Diamond-based RF power transistors: Fundamentals and applications

The present status of diamond-based transistors for high-frequency and high-power applications is reviewed. We have achieved the drain current density of 550 mA/mm, cut-off frequencies for current gain (fT) and power gain (fMAX) of 45 GHz and 120 GHz, respectively, and output-power density of 2.1 W/mm at 1 GHz in class-A operation of a field-effect transistor (FET) with hydrogen (H)-surface-terminated diamond. We have found that gate capacitance can be separated into depletion-layer capacitance and insulator capacitance. Concerning a stability of H-surface termination, no apparent decrease in the current for an FET without a gate contact was observed, but gate bias stress results in a slight decrease in the drain current and simultaneously an increase in the gate leakage current.