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A novel method to model the effect of local workfunction variation in high-k metal gate nanoscale transistors is proposed. Impact of variability in metal grain granularity on device performance is studied using ab initio density functional theory calculations and device simulations, which show that different metal grain orientations (GOs) can result in large (≥100 mV) variation in metal gate effective work function. Probabilities of occurrence of each GO and the grain size are used to estimate the work-function variations. Full 3-D device simulations are performed to study the effect of metal grain granularity on FinFET and planar MOSFET behavior. Simulated mismatch trends are shown to be in good agreement with the grain diameters and device geometries.