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Sources responsible for local and inter-die threshold voltage (Vt) variability in undoped ultra-thin FDSOI MOSFETs with a high-k/metal gate stack are experimentally discriminated for the first time. Charges in the gate dielectric and/or TiN gate workfunction fluctuations are determined as major contributors to the local Vt variability and it is found that SOI thickness (TSi) variations have a negligible impact down to TSi=7 nm. Moreover, TSi scaling is shown to limit both local and inter-die Vt variability induced by gate length fluctuations. The highest matching performance ever reported for 25 nm gate length MOSFETs is achieved (AVt=0.95 mV.mum), demonstrating the effectiveness of the undoped ultra-thin FDSOI architecture in terms of Vt variability control.