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We explore the breakdown of universal mobility behavior in sub-100-nm Si MOSFETs, using a novel three-dimensional (3-D) statistical simulation approach. In this approach, carrier trajectories in the bulk are treated via 3-D Brownian dynamics, while the carrier-interface roughness scattering is treated using a novel empirical model. Owing to the high efficiency of the transport kernel, effective mobility in 3-D MOSFETs with realistic Si-SiO2 interfaces reconstructed from a Gaussian or exponential correlation function can be simulated in a statistical manner. We first demonstrate a practical calibration procedure for the interface mobility and affirm the universal behavior in the long channel limit. Next, effective mobility in ensembles of MOSFETs with a gate length down to 10 nm is investigated. It is found that the random-discrete nature of the Si-SiO2 interface leads to a distribution of carrier mobility below the interface, which can deviate considerably from universal mobility curves when Lgate<6Λ, where Λ is the correlation length for the SiO2 interface.