A compact optical modulator based on the hybrid integration of a III−V photonic crystal-based active region in a silicon-on-insulator waveguide is proposed and designed. An extinction ratio up to 20 dB is obtained in a 18.75 µm-long device. A 3 dB cut-off frequency of 15 GHz, and an energy consumption of 19 fJ/bit are estimated by an equivalent small-signal electrical circuit, while the optical bandwidth is larger than 10 nm, which is higher than for typical low power consumption silicon modulators.