Phosphorus passivation of the SiO 2/4H-SiC interface lowers the interface trap density and increases the field effect mobility for N-channel MOSFETs to twice the value of 30-40cm 2/V-s obtained using standard NO nitridation. Passivation using P 2O 5 introduced with an SiP 2O 7 planar diffusion source (PDS) converts the oxide layer to phosphosilicate glass (PSG) which is a polar material. BTS (bias-temperature-stress) measurements with MOS capacitors and FETs show that the benefits of reduced interface trap density and increased mobility are offset by unstable flat band and threshold voltages. This instability can be removed by etching away the PSG oxide and depositing a replacement SiO 2 layer. However, trap densities for etched MOS capacitors are "NO-like" (i.e., higher), which would lead one to expect a lower mobility if MOSFETs are fabricated with the PSG / etch / deposited oxide process.