High quality GaN epitaxial layers were grown on 6H–SiC substrates by using low-pressure metalorganic chemical vapor deposition method. Samples employing a three-periodGaN/Al0.08Ga0.92N(100 Å/100 Å)as a buffer layer produce a good quality GaN epitaxial layer, with mobility and carrier concentration of612 cm2/V⋅sand1.3×1017 cm−3(at 300 K), respectively. The enhanced electron mobility in theAl0.08Ga0.92N/GaNheterostructures is also observed. By using the van der Pauw method of Hall measurement, the sheet carrier density and mobility at 4.2 K for theAl0.08Ga0.92N/GaNheterostructure are5.8×1012 cm−2and5300 cm2/V⋅s,respectively. Strong SdH (Shubnikov–de Haas) oscillations were observed to confirm the two-dimensional electron gas (2DEG) phenomenon at the AlGaN/GaN top heterointerface. In addition, an extra SdH oscillation also resulted from the high-quality 2DEG channel of the GaN/AlGaN bottom heterointerface. ©1997 American Institute of Physics.