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Using high-quality polycrystalline chemical-vapor-deposited diamond films with large grains (∼100 μm), field effect transistors (FETs) with gate lengths of 0.1 μm were fabricated. From the RF characteristics, the maximum transition frequency fT and the maximum frequency of oscillation fmax were ∼ 45 and ∼ 120 GHz, respectively. The fT and fmax values are much higher than the highest values for single-crystalline diamond FETs. The dc characteristics of the FET showed a drain-current density IDS of 550 mA/mm at gate-source voltage VGS of -3.5 V and a maximum transconductance gm of 143 mS/mm at drain voltage VDS of -8 V. These results indicate that the high-quality polycrystalline diamond film, whose maximum size is 4 in at present, is a most promising substrate for diamond electronic devices.