Threshold voltage mismatch and intra-die leakage current in digital CMOS circuits

Due to device and voltage scaling scenarios for present and future deep-submicron CMOS technologies, it is inevitable that the off-state current (I_off) of MOSFET transistors increases as the technology minimum dimensions scale down. Experimental evidence shows that the leakage current distribution of modern deep-submicron designs not only has a higher mean value but it also presents a larger variability as well. In this paper, we investigate the impact of threshold voltage mismatch as one plausible source for this increased variability. In digital circuit design, it is commonly assumed that the threshold voltage difference (mismatch) of static CMOS cells is negligible. However, threshold voltage mismatch (ΔV_to) has a two-sided effect on the off-state current. Namely, the total cell´s current can increase or decrease depending upon the direction of the V_t mismatch shift. This effect can be so severe that I_off can increase by more than one order of magnitude with respect to its nominal value due only to V_to mismatch. We further show through experimental results that the V_to mismatch of paired transistors working in the subthreshold regime can be worse by a factor of two as compared to transistors working in the saturation or linear regions. A factor of two larger spread is obviously quite devastating in terms of area, speed, and power consumption, should it be desired to attain the same I_off level as for a V_to mismatch characterized out of the subthreshold regime.