Since power devices require an electrically active, thick n-type silicon layer with high resistivity and a large area, their electrical characteristics are extremely sensitive to contamination. If transition metals diffuse into the wafers during the high temperature steps required for device fabrication, an uncontrolled increase in leakage Current and on state voltage can be observed. Further more, current filamentation and instabilities of the electrical data can occur. As a consequence of the low doping level of the n-base, the blocking voltage and the failure rate due to cosmic radiation are sensitive to contaminating atoms acting as donors or accepters. To obtain information about the sources and the extent of the contamination, a contamination check was performed after each high temperature step. Moreover, the dependence of the carrier lifetime on temperature and injection level was analyzed for typical operation conditions of power devices. The results proved to be important in finding ways of keeping contamination and silicon defect densities as low as possible and to ensure that good electrical data with adequate stability could be obtained.