Hot electron energy relaxation in gallium nitride

We have used pulsed electrical transport measurements to investigate the en ergy relaxation by electrically heated electrons in a series of GaN epilaye rs. We have determined the energy loss rates per electron in the electron t emperature range 1.5 to 375 K. For electron temperatures above about 140 K we find that optic phonon emission is the dominant energy relaxation proces s. We obtain values for the longitudinal optic (LO) phonon energy and scatt ering time (tau) of 92 meV (+/- 1 meV) and 0.53 fs, respectively. The value for tau is consistent with numerical estimates of the Frohlich coupling co nstant in GaN. In a number of samples we also observe a peak in device resi stance at an electric field of around 300 kV m(-1) At this field the carrie r drift velocity is approximately equal to the velocity of sound (approxima te to 6000 ms(-1)) and Re suggest that the resistance increase is due to in creased phonon scattering associated with 'Cerenkov' phonon emission.