A closed-form stability model for cross-coupled inverters operating in sub-threshold voltage region

A cross-coupled inverter which is an essential element of on-chip memory subsystems plays an important role in synchronous LSI circuits. In this paper, an analytical stability model for a cross-coupled inverter operating in a sub-threshold voltage region is proposed. The proposed model analytically shows that the minimum operating voltage of the cross-coupled inverter distributes normally in a high-s region if the distribution of the threshold voltage is Gaussian. The minimum supply voltage at which the yield of the cross-coupled inverter becomes a specific value can be accurately derived by a simple calculation using the model. Monte-Carlo simulation assuming a commercial 28 nm process technology demonstrates the accuracy and the validity of the proposed model. Based on the model, this paper shows strategies for variation tolerant memory design.