Ensemble Monte Carlo simulations have been carried out to investigate the effects of upper valleys on the characteristics of wurtzite GaN MESFETs. Electronic states within the conduction band valleys at the Γ1, U, M, Γ3 and K are represented by non-parabolic ellipsoidal valleys centred on important symmetry points of the Brillouin zone. The following scattering mechanisims, i.e, impurity, polar optical phonon, acoustic phonon, alloy and piezoelectric are inculded in the calculation. Ionized imurity scattering has been treated beyound the Born approximation using the phase-shift analysis. The simulation results show that on the drain side of the gate region, hot electrons attained enough energy to be scattered into the upper satellite conduction valleys. Approximately %20 of the electrons occupy the higher valleys (mainly U and M valley). The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible, and the predicted drain current and other electrical characteristics for the simulated device including upper valleys show much closer agreement with the available experimantal data.

Temperature and doping dependencies of electron mobility in InAs, AlAs and AlGaAs structures have been calculated using an ensemble Monte Carlo simulation. Electronic states within the conduction band valleys at the Γ, L and X are represented by non-parabolic ellipsoidal valleys centred on important symmetry points of the Brillouin zone. The simulation shows that intervalley electron transfer plays a dominant role in high electric fields leading to a strongly inverted electron distribution and to a large negative differential conductance. Our simulation results have also shown that the electron velocity in InAs and AlAs is less sensitive to temperature than in other III-V semiconductors like GaAs and AlGaAs. So InAs and AlAs devices are expected be more tolerant to self-heating and high ambient temperature device modeling. Our steady-state velocity-field characteristics are in fair agreement with other recent calculations.