We have fabricated and characterized a novel Si-based light modulator working at the standard communication wavelength of 1.5 μm. It consists of a three-terminal bipolar mode field effect transistor integrated with a silicon rib waveguide on epitaxial Si wafers. The modulator optical channel is embodied within its vertical electrical channel. Light modulation is achieved moving a plasma of carriers inside and outside the optical channel by properly biasing the control electrode. The carriers produce an increase of the Si absorption coefficient. The devices have been fabricated using clean-room processing. Detailed electrical characterization and device simulations confirm that strong conductivity modulation and plasma formation in the channel are achieved. The plasma distribution in the device under different bias conditions has been directly derived from emission microscopy analyses. The device performances in terms of modulation depth will be presented.