Abstract
Monte Carlo simulations of high-field transport are used to calculate the spectral density of velocity fluctuations and the diffusion coefficients of hot electrons in GaN and InN. The results of the simulations are complemented by the investigation of the velocity relaxation time through the balance equation method. It is found that the low-frequency noise spectrum remains white up to 100-300 GHz, so that the steady-state diffusion coefficient can be considered as constant in practically the whole millimetre wavelength range. It is shown that the main features of the field dependence of the longitudinal and transverse diffusion coefficients such as initial decrease of the diffusion coefficient followed by a peak near the threshold field for the intervalley transfer, etc are described well both qualitatively and quantitatively by simplified approaches based on velocity relaxation times.