TABLE OF CONTENTS We have analyzed the steady-state, transient time-dependent and spatially-dependent electron transport in the group III-nitride materials at high and ultra-high electric fields. Particularly, various transport regimes were investigated under the high electron concentration and the low concentration conditions.
When the electron concentration is high, the transport is characterized by cooperative dynamics of the electrons. Specifically, their distribution functions assume the shifted Maxwellian form with two parameters, the electron temperature and the drift velocity. These parameters should be found from the momentum and energy budgets common for all electrons. We have derived time-dependent differential equations for these parameters assuming that the optical phonon scattering is the dominant relaxation mechanism. Considering the importance of energy band structure for high-field transport, we performed a general investigation of two models for the conduction band, the single-valley model and multi-valley model, both of which take into account the nonparabolicity effects.
For the one-valley model, we found that the nonparabolicity parameter ? plays a crucial role. At v=0, two stationary solutions exist at the electric fields less than the threshold value ? th . One of these solutions corresponds to a stable steady-state regime, while the other is unstable. At the fields larger than ? th , no stationary solutions exist at all. Then, at a certain interval of the nonparabolicity parameters there are three solutions. One of the solutions is stable in a finite field interval restricted by the value close to ? th ,
