An Analytical Model for Negative Bias Temperature Instability

Negative bias temperature instability (NBTI) in PMOS transistors has become a significant reliability concern in present day digital circuit design. With continued scaling, the effect of NBTI has rapidly grown in prominence, forcing designers to resort to a pessimistic design style using guard-banding. Since NBTI is strongly dependent on the time for which the PMOS device is stressed, different gates in a combinational circuit experience varying extents of delay degradation. This has necessitated a mechanism of quantizing the gate-delay degradation, to pave the way for improved design strategies. Our work addresses this issue by providing a procedure for determining the amount of delay degradation of a circuit due to NBTI. An analytical model for NBTI is derived using the framework of the reaction-diffusion model, and a mathematical proof for the widely observed phenomenon of frequency independence is provided. Simulations on ISCAS benchmarks under a 70nm technology show that NBTI causes a delay degradation of about 8% in combinational logic based circuits after 10 years (ap 3 times 108s)