To obtain semi-insulating SiC by vanadium and boron co-doping during PVT growth, a detailed understanding of the dopant (B,V) incorporation is required. Crystal growth of 1.4 " 6H-SiC on either Si or C face, doped with boron or vanadium, respectively, was performed. For reference purposes, also nominally undoped SiC crystals were grown. It is shown that in nominally undoped crystals nitrogen remains residual impurity acting as a finite source during growth. An exponential decrease of charge carrier concentration in growth direction from 2 x 10(17) to 8 x 10(15) cm(-3) was observed. In boron doped crystals, charge carrier concentration increases with growth time because of nitrogen decrease and boron loss from the powder charge due to evaporation. However, lateral inhomogeneity throughout a wafer is as low as Deltap/p = 15%. Factors influencing the transfer coefficient of boron are discussed. Semi-insulating behavior was achieved by vanadium doping as shown with electron spin resonance and optical absorption. V content of the source as well as V incorporation in the crystal decrease with growth time. If the solubility limit of V in SiC is exceeded, defects consisting of vanadium-rich precipitates and planar voids occur. The vanadium transfer coefficient depends on the partial pressure of V species and measures about 0.001.