Evaluation of Digital Circuit-Level Variability in Inversion-Mode and Junctionless FinFET Technologies

In this paper, we develop an evaluation framework to assess variability in nanoscale inversion-mode (IM) and junctionless (JL) fin field-effect transistors (FinFETs) due to line edge roughness (LER) and random dopant fluctuation (RDF) for both six transistor (6T) static random access memory (SRAM) design and large-scale digital circuits. From a device-level perspective, JL FinFETs are severely impacted by process variations: up to 40% and 60% fluctuation in threshold voltage is observed from LER RDF. Conversely, results show that variability-induced shifts and broadening of timing and power in large-scale digital circuits are not significant and can be accommodated in the design budget. However, we find that LER has a large impact on static noise margin analysis of 6T SRAMs. Required V_ccmin values for SRAMs using JL devices reach up to 2× those implemented in conventional IM technologies. The yield for JL SRAM is completely compromised in the presence of realistic levels of LER and RDF. Fortunately, the impact of variability is somewhat reduced with scaling for JL designs; both LER and RDF induce less variation for the 15-nm node compared with the 32-nm node. The observed reduction in V_ccmin with technology scaling suggests that digital circuits implemented with JL FinFETs may eventually offer the same level of operability as those based on IM FinFETs, especially in the presence of circuit-level SRAM robustness optimizations.