We present quantitative calculations of the electron drift mobility in wurtzite (WZ) and zincblende (ZB) structuren-type AlGaN/GaN modulation-doped field-effect transistors. The two-dimensional character of the quantum confined carriers as well as spontaneous and piezoelectric electric field effects are fully taken into account. For given doping concentration, we find that the internal electric fields lead to a much stronger carrier confinement and higher channel densities than in standard III–V materials. For high qualityn-type heterostructures, we predict a room temperature mobility at high densities close to2000 cm2/V s.©1998 American Institute of Physics.