This letter reports the small-signal and large-signal performances at high drain voltage (VDS) ranging up to 60 V for a 0.5 μ m gate length two-dimensional hole gas diamond metal-oxide-semiconductor field-effect transistor with a 100-nm-thick atomic-layer-deposited Al2O3 film on a IIa-type polycrystalline diamond substrate with (110) preferential surfaces. This diamond FET demonstrated a cutoff frequency (fT) of 31 GHz, indicating that its carrier velocity was reaching 1.0× 107 cm/s for the first time in diamond. In addition, a fT of 24 GHz was obtained at VDS=-60 V, thus giving a f T× VDS product of 1.44 THz · V. This diamond FET is promising for use as a high-frequency transistor under high voltage conditions. Under application of a high voltage, a maximum output power density of 3.8 W/mm (the highest in diamond) with an associated gain and power added efficiency were 11.6 dB and 23.1% was obtained when biased at VDS=-50 V using a load-pull system at 1 GHz.
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