The effect of compact modelling strategy on SNM and Read Current variability in Modern SRAM

Abstract

It has been shown that sub 100nm SRAM is particularly sensitive to stochastic device variability. In this paper we consider two correlated figures of merit for SRAM, Static Noise Margin (SNM) and Read Current. For the purposes of this paper 1,000 3D atomistic simulations of microscopically different 25nm P and N bulk MOSFETs were performed, and statistical compact models were then extracted for each device. Using these models simulations are performed to calculate the SNM and Read Current distributions of SRAM cells constructed using devices from the device ensemble. Variability in device performance has been then introduced via Gaussian or skewed Gaussian threshold voltages (Vt) and by using values of Vt extracted directly from the individual device compact models and the results of these simulations are then compared to the baseline simulations using fully extracted models. The results clearly demonstrate the errors that can be introduced in the estimation of SNM and Read Current distribution of a 6T SRAM cell when statistical device variability is not correctly modelled.