A Low Vth SRAM Reducing Mismatch of Cell-Stability with an Elevated Cell Biasing Scheme

Abstract

A lower-threshold-voltage (LVth) SRAM cell with an elevated cell biasing scheme, which enables to reduce the random threshold-voltage (Vth) variation and to alleviate the stability-degradation caused by word-line (WL) and cell power line (VDDM) disturbed accesses in row and column directions, has been proposed. The random Vth variation (σVth) is suppressed by the proposed LVth cell. As a result, the LVth cell reduces the variation of static noise margin (SNM) for the data retention, which enables to maintain a higher SNM over a larger memory size, compared with a conventionally being used higher Vth (HVth) cell. An elevated cell biasing scheme cancels the substantial trade-off relationship between SNM and the write margin (WRTM) in an SRAM cell. Obtained simulation results with a 45-㎚ CMOS technology model demonstrate that the proposed techniques allow sufficient stability margins to be maintained up to 6σ level with a 0.5-V data retention voltage and a 0.7-V logic bias voltage.