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We propose and analyze a novel compact electrooptic modulator on a silicon-on-insulator (SOI) rib waveguide. The device confines both optical field and charge carriers in a micron-size region. The optical field is confined by using a planar Fabry-Perot microcavity with deep Si/SiO2 Bragg reflectors. Carriers are laterally confined in the cavity region by employing deep-etched trenches. The refractive index of the cavity is varied by using the free-carrier dispersion effect produced by a p-i-n diode. The device has been designed and analyzed using electrical and optical simulations. Our calculations predict, for a 20-μm-long device, a modulation depth of around 80% and a transmittance of 86% at an operating wavelength of 1.55 μm by using an electrical power under dc conditions on the order of 25 μW.