Exploration and evaluation of hybrid TFET-MOSFET monolithic 3D SRAMs considering interlayer coupling

This paper investigates and evaluates 7T hybrid TFET-MOSFET monolithic 3D SRAM cells considering interlayer coupling for ultra-low voltage operation using TCAD mixed-mode simulations. The planar (2D) 7T hybrid TFET-MOSFET SRAM cell is shown to exhibit equal leakage, better stability and performance compared with the conventional 2D 8T MOSFET SRAM at ultra-low voltage (Vdd ≤ 0.3V). The interlayer coupling, where the front-gate of the bottom tier device alters the back gate bias of the upper tier device, and various stacking and layout arrangements are examined and exploited to improve the stability and performance of monolithic 3D SRAMs. An optimized 3D 7T hybrid SRAM design is shown to exhibit 80% write static noise margin (WSNM) improvement and 24% cell write performance improvement, whereas optimized 3D 8T MOSFET SRAM exhibits 66% WSNM improvement and 33% cell write performance improvement over the planar design. Furthermore, 3D SRAM designs are shown to reduce the SRAM cell area by nearly 40%.