We present a fully integrated Mach-Zehnder interferometer in silicon-on- insulator technology. Modulation of the index of refraction is achieved through the plasma dispersion effect resulting in a bandwidth in the 10 MHz range. A particular and innovative design makes this device completely compatible with CMOS technology allowing electronic functions to be integrated on the same substrate. Measurement results, limitations due to thermooptic effect and absorption related to charge injection together with further improvements are discussed.