On measurements of the electrons and holes impact-ionization coefficients in 4$H$–SiC

All published results of measurements (at 300 K) of the impact ionization coefficients for electrons $\alpha_{n}$ and holes $\alpha_{p}$ in 4$H$–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 $\times$ 10$^{6}$ cm$^{-1}$, $E_{n}$ = 25.6 MV/cm, $a_{p}$ = 5.31 $\times$ 10$^{6}$ 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.