High-quality GaN/AlGaN high-electron-mobility transistors (HEMT) characterized by room temperature mobilities of similar to 1000 cm(2) V-1 s(-1) and sheet electron densities in the range of 3 x 10(12)-2 x 10(13) cm(-2) have been grown by reactive molecular-beam epitaxy on insulating C-doped GaN template layers. Growth data and mobility values resulting from over 50 HEMT growth experiments on 2 in. diameter sapphire wafers are presented to show the remarkable overall high yield and reproducibility of the HEMT structures grown by this method. The use of insulating C-doped GaN buffer layers has greatly increased reproducibility of the device structures by ensuring device isolation through controlled carbon doping. Moreover, an undoped GaN channel layer of remarkably low defect density and high mobility can be grown on the C-doped GaN template with high reproducibility. Precise control of the growth temperature was key to achieving the high quality and reproducibility of the structures.