Static Noise Margin of Ultrathin-Body SOI Subthreshold SRAM Cells-An Assessment Based on Analytical Solutions of Poisson's Equation

Abstract

This paper investigates the static noise margin (SNM) of ultrathin-body (UTB) SOI SRAM 6T/8T cells operating in the subthreshold region using analytical solutions of Poisson's equation validated with TCAD simulations. An analytical framework to calculate the SNM for UTB SOI SRAMs operating in the subthreshold region is presented. Our results indicate that for improving both read SNM (RSNM) and write SNM (WSNM), the back-gating technique is more effective in the subthreshold region than in the superthreshold region. The 6T UTB SOI subthreshold SRAM cell with the back-gating technique by increasing the strength of the pull-up transistors and decreasing the strength of the pass-gate transistors shows comparable RSNM with the 10T bulk subthreshold SRAM and an improvement in RSNM variation. Due to better electrostatic integrity, the back-gating technique (pull-up transistors with positive back-gate bias, pull-down/pass-gate transistors with negative back-gate bias) mitigates the 6T UTB SOI SRAM RSNM variation significantly with some improvement in RSNM. Increasing cell beta-ratio shows a limited improvement on RSNM and has no benefit on the SNM variability for the subthreshold operation. The UTB SOI 8T SRAM cell exhibits RSNM 2 x larger than the 6T SRAM cell in the subthreshold region. Both negative bit-line voltage (VBL) and boosted word-line voltage (VWL) are more effective than lower cell supply voltage to improve WSNM, and negative VBL shows a larger improvement in WSNM than boosted VWL.