Although switching techniques based on charge injection in silicon have progressed greatly in recent years, switching energies are still above 10 fJ/bit, which is considered the threshold for practical implementation in on-chip optical interconnects. This is due primarily to silicon's relatively weak electro-optic response, as well as the large physical extent of existing switching geometries, both of which increase the energy required to achieve switching. By using a resonant approach in which the optical mode is spatially tightly confined, however, the volume of active material is decreased, resulting in reduced switching energy. In this paper we report on the use of a thin MOS capacitor to inject charge into a resonator based on a photonic crystal microcavity. By injecting charge only into the volume in which the optical mode is localized, switching energy can be reduced below 1 fJ/bit. The index shift available (Δn ~ 0.001) allows the use of a relatively low-Q resonator (Q ~ 550), enabling high optical bandwidth of 100 Gbps with a device footprint below 25 μm2.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.