Variability resilient low-power 7T-SRAM design for nano-scaled technologies

Touqeer Azam,  Binjie Cheng,  David R.S. Cumming
University of Glasgow


High variability in nano-scaled technologies can easily disturb the stability of a carefully designed standard 6T-SRAM cell, causing access failures during a read/write operation. We propose a 7T-SRAM cell to increase the read/write stability under large variations. The proposed design uses a low overhead read/write assist circuitry to increase the noise immunity. Use of an additional transistor and a floating ground allows read disturb free operation. While the write assist circuitry provides a floating ground during a write operation that weakens cell storage by turning off the supply voltage to ground path of the cross-coupled inverter pair. This allows a high speed/low power write operation. Monte Carlo simulations indicate a 200% increase in the read stability and a boost of 124% in write stability compared to a conventional 6T-SRAM design, when subjected to random dopant fluctuations, line edge roughness, and poly-granularity variations. HSPICE simulations of a 45nm 64x32 bit SRAM array designed using standard 6T and proposed 7T SRAM cells indicate a 31% improvement in write speed/write power, read power decreases by 60%, and a 44% reduction in the total average power consumption is achieved with the proposed design.