Measurements of impact ionization coefficients of electrons and holes in 4H-SiC and their application to device simulation

This article is intended to review measurements and modeling of anisotropic impact ionization coefficients of electrons and holes in 4H-SiC. Owing to the hexagonal crystal structure, the carrier transport in 4H-SiC shows anisotropy, such as anisotropic mobility and anisotropic impact ionization coefficients. The anisotropy of impact ionization coefficients brings about the difference in the breakdown voltage of p(+)n diodes on Si-face and A-face. Experiments showed that the breakdown voltage of a p(+)n diode on A-face is 62 percent of that on Si-face. This difference in breakdown voltage corresponds to 81 percent of anisotropy in breakdown fields. The impact ionization coefficients parallel and perpendicular to c-axis were extracted from measurements of the breakdown voltage and the multiplication factor of holes in high electric fields. The analytical model of the anisotropic impact ionization coefficients, which interpolates impact ionization coefficients parallel and perpendicular to c-axis, was derived, considering the energy transport in the avalanche phenomena. The effects of anisotropic impact ionization coefficients on the avalanche breakdown in 4H-SiC power devices are examined. We show that the avalanche breakdown voltage of a power device is affected by the anisotropy of impact ionization coefficients, because avalanche breakdown occurs at the field crowding point at peripheral edge of power device, where the lateral field dominates. Finally, we show that the optimized two-zone junction termination structure suppresses the degradation of the breakdown voltage caused by anisotropy of the impact ionization coefficients. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim