Silicon as a photonic medium has a unique advantages. It is transparent in the range of optical telecommunications wavelengths (1.3 and 1.55µm) and has a high index of refraction, which allows for the fabrication of high-index-contrast submicrometer structures. The key elements for the development of silicon microphotonics is the silicon-based optical modulators which have earned a great interest in the recent years. High-speed optical modulation can be achieved by free carrier concentration variations. Electron and hole injection in PIN diodes has been widely used but generally leads to bandwidths lower than a few 100MHz.