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 alpha (n) and holes alpha (p) in 4H-SiC are analyzed. It is shown that the most plausible approximations of dependences of alpha (n, p) on electric-field strength E have the usual form alpha (n, p) = a (n, p) exp(-E (n, p) /E) at fitting-parameter values of a (n) = 38.6 x 106 cm(-1), E (n) = 25.6 MV/cm, a (p) = 5.31 x 106 cm(-1), and E (p) = 13.1 MV/cm. These dependences alpha (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 alpha (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.