NBT STRESS AND RADIATION RELATED DEGRADATION AND UNDERLYING MECHANISMS IN POWER VDMOSFETS

In this paper we provide an overview of instabilities observed in commercial power VDMOSFETs subjected to irradiation, NBT stress, and to consecutive exposure to them. The results have indicated that irradiation of previously NBT stressed devices leads to additional threshold voltage shift, while NBT stress effects in previously irradiated devices depend on the gate bias applied during irradiation and on the total dose received. This points to the importance of the order of applied stresses, indicating that for proper insight into the prediction of device behaviour not only harsh conditions, but also the order of exposure have to be considered. It has also been shown that changes in the densities of oxide trapped charge and interface traps during spontaneous recovery after each of applied stresses can be significant, thus leading to additional instability, even though the threshold voltage seems to remain stable, pointing to the need for clarifying the responsible mechanisms .