Room-temperature Hall mobilities exceeding 900 cm2/V s are obtained for AlGaN/GaN heterostructures on (111) Si by single-temperature flow modulation organometallic vapor phase epitaxy. Thin pseudomorphic AlGaN top layers exhibit a 1.5 nm surface roughness and induces a two-dimensional electron gas sheet carrier concentration of 1.0×1013 cm−2. The GaN buffer layer has a background carrier concentration of 1.0×1015 cm−3, 130 arcsec x-ray diffraction full width at half maximum, and a low-temperature photoluminescence linewidth of 10 meV. An AlN nucleation layer provides static electrical isolation between the AlGaN/GaN and the conducting Si substrate. Large crack-free areas of high-crystalline-quality epitaxial material are obtained and have been successfully used for transistor fabrication.