In this chapter is has been shown that the recovery of a multi-layer high-k pMOSFET structure after NBTI stress can be well modeled on the basis of a refined multi-state defect model, in which the effective rates of the model over two metastable states determine the temporal occupancy of a single defect. Since the area of the devices investigated here is large, also a large number of oxide defects having different defect quanitities like energies, barriers inbetween, and positions inside the oxide are assumed. Due to the complexity of the device also quantum mechanical effects like subbands are incorporated, which is necessary to explain the heavier stress conditions. When the effective rates of all the defects for all the contributing subbands at certain times are summed up, the defect band occupation can be calculated giving the overall degradation. This is done for measurement data covering a large range of accelerated stress conditions yielding excellent agreement.