NBTI/PBTI-aware wordline voltage control with no boosted supply for stability improvement of half-selected SRAM cells

Abstract

Negative Bias Temperature Instability (NBTI) and Positive Bias Temperature Instability (PBTI) are the two important reliability issues that degrade the SRAM cell stability over time. In this paper, we analyze the impact of NBTI and PBTI on 8T SRAM cells, and propose a stability improvement technique without using boosted supply voltage. In decoupled SRAM cells, the cell stability is limited by the stability of the half-selected cells, whose stability is significantly affected by NBTI and PBTI. The proposed technique lowers the WWL voltage to reduce the amount of disturbance and compensate the degraded cell stability. Lowering the WWL voltage doesn't affect the write margin substantially since the write margin is improved with NBTI and PBTI and the lowered WWL will produce a write margin similar to the original one.