-gate diamond MISFETs. The highest cutoff frequency of 23 GHz and the maximum frequency of oscillation of 25 GHz are realized in the 0.2-μm-gate diamond MISFET. From the intrinsic transconductances or the cutoff frequencies, the saturation velocities are estimated to be 4×10⁶ cm/s in the submicrometer-gate FETs. They are reduced by gate-drain capacitance and source resistance.