The Study of Abnormal Degradation in High-Voltage P-Type Mosfets with N+ Polysilicon Gate during AC Stress

We investigated the degradation characteristics of High-voltage (HV) P-type Metal-oxide-semiconductor field effect transistors (P-MOSFETs) during negatively AC stress. The threshold voltage is shifted by degradation during the highly-negative gate bias, and the performance of transistors is degraded. In HV P-MOSFET with N+ polysilicon gate, there are two kinds of degradations during AC stress. One is the early degradation by Negative Bias Temperature Instability (NBTI), and the other is abnormal and drastic degradation after NBTI. To understand this abnormal degradation, we measured two samples with different gate materials such as N+ and P+ polycrystalline silicon and compared their degradation chracteristics. Thus we noticed that the abnormal degradation occurs only with N+ polysilicon gate in spite of lower oxide E-field. In this work, we studied the chracteristics of abnormal degradation and discussed a suitable mechanism of this phenomenon.