The next generation of wireless infrastructure will rely heavily upon wide band gap semiconductors owing to their unique materials properties, including: their large bandgap, high thermal conductivity, and high breakdown field. To facilitate implementation of this next generation, a significant effort is required to make SiC MESFET and GaN HEMT microwave devices more suitable for widespread application. Currently, the interest in high-purity semi-insulating (HPSI) 4H-SiC is critically tied to its influence on microwave devices, whether performance or affordability. To address these issues, we have developed high-purity 3-inch and 100 mm 4H-SiC substrates with low micropipe densities (as low as 1.4 cm(-2) in 3-inch and <60 cm(-2) in 100 mm) and uniform semi-insulating properties (>10(9) Omegacm) over the full wafer diameter. These wafers possess typical residual shallow level contamination less than 1x10(16) cm(-1) (5x10(15) nitrogen and 3x10(15) boron) with best nitrogen values of 3x10(14). In this paper, we will report on the development of our HPSI growth process focusing on the specific areas of the assessment of semi-insulating character and device applicability.