A Monte Carlo study of the high-field electron transport in 4H–SiC is presented using a new analytic band model. The band model consists of two analytical bands that include band bending at the Brillouin zone boundaries. The band bending is very important in 4H–SiC and 6H–SiC and has to be taken into account in order to have an accurate model at high electric fields. Numerical calculation of the density of states given by the new model has been used in order to model the energy dependence of the scattering mechanisms accurately. The new model predicts a lower saturation velocity in the c direction (peak velocity 1.8×107 cm/s) than in perpendicular directions (peak velocity 2.1×107 cm/s). This is directly related to the strong band bending in the c direction. This effect is also responsible for a much more pronounced velocity peak in the c direction compared to perpendicular directions. In the low-field region the mobility is higher in the c direction (mobility ratio near 0.8), which is in agreement with experimental results.

A study of electron transport in 6H-SiC is presented using a full band Monte Carlo simulation model. The Monte Carlo model uses four conduction bands obtained from a full potential band structure calculation based on the local density approximation to the density functional theory. Electron–phonon coupling constants are deduced by fitting the Monte Carlo simulation results to available experimental data for the mobility as a function of temperature. The saturation velocity perpendicular to the c axis is found to be near 2.0×107 cm/s, which is in good agreement with the experimental data available. In the c-axis direction the saturation velocity is much lower (4.5×106 cm/s). There are no direct experimental results available for the saturation velocity in the c-axis direction. A comparison between two-dimensional simulations of a 6H-SiC permeable base transistor, using transport parameters obtained from the Monte Carlo simulations, and experimental I–V characteristics confirms the low value. The physical mechanism behind this result can be explained in terms of the small group velocity in the c-axis direction for reasonable energy levels in combination with band structure effects that limits the energy range that an electron can reach by drift. This effect reduces the mean energy of the carriers for an electric field applied along the c axis and at 1.0 MV/cm the difference in mean energy compared with perpendicular directions is almost one order of magnitude. The mean energy increases with increasing temperature for electric fields in the c-axis direction, while the situation is reversed in perpendicular directions. In general the impact ionization coefficient has the same temperature dependence as the mean energy and this indicates that the impact ionization coefficient for electrons has a positive temperature derivative along the c axis. This may be a serious drawback in the design of high power vertical metal–semiconductor field effect transistors.

The time evolution of Bloch electrons (holes) moving in a constant electric field has been studied for GaN and 2H-SiC using a numerical model based on realistic band structures. The large band gap of GaN and the SiC polytypes provide much larger critical fields than in conventional semiconductors, which allows device operation at very high electric fields. At sufficiently high electric fields the carriers may change band during drift due to tunneling. GaN has a direct band gap, while the hexagonal SiC polytypes have indirect band gaps. In spite of this difference the valence band structure is very similar due to the wurtzite symmetry. In this work the GaN and the 2H-SiC polytype are considered as wurtzite prototype semiconductors in order to study valence band to band tunneling in wurtzite semiconductors for electric fields directed along the c axis. A large valence band to band tunneling probability was found for both materials at electric fields above 400 kV/cm. This shows the importance of considering band to band tunneling in studies of high field hole transport in wide band-gap hexagonal semiconductor materials. The proposed numerical approach can be used to enhance the interband tunneling models used in Monte Carlo simulation of carrier transport in hexagonal semiconductors.

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