Comparison of 4T and 6T FinFET SRAM Cells for Subthreshold Operation Considering Variability—A Model-Based Approach.

This paper investigates the cell stability of recently introduced four-transistor (4T) and conventional six-transistor (6T) fin-shaped field-effect transistor static random access memory (SRAM) cells operating in a subthreshold region using an efficient model-based approach to consider the impact of device variations. Compared with the 6T cell, this paper indicates that 4T SRAM cells exhibit a better nominal read static noise margin (RSNM) because of the reduced read disturb. For 4T cells, the nearly ideal values of Vwrite, 0 and Vwrite, 1 guarantee the positive nominal write static noise margin (WSNM) for selected cells. For half-selected cells on the selected bit line, a sufficient margin is observed between write time (for selected cells) and write disturb (for half-selected cells). Using the established model-based approach, the variability of subthreshold 6T and 4T SRAM cells is assessed with 1000 samples. Our results indicate that the 4T driverless cell with a larger \mu\RSNM and a slightly worse \sigma\RSNM shows a comparable \mu/\sigma ratio in RSNM with the 6T cell. Furthermore, for a given cell area, 4T SRAM cells using relaxed device dimensions with reduced \sigma\RSNM can outperform the 6T cell. For write operation, 4T SRAM cells exhibit a superior WSNM, whereas the design margin between write time and write disturb needs to be carefully examined to ensure an adequate margin considering device variability. [ABSTRACT FROM PUBLISHER]