4H-SiC n-channel power MESFETs with nitrogen-doped epitaxially grown channel and nitrogen n+-implanted source/drain ohmic contact regions, with and without p-buffer layer fabricated on semi-insulating substrates exhibited hysteresis in the drain I-V characteristics of both types of devices at 300 K and 480 K due to traps. However, thermal spectroscopic measurements could detect the traps only in the devices without p-buffer. In this study the two-dimensional device simulator, MediciTM, and optical admittance spectroscopy (OAS) measurements are used to help resolve the discrepancy in the initial experimental characterization results and interpret the results. Device simulations also showed hysteresis in the drain I-V curves of both types of devices at 300 K and 480 K. Simulations suggest that, in addition to the SI substrate traps, which are known to be major cause of hysteresis in MESFET drain I-V characteristics, acceptor traps due to source/drain residual implant lattice damage could also contribute to the hysteresis observed in the drain I-V characteristics of the experimental MESFETs. Although surface traps are known to cause hysteresis in the I-V curves of MESFETs, their presence was not observed in the experimental devices. The results of the OAS measurements showed several peaks in the spectra of the devices without p-buffer, while in the spectra of the devices with p-buffer the peaks were generally non-existent or reduced. This demonstrates that the peaks observed in the OAS spectra are largely due to substrate traps and that the p-buffer layer is effective in isolating the channel from the substrate. A peak centered around 1.51 eV below the conduction band, which has also been observed in the literature after He+-implantation, is consistently observed in the spectra of both types of devices although it appears reduced in the spectra of the devices with buffer. In this dissertation it is shown that it is likely the traps responsible for this peak could contribute to the hysteresis observed at 300 K and could be solely responsible for the hysteresis observed at high temperatures such as 480 K, since simulations suggest that hysteresis due to semi-insulating substrate traps disappear at high temperatures such as 480 K.