  We show that density-dependent velocity saturation in a GaN High Electron
Mobility Transistor (HEMT) can be related to the stimulated emission of
longitudinal optical (LO) phonons. As the drift velocity of electrons
increases, the drift of the Fermi distribution in reciprocal space produces
population inversion and gain for the LO phonons. Once this gain reaches a
threshold value, the avalanche-like increase of LO emission causes a rapid loss
of electron energy and momentum and leads to drift velocity saturation. Our
simple model correctly predicts both the general trend of the saturation
velocity decreasing with increasing electron density and the values of
saturation velocity measured in our experiments.
