Impact of NMOS/PMOS Imbalance in Ultra-Low Voltage CMOS Standard Cells

In this paper, the impact of the NMOS/PMOS imbalance on Ultra-Low Voltage (ULV) circuits and their design is discussed within a unitary framework for the first time. Variations are shown to dramatically affect imbalance due to the long-tailed probability density and high variability. The impact of the imbalance on the minimum supply voltage VDD,min ensuring correct gate switching is studied analytically. The results theoretically justify the experimental results in [1], which agree very well with the predictions. The impact of the imbalance on the leakage energy in VLSI systems is also analyzed through a simple but representative example. An analytical model is presented to predict such leakage energy increase due to imbalance. Extensive results in 65-nm CMOS are shown to agree with the design considerations and quantitative models presented.