SRAM bitcell design for low voltage operation in deep submicron technologies

As technology scales down, an increasing number of transistors can be integrated into a single chip but process variation becomes more serious. SRAM is one of the key components in a SoC and it occupies a large portion of the SoC. Thus, the SRAM bitcell is typically designed using very small transistors for high integration, which limits the minimum operating voltage (V_CCmin) of the SoC because of the large threshold voltage (V_th) mismatch between paired transistors caused by small feature size. As process technology scales down to sub-32nm technology, the 6T SRAM bitcell that is currently used may not achieve proper stability, write-ability, and read-ability at the required operating voltage. In this paper, several approaches are investigated to resolve the issue, such as upsized 6T SRAM bitcell, 8T SRAM bitcell, read- and write-preferred bitcells, and read- and write-assist circuits. HSPICE simulations are performed using PTM 32nm model parameters.