Different SiC thermal oxide passivation techniques were characterized using UV-induced hysteresis to estimate the fixed charge, Q(f), and interface-trapped charge, Q(it). Steam-grown oxides have a fixed charge density of Q(f)=1x10(12) cm(-2), and a net interface-trapped charge density of Q(it)=4x10(11) cm(-2). Addition of a thin low-pressure chemical-vapor deposited (LPCVD) silicon nitride layer decreased these parameters to Q(f)=2x10(11) cm(-2) and Q(it)=4x10(10) cm(-2). Dry oxide shows a fixed charge density, Q(f)-=3x10(12) cm(-2) and interface-trapped charge density, Q(it)=4x10(11) cm(-2) which changes to Q(f)=+7x10(10) cm(-2) and Q(it)=1x10(10) cm(-2) with the addition of a LPCVD silicon nitride cap. Dry thermal oxide with a silicon nitride cap was used to passivate SiC MESFETs to achieve a power-added efficiency of 60% in pulsed operation at 3 GHz in Class AB bias conditions.