  The analysis of all published results on electrons and holes impact
ionization coefficients in 4H-SiC at 300K have been performed. It is shown that
the most plausible approximation of dependencies $\alpha_{n,p}$ on the electric
field strength $E$ have a conventional kind
$\alpha_{n,p}=a_{n,p}\exp(-E_{n,p}/E)$ with the values of the fitting
parameters $a_n=38.6\cdot 10^6$ 1/cm, $E_n=25.6$ MV/cm, $a_p=5.31\cdot 10^6$
1/cm, $E_p=13.1$ MV/cm. These dependencies $\alpha_{n,p}(E)$ have been used to
calculate the maximum field strength $E_b$ and the thickness of the space
charge region $w_b$ at the breakdown voltage $U_b$. A set of new formulas are
presented for calculation of $\alpha_{n,p}(E)$ from results of avalanche
multiplication coefficients and excess noise factor measurements at one-side
lighting of photodiodes with stepped doping.
