A high-permittivity (high-k) material is applied as the gate dielectric layer in a silicon metal-oxide-semiconductor (MOS) capacitor to form a special electro-optic (EO) modulator. Both induced charge density and modulation efficiency in the proposed modulator are improved due to the special structure design and the application of the high-k material. The device has an ultra-compact dimension of 691 \mu m in length.