Substrate temperature uniformity on an Applied EPI 4 in. GEN-III single-wafer reactor has been improved with the use of a two-zone substrate heater, Varying the outer filament temperature can effectively change the temperature profile across wafers. The best uniformity in sheet resistance, source-to-drain saturation current in pseudomorphic high electron mobility transistor, photoluminescence peak wavelength and intensity for growth on 4 in. wafers was obtained by setting the inner and outer filaments at the same temperature. The source-to-drain saturation current and photoluminescence intensity uniformity for 4 in. pseudomorphic high electron mobility transistor grown on the GEN-III reactor compared very favorably with those grown on multiwafer production reactors. A total wavelength variation of less than 2 nm across 4 in. AlGaAs calibration wafers grown at 680 degreesC has been achieved using the dual-zone heater. Improved uniformity was also demonstrated with lattice-matched growth on 4 in. InP substrates.