DescriptionSilicon carbide is the only wide band gap semiconductor that has a native oxide, and a leading candidate for development of next-generation, energy efficient, high power metal-oxide-semiconductor field effect transistors (MOSFETs). Progress in this technology has been limited by the semiconductor-dielectric interface structure and its effect on the inversion layer mobility. The major objective of this dissertation is to study and improve 4H-SiC MOSFET interface structure, defect states and inversion layer mobility on the (11-20) crystal face of SiC (a-face), employing nitrogen and phosphorous passivation. We also use these results to explore the effect of reactive ion etching on the a-face, an important aspect of processing optimum power devices. We correlate electrical measurements, i.e. current-voltage (I-V) and capacitance-voltage (C-V) with physical characterization including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS) and medium energy ion scattering (MEIS). A significant phosphorus induced inversion layer mobility enhancement of ~125 cm2/V-s is achieved, and the revisited effect of NO on the a-face of 4H-SiC yields an impressive mobility of ~85 cm2/V-s. These results indicate that N and P improves the interface both by passivation and by interfacial counter doping, with the latter mechanism more effective on the a-face than the Si-face. Interface trap density (Nit) and the mobility-temperature dependence both indicate coulomb scattering is no longer the limiting factor for the N and P annealed a-face inversion layer mobility. The second major part of this dissertation reports the use of hydrogen annealing to implement the successful recovery of the a-face (11-20) crystal structure and the inversion layer mobility following degradation by reactive ion etching (RIE). The results impact the processing of SiC trench MOSFETs where the a-face sidewall forms a significant portion of the conducting semiconductor channel.