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

Abstract

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 V(write, 0) and V(write, 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.