Semi-insulating SiC is presently the substrate of choice for high power AlGaN/GaN HEMTS due to its good thermal conductivity, large diameter and near lattice-match to GaN. During the past several years, II-VI has been developing and improving semi-insulating 6H-SiC substrate manufacturing technology through a joint Air Force Research Laboratory (AFRL) / Missile Defense Agency (MDA) funded initiative. Presently, large-diameter 6H SiC single crystals at are grown using an Advanced Physical Vapor Transport (PVT) sublimation process. During this effort, substrate diameter was scaled from 75mm to greater than 100mm, while at the same time dramatically improving their crystalline quality and manufacturability. X-ray rocking curve analysis of polished 100mm 6H wafers showed edge-to-edge lattice curvature (ΔΩ) ~ 0.02° (Fig.1) and FWHM of between 14 and 42 arc-seconds (Fig. 2). Typical micropipe densities of 100mm 6H material now approach 1 cm-2 or below, while dislocation densities of < 104 cm-2 have been demonstrated. Stable semi-insulating properties are readily achieved by compensation with vanadium, which results in highly uniform boule resistivity, on the order of 1011 Ohm-cm (Fig. 3,4). Productivity increased by 2.5x (Fig. 5) and total square inches of commercially shipped substrates increased by a factor of > 3 (Fig.6). Manufacturing results will be presented, including defects and yield improvements, material quality, cycle time reduction and process throughput.