We fabricated and characterized an electro-optic Si-based amplitude light modulator working at 1.5 mu m. It is a Bipolar Mode Field-Effect transistor (BMFET) integrated within a Si rib waveguide. The devices, 100 tin long, were fabricated using epitaxial Si wafers and standard clean room processing. The light is absorbed during its travel in the device optical channel when a plasma of free carriers, electrically driven, is generated and placed inside the channel. We experimentally monitored the plasma formation and localization in the device using standard Emission Microscopy analysis. The optical characterization in static conditions provides a modulation depth of similar to 90%, well above the 25% minimum required to consider a device a modulator. Furthermore, dynamical measurements show a modulation depth of 65% at 100KHz of operation frequency. Finally, an experimental evidence of a frequency threshold, at about 500 KHz, is observed in the plasma behavior. Theoretical considerations and experimental data suggest that at frequencies below threshold the dominant phenomenon is the plasma generation/recombination, while above threshold the carrier drift leads the plasma motion and redistribution in the device channel.