Effect of anisotropic material properties on the forward voltage drop in 6H- and 4H-SiC power diode structures

Abstract

It is demonstrated by numerical simulation that the anisotropic material properties of 6H-SiC can have an important effect on the forward voltage drop of 10 kV 6H-SiC power diodes. A pronounced difference in the carrier distribution was seen for substrates with the surface normal parallel or orthogonal to the c-axis. In fact, it is found that current conduction along the low-mobility c-axis in 6H-SiC (i.e. when the normal to the substrate surface is parallel to the c-axis) can lead to a lower forward voltage drop than if the main current conduction is in the high-mobility direction (orthogonal to the c-axis). Depending on the device geometry the c-axis orientation can be used in the optimization of the forward voltage drop. The effect of anisotropy is expected to be of importance also for on-state losses in other bipolar 6H-SiC devices (e.g. IGBTs). For the design of 4H-SiC bipolar power devices the effect of anisotropic conduction properties is not likely to be critical.