In this paper we show the progression in the development of semi-insulating SiC grown by the sublimation technique from extrinsically doped material to high purity semi-insulating (HPSI) 4H-SiC bulk crystals of 2-inch and 3-inch diameter without resorting to the intentional introduction of elemental deep level dopants, such as vanadium. Secondary ion mass spectrometry, optical absorption, deep level transient spectroscopy and electron paramagnetic resonance data suggest that the semi-insulating behavior in HPSI material originates from deep levels associated with intrinsic point defects. While high temperature resistivity measurements on different high purity 4H-SiC samples indicate activation energies ranging from 0.9 to 1.6 eV, HPSI wafers with homogeneous activation energies near mid-gap are demonstrated. The room-temperature thermal conductivity of this material approaches the theoretical maximum of similar to5 W/cmK. Additionally, HPSI substrates exhibit micropipe densities as low as 8 cm(-2) over the full diameter of a 3-inch wafer. MESFETs produced on HPSI wafers are free of backgating effects and have resulted in the best combination of power density and efficiency reported to date for SiC MESFETs of 5.2 W/mm and 63% power added efficiency (PAE) at 3.5 GHz.