We report on the measurements of the electron mobility in modulation-doped Al0.2Ga0.8N–GaN heterostructures grown on sapphire, conducting 6H–SiC, and insulating 4H–SiC substrates as a function of the sheet electron density, ns, at the heterointerface in a wide temperature range. The mobility increases with an increase in ns up to approximately 1×1013 cm−2 and decreases with a further increase in ns. This is explained by the electron spillover at high values of ns from the two-dimensional states at the AlGaN/GaN heterointerface into the delocalized states in the doped GaN channel. The maximum electron Hall mobility in excess of 2000 cm2/V s at room temperature and 11 000 cm2/V s at 4.2 K was measured in the heterostructures grown on 6H–SiC at the values of ns close to 1013 cm−2 and 7×1012 cm−2, respectively. © 1999 American Institute of Physics.