Negative Bias Temperature Instability (NBTI) is a major reliability issue for p-channel MOSFETs and is observed when the transistor is stressed with negative gate voltages at elevated temperatures. This stress creates interface and oxide traps at the Si/SiO2 interface which degrades the overall device performance.
The main impact on device performance due to NBTI stress is the increase in the threshold voltage, the degradation of mobility, drain current and transconductance. The degradation of drain current (ON-current) affects the performance of both analog and digital circuits. In digital circuits, MOSFETs mainly work as switches and the degradation of ON-current causes an increase in the delay time and lowers the switching speed. In analog circuits, most analog operations need matched device pairs and the mismatch in device performance from NBTI stress results in the circuit failure.
In order to capture the degradation of device performance in circuits, it is essential to have physics based compact models of NBTI (see figure). Only simple models are available and these fail to reproduce the dynamics of the degradation, particularly the recovery behavior. A detailed analysis from the sub-threshold region to the linear region of operation has been done to find the parameters that are mainly responsible for the change in threshold voltage. The degradation of mobility due to NBTI stress in this region is mainly due to coulomb scattering. The contribution of mobility to the degradation of drain current is less than 10% of the total threshold voltage shift. The main contribution of threshold voltage shift is from the change in occupancy at the interface due to trapped charges. The sub-threshold swing parameter, which is a function of the interface/oxide capacitance, is responsible for the change in the sub-threshold slope of the device with the application of NBTI stress.