Effect of variations and variation tolerance in logic circuits

Variations in process parameters affect the operation of integrated circuits (ICs) and pose a significant threat to the continued scaling of transistor dimensions. This fluctuation in device geometries might prevent them from meeting timing and power criteria and degrade the parametric yield. Process limitations are not exhibited as physical disparities only; transistors experience temporal device degradation as well. On top of it, power management techniques like voltage scaling, dual V _TH, further magnify the variation-induced reliability issues. On the other hand, conventional resiliency techniques like transistor upsizing and supply voltage boosting typically increase the power consumption. Low-power dissipation and process variation tolerance therefore impose contradictory design requirements. Such issues are expected to further worsen with technology scaling. To circumvent these non-idealities in process parameters, we describe two approaches: (1) variation-tolerant circuit designs and (2) circuits that can adapt themselves to operate correctly under the presence of such inconsistencies. In this chapter, we first analyze the effect of process variations and time-dependent degradation mechanisms on logic circuits. We consider both die-to-die and within-die variation effects. Next, we provide an overview of variation-tolerant logic design approaches. Interestingly, these resiliency techniques transcend several design abstraction levels - however in this chapter, we focus on circuit level techniques to perform reliable computations in an unreliable environment.