Optoelectronic devices are considered the innovative element for the next generation of microelectronic integrated circuits. For this purpose, both active and passive devices-extremely miniaturized-must be implemented. We fabricated and electro-optical Si-based light intensity modulator working at 1.5 mu m using a bipolar mode field-effect transistor integrated within a Si rib waveguide. The principle of operation is the light absorption by a plasma of free carriers that can be opportunely moved inside or outside of the device optical channel by properly changing the control bias. The devices, only 100 mu m long, were fabricated using epitaxial Si wafers and standard clean room processing. The optical characterization at 1.48 mu m in static conditions shows a modulation of similar to 90% while the dynamic electrical characterization provides a switching time of approximate to 10 ns (foreseen modulation frequency of hundreds of MHz). A modulation depth above 25% is observed for modulation frequency up to 300 kHz. (c) 2005 American Institute of Physics.