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The band gap and electric permeability changes across the Si/SiO2 interface of a MOS device were calculated based on the density function theory. The band gap changes non-abruptly at the interface with a transition region of 5 A toward the oxide. The calculated dielectric constants change almost abruptly at Si/SiO2 interface. Thus-obtained band gap profile and spatial behavior of dielectric constants were used in a device simulation of an ultra-thin-body n-type double-gate MOSFET. The potential profile in the channel was calculated by self-consistently solving the two-dimensional Poisson-Schrodinger equations. The gate leakage current was calculated accurately using the three-terminal non-equilibrium Green's function approach. The effect of the band gap transition across the Si/SiO2 interface on the device performance is investigated in detail.