Applicability of Fermi golden rule and possibility of low-field runaway   transport in nitrides
  In order to justify applicability of the standard approach of perturbation theory for the description of transport phenomena in wide-band polar semiconductors with strong electron-phonon interactions, we have compared dependences of energy losses to the lattice on the electron drift velocity obtained for different materials in the frameworks of (a) a perturbative approach based on calculation of the scattering rates from Fermi's golden rule and (b) a non-perturbative approach based on the path-integral formalism of Thornber and Feynman. Our results reveal that despite strong electron-phonon coupling in GaN and AlN such that intercollision times become of the order of the period of phonon oscillation, standard perturbative treatment can still be applied successfully for this type of material. Our findings also indicate possibility for unique long-distance runaway transport in nitrides which may occur at the pre-threshold electric fields. Polaron ground state energy and effective masses are calculated for GaN and AlN as well as for GaAs and Al_2 O_3. 
S. M. Komirenko, K. W. Kim, M. A. Stroscio and M. Dutta