Impact of NBTI and PBTI in SRAM bit-cells: Relative sensitivities and guidelines for application-specific target stability/performance

Abstract

The stability and performance characteristics of Static Random Access Memories (SRAMs) are known to degrade with time due to the impact of Negative and Positive Bias Temperature Instabilities (NBTI (in PFET) and PBTI (in NFET)). In this work, we provide insights into relative sensitivities of these phenomena on speed and stability of SRAM cells. Relative impact on access time, stability, and tolerability of one phenomenon over another has been studied for different application specific (high-performance or low-power) SRAM cells. We show that high-performance SRAM cells should have lower VT drift due to PBTI compared with dense cells to contain READ stability and access time. Further, worst-case static stress poses tighter process constraints compared with alternating stress.