Full band Monte Carlo simulation of electron transport in 6H-SiC

Abstract

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.