The growth of AlGaN/GaN modulation-doped field-effect transistors (MODFETs) by ammonia-molecular-beam epitaxy on sapphire substrates is reported. C-doped GaN (2 mu m thick) was used as the insulating buffer layer in the device structures. The MODFET structure was completed by the subsequent growth of 2000 Angstrom of undoped GaN as the channel layer and 130 Angstrom of AlxGa1-xN(0.1 less than or equal to x less than or equal to 0.3) as the donor barrier layer. Sheet carrier densities of up to 2 x 10(13)cm(-2) with mobility of similar to 1000 cm(2)/Vs have been achieved even without doping the AlxGa1-xN barrier, indicating a large piezoelectric effect and excellent interface quality. The MODFET layers grown exhibited a unique surface morphology showing very Rat plateaus with rms roughness of 0.8 nm on the plateaus and rms roughness of 8 nm over a larger area. A 100-mu m-wide device with a 1 mu m gate length exhibited a maximum de current drive of 0.9 A/mm, a pe:lk transconductance of 160 mS/mm, a current gain cutoff frequency of 15.6 GHz. and a maximum oscillation frequency of 49.4 GHz. The high de and rf performance is attributed to the high two-dimensional electron mobility, high sheet charge density, and insulating property of the C-doped GaN buffer. [S0734-2101(00)05402-6].