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A novel Si-based metal-oxide-semiconductor (MOS) electrooptic phase modulator including two shunt oxide layer capacitors integrated on a silicon-on-insulator (SOI) waveguide is simulated and analyzed. The refractive index near the two gate oxide layers is modified by the free carrier dispersion effect induced by applying a positive bias on the electrodes. The theoretical calculation of free carrier distribution coupled with optical guided mode propagation characteristics has been carried out. The influence of the structure parameters such as the width and the doping level of the active region are analyzed. A half-wave voltage Vπ=4 V is demonstrated with an 8-mm active region length and a 4-μm width of an inner rib under an accumulation mode. When decreasing the inner rib width to 1 μm, the phase modulation efficiency is even higher, and the rise and fall times reach 50 and 40 ps, respectively, with a 1.0×1017 cm-3 doping level in the active region.