Mesoscopic hydro-thermodynamics of phonons in semiconductors: heat transport in III-nitrides

It is presented a generalized hydro-thermodynamics (called mesoscopic hydro-thermodynamics MHT) of phonons in semiconductors, driven away from equilibrium by external forces, derived by the method of moments from a generalized Peierls-Boltzmann kinetic equation built in the framework of a non-equilibrium statistical ensemble formalism. The resulting MHT involves the enormous set of coupled evolution equations for the densities of the quasi-particles (phonons) and their energy together with their fluxes of all orders. The handling of them requires the introduction of a contraction of description what defines MHT’s of different orders. We illustrate the matter analyzing heat transport by phonons in GaN within the framework of a MHT of first order to obtain a generalized Guyer-Krumhansl equation from which it is analyzed the effect of geometry on the heat transport. It is described the influence of size (from bulk to nanometric scales) on the reduction of the thermal conductivity and the improving of the figure of merit of thermoelectric devices.