Variation-tolerant ultra low-power heterojunction tunnel FET SRAM design

Steep sub-threshold Interband Tunnel FETs (TFETs) are promising candidates for low supply voltage applications with higher switching performance than traditional CMOS. Unlike CMOS, TFETs exhibit uni-directional conduction due to their asymmetric source-drain architecture, and delayed output saturation characteristics. These unconventional characteristics of TFETs pose a challenge for providing good read/write noise margin characteristics in TFET SRAMs. We provide an analysis of 8T and 10T TFET SRAM cells, including Schmitt-Trigger (ST) based cells, to address these shortcomings. By benchmarking a variety of TFET-based SRAM cells, we show the utility of the Schmitt-Trigger feedback mechanism in improving the read/write noise margins, thus enabling ultra low-V_CC operation for TFET SRAMs. We also propose a variation model for studying the impact of device-level variation on TFET SRAM cells. We show that the TFET ST SRAM cell has sufficient variation tolerance to operate at low-V_CC, and is a very promising cell to achieve a V_CC-min of 124mV. The TFET ST cell operating at its V_CC-min provides a 1.2x reduction in dynamic energy and 13x reduction in leakage power compared to the best CMOS-based SRAM implementation operating at it's V_CC-min, while giving better performance at the same time.