Mitigating the Impact of NBTI and PBTI Degradation

Abstract

Modern CMOS devices encounter a major problem that alters the threshold voltages of the NMOS and PMOS. Negative Bias Temperature Instability (NBTI) and Positive Bias Temperature Instability (PBTI) are common ageing phenomena observed in PMOS and NMOS devices, respectively. Due to operating temperature and stress time, NBTI and PBTI create a decrease in drain-to-source current and an increase in propagation delay. Threshold voltage is an important parameter due to exponential dependence on delay and leakage power. Threshold voltage variations produce adverse effects on operation frequency. These phenomena accrue in pull-up/pull-down transistors in stack and vastly degrade CMOS performance. This paper discusses the various factors responsible for NBTI and PBTI and the challenges associated with modeling these effects due to the recovery mechanism exhibited by the transistors when the stress is removed. Therefore, we propose an algorithm to reduce the effects of NBTI and PBTI that will reduce the stress time for each transistor through the relative repositioning of the transistors based on the signal probability.