On Measurements of the Electrons and Holes Impact-Ionization Coefficients in 4H–SiC

All published results of measurements (at 300 K) of the impact ionization coefficients for electrons α n and holes α p in 4H–SiC are analyzed. It is shown that the most plausible approximations of dependences of α n, p on electric-field strength E have the usual form α n, p = a n, p exp(–E n, p /E) at fitting-parameter values of a n = 38.6 × 106 cm–1, E n = 25.6 MV/cm, a p = 5.31 × 106 cm–1, and E p = 13.1 MV/cm. These dependences α n, p (E) are used to calculate the highest field strength E b and thickness w b of the space-charge region at the breakdown voltage U b . A number of new formulas for calculating α n, p (E) are obtained from the results of measuring the avalanche-multiplication coefficients and the excess-noise factors under the single-sided illumination of photodiodes with stepped doping.