The velocity-held characteristics in zinc-blende GaN are calculated fr
om the Boltzmann equation, using realistic energy bands taken from ab
initio theory. The drift velocity and the high-field negative differen
tial resistance are shown to be largely determined by the inflection p
oint in the bands centered around the Gamma valley, instead of the usu
al intervalley scattering. We analyze the relative importance of these
competing mechanisms for GaN and Al0.5Ga0.5N. The importance of this
anomaly to device properties is also discussed. (C) 1997 American Inst
itute of Physics.