Provided is an optical device with improved phase shift and propagation loss of light without decreasing the dynamic characteristics of the optical device. The optical device includes a first semiconductor layer which is doped with a first type of conductive impurities and has a uniform thickness; a gate insulating layer which has a ? shape and is formed on a portion of the first semiconductor layer and has a thin center portion; and a second semiconductor layer which covers an upper surface of the gate insulating layer and is doped with a second type of conductive impurities opposite to the first type of conductive type impurities.