Semi-insulating SiC crystals were grown by sublimation Physical Vapor Transport technique and semi-insulating 6H-SiC substrate wafers up to 2 " in diameter were fabricated. Resistivity measurement and high temperature Hall measurement results showed that a large segment of a SiC boule grown in this process was semi-insulating. To evaluate these SI SiC substrates, GaN thin films and device structures were grown on the semi-insulating SiC substrates and device characterization results are presented.