A silicon optical modulator operating at high speed and low voltage is proposed by using a Schottky diode. The optical modulation is achieved by the intensity change of guiding light due to free-carrier absorption, not conventional interference effects. The rib waveguide structure of the modulator has a height of 340 nm, a etch depth of 150 nm, a width of 4.8 μm, and a modulation length of 500 μm. It was designed to maximize the free carrier injection by a Schottky contact on the rib waveguide center. The center of the rib waveguide is lightly doped with phosphorus of 1016 cm -3, and the sides are heavily doped with phosphorus of 10 20 cm-3 to improve modulation depth by injecting free carriers into the center of the rib waveguide. This design allowed a high overlap between the optical mode and carrier density variations in the center of the waveguide. To achieve high speed operation, travelling-wave type electrodes were designed to allow co-propagation of electrical and optical signals along the waveguide. The device simulated results demonstrate a 3.3 dB modulation depth for a 500 μm modulation length with 3 Vpp driving voltages. We demonstrated a Schottky modulator operating Si EAM at 3 Vpp with a 3 dB bandwidth of 7 GHz.