A high purity physical vapor transport (PVT) process has been developed successfully at Dow Coming Corporation to produce large area, semi-insulating 6H-SiC wafers without the addition of dopants during growth. In this study, a variety of techniques including COREMA, Hall effect measurement and two terminal I-V test, etc. were used to characterize these semi-insulating 6H-SiC wafers. Activation energy information was also obtained by temperature dependent resistivity measurement. SIMS (Secondary Ion Mass Spectroscopy) and GDMS (Gas Discharge Mass Spectroscopy) analyses were performed to identify the possible doping impurities and their concentrations in SiC crystals. Based on these data, the impact of residual dopants on the semi-insulating properties of SiC material is discussed.