Applicability of the Fermi golden rule and the possibility of low-field runaway transport in nitrides

We investigated electron transport characteristics of wide-band polar semic onductors with intermediate strength of the electron-phonon interaction. El ectron energy loss to the lattice was calculated as a function of electron velocity for various materials in the frameworks of (a) a perturbative appr oach based on the calculation of 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 suggest that the standard perturbative t reatment can be applied to GaN and AIN despite the relatively strong electr on-phonon coupling in this material system, with intercollision times of th e order of the period of the phonon oscillation. Our findings also indicate the possibility for unique long-distance runaway transport in nitrides whi ch may occur at the pre-threshold electric fields. The polaron ground-state energy and effective masses are calculated for GaN and AIN as well as for GaAs and Al2O3. An expression for the Frohlich coupling constant for wurtzi tes is derived.