4H-SiC MESFETs were fabricated using a bilayer dry thermal oxide/low-pressure chemical vapor deposited (LPCVD) silicon nitride for surface passivation. The passivation dielectric consists of a 20 nm thick dry thermal oxide covered by a 45 nm thick LPCVD silicon nitride layer. Devices utilize a recessed-channel architecture with 0.6 micron T-gates. Devices with the bilayer SiO2/SiNx passivation achieved a ft=9.3 GHz and fmax=15.5 GHz (WG=1.5 mm). The device transconductance was 34 mS/mm, drain current density was 235 mA/mm, and pinchoff voltage was –8V. Devices were load-pull characterized at 3 GHz with a 10% duty cycle and 100 μs repetition rate and a Class AB quiescent bias of IDS=100 mA/mm, and VDS=30V. Large devices with a 9.6 mm gate-periphery deliver an output power of 43.2 dBm (20.9 W=2.2W/mm) with a power-added-efficiency of 59% at a gain of 8.8 dB.