Theory of high field transport and impact ionization in III-Nitride semiconductors

High field electron and hole transport in wurtzite phase GaN is studied using an ensemble Monte Carlo method. The model includes the details of the full band structure derived from nonlocal empirical pseu-dopotential calculations. The nonpolar carrier-phonon interaction is treated within the framework of the rigid pseudoion approximation using ab initio techniques to determine the phonon dispersion relation. The impact ionization transition rate is computed based on the calculated electronic structure and the corresponding wave-vector dependent dielectric function. The complex band structure of wurtzite GaN requires the inclusion of band-to-band tunnelling effects that are critical at high electric fields. The electric-field-induced interband transitions are investigated by the direct solution of the time-dependent multiband Schroedinger equation. The multiband description of the transport predicts a considerable increase of the impact ionization coefficients compared to the case in which tunnelling is not considered.