New circuit architecture for electro-optic modulator based on free-carrier dispersion effect is invented, in which the waveguide capacitor of the modulator is embed in the circuits and physically layout together with transistors, the switching of the modulator occurs in transistors, and as the result, the electro-optical modulation occurs in the waveguide capacitor. The invented modulator is not one physical device, it is actually a circuit. Several circuit design techniques are imported, leading to several new modulator circuits that have very high operation speed and very small power consumption. Several new waveguide capacitor structures are also invented that allow high efficient modulator circuits to be built.