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Channel recessed 4H-SiC MESFETs have demonstrated excellent small signal characteristics and the effect of Si3N4 passivation on these devices has been studied in this work. A saturated current of 250-270 mA/mm and a maximum transconductance of 40-45 mS/mm were measured for these devices. The 3-terminal breakdown voltage Vds ranges from 120 V to more than 150 V, depending on gate-drain spacing. 2×200 μm devices with 0.45 μm gate length show high Ft of 14.5 GHz and Fmax of 40 GHz. After Si3N4 passivation, the output power and PAE were increased by 40% and 16%, respectively, for CW power measurement. Other measurements, such as, the change in surface potential and the dispersion of the drain current make it clear that the passivation of SiC MESFETs reduces the surface effects and enhances the RF power performance by suppressing the instability in DC characteristics.