Recovery of the NBTI-Stressed Ultrathin Gate p-MOSFET: The Role of Deep-Level Hole Traps

Under a static negative-bias temperature stress, the negative threshold-voltage V_t shift (extracted from the dc current-voltage characteristic) of the direct-tunneling gate p-MOSFET is found to be substantially larger than that calculated based on the interface-state density measured using the charge-pumping method. Device-recovery characteristics from bipolar gate stress show that interface states alone cannot entirely account for the V_t shift, and indicate that a substantial number of positive oxide charges are also generated during stress. Stability of the increased V_t shift under a negative dc gate biasing and unipolar ac gate pulsing implies that these positive charges are deep-level hole traps with energy states above the Si conduction band edge. Because the defect states are outside the energy window of direct electron tunneling, their long relaxation time plays an important role in the slow recovery transient of the p-MOSFET