On the efficacy of write-assist techniques in low voltage nanoscale SRAMs

Read and write assist techniques are now commonly used to lower the minimum operating voltage (V_min) of an SRAM. In this paper, we review the efficacy of four leading write-assist (WA) techniques and their behavior at lower supply voltages in commercial SRAMs from 65nm, 45nm and 32nm low power technology nodes. In particular, the word-line boosting and negative bit-line WA techniques seem most promising at lower voltages. These two techniques help reduce the value of WL_crit by a factor of ~2.5X at 0.7V and also decrease the 3?? spread by ~3.3X, thus significantly reducing the impact of process variations. These write-assist techniques also impact the dynamic read noise margin (DRNM) of half-selected cells during the write operation. The negative bit-line WA technique has virtually no impact on the DRNM but all other WA techniques degrade the DRNM by 10-15%. In conjunction with the benefit (decrease in WL_crit) and the negative impact (decrease in DRNM), overhead of implementation in terms of area and performance must be analyzed to choose the best write-assist technique for lowering the SRAM V_min.