We have performed a material study of 4H SiC PN diodes after they have been exposed to long term forward voltage operation. After this operation an increase in forward voltage drop was observed on some of the diodes. Using cathodeluminescence, photoluminescence lifetime mapping and synchrotron white beam X-ray topography we have found that structural defects are created in the epitaxial layers during the operation. The structural defects are interpreted as stacking faults in the;CH SiC basal plane, propagating through the entire n- base layer. The stacking faults, or defects associated with these, are acting as recombination centers reducing the recombination emission intensity and the carrier lifetime in the material.