Low Power Loadless 4T SRAM cell based on degenerately doped source (DDS) In0.53Ga0.47As Tunnel FETs

Abstract

We propose a Loadless 4T SRAM cell using degenerately doped source (DDS) p-channel In0.53Ga0.47As Tunnel FETs (TFETs) as dual purpose access/load devices and low leakage steep sub-threshold n-channel TFETs as drive devices. A Loadless 4T CMOS SRAM cell [1] has the requirement that the leakage current of the PMOS access transistors should be larger than the leakage current of the NMOS drive transistors to maintain state. In this paper, we introduce a p-type TFET with a degenerately doped source, which has a kT/q sub-threshold slope, compared to an n-type TFET which has a sub-kT/q slope. This difference in sub-threshold behaviour of the DDS PTFET and DDS NTFET helps to maintain the IOFF ratio which is required for cell stability. We explain the temperature dependent sub-threshold characteristics of the In0.53Ga0.47As (DDS) p-TFET by analyzing the position of the Fermi level in the source of the p-TFET as a function of source doping. Further, the asymmetric source drain architecture of the TFETs is exploited to solve the adjacent bit flip problem of unselected Loadless 4T SRAM cells during a column write operation. Finally, we include a comparison of the leakage energy and cell access time of the TFET based SRAM cell and benchmark its performance relative to state-of-the-art CMOS based 6T SRAM cells.