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Many components of variability become larger percentage design factors with decreasing feature size. Hence, the small transistors in SRAM cells are particularly sensitive to these variations. The SRAM cell transistors in sub-100-nm designs may contain fewer than 100 channel dopant atoms. To achieve a robust design with such variability, one must enhance the normal static-noise-margin and write-trip-point analysis, often with Monte Carlo simulations using statistical transistor models including the process and mismatch fluctuations. Similar challenges exist for the sense amplifiers normally used with SRAM arrays. Except with very low speed designs, yield to speed can be substantially reduced by variations between nominally matched sense amplifier transistors as well as by the variability resulting in a very worst memory cell low read current. This also increases the hazards of delay timing with dummy paths and dummy cells and increases the need for at-speed testing prior to repair.
Date of Conference: 7-11 Nov. 2004