II-VI has developed an Advanced PVT (APVT) process for the growth of nominally undoped (vanadium-free) semi-insulating 2" and 3" diameter 6H-SiC crystals with room temperature resistivity up to 10(10) &UOmega;(.)cm. The process utilizes high-purity SiC source and employs special measures aimed at the reduction of the impurity background. The APVT-grown material demonstrates concentrations of B and N reduced to about 2-10(15)cm(-3). Wafer resistivity has been studied and correlated with Schottky barrier capacitance, yielding the density of deep compensating centers in 6H-SiC in the low 10(15) cm(-3) range for both n-type and p-type material. The nearly equal density of deep donors and deep acceptors indicates that the centers responsible for the intrinsic compensation can be amphoteric. The EPR density of spins from free carbon vacancies is about 10(14) cm(-3). It is also hypothesized that impurity-vacancy complexes can be present in the undoped material and participate in compensation.