Hot phonon-plasmon modes in GaN

Hot phonon effects are of interest for heterostructure field effect transistors where heat dissipation depends on the lifetime of the longitudinal optical mode. Previous treatments have ignored the coupling of plasmon and phonons. Here, a bulk-phonon model is used to examine the effects of hot phonon-plasmon coupled modes based on the random-phase Lindhard dielectric function with nondegenerate statistics and a nonparabolic conduction band. Comparison of the results concerning power dissipation and drift velocity with those for bare phonons assuming a constant phonon lifetime indicate no striking differences. The phonon lifetime is known to exhibit a temperature and electron density dependence in GaN. Incorporating the observed dependence of lifetime on electron density profoundly affects the hot phonon effect. We find that the drift velocity, as expected, becomes limited by the production of hot phonons at high electric fields as the electron density is increased but this reduction in velocity is ameliorated by the concomitant reduction in the phonon lifetime with increasing density. The result is that hot phonon effects become insensitive to electron density at high fields.