  Unusually large spontaneous and piezoelectric fields in the III-V nitrides
have led to the making of an entirely new class of two-dimensional electron
gas. Fluctuation from a perfectly periodic binary structure in highly polar
semiconductor alloys present the same physical situation as a random
distribution of microscopic dipoles. The excess dipole distribution in the
barrier layers is evaluated by a method similar to the virtual crystal
approximation. It is shown that the mobility of electrons in the
two-dimensional electron gas formed in highly polar heterostructures is
intrinsically limited by scattering from such dipoles.
