A Si-based light modulator working in the near infrared at frequencies up to 300 kHz is proposed. The device proposed is a bipolar mode field effect transistor (BMFET) integrated in an Si rib waveguide. The operation principle is the light dispersion/absorption by a plasma of free carriers that can be moved in or out of the optical path. It works as a pure amplitude light modulator; hence, it can be shrunk. Its small dimensions, only 100 µm in length and less than 50 µm in width in the proposed version, make it suitable for monolithic integration for applications where the major constraints are the device dimensions. As an example, the device application to an integrated gas sensing or chemical absorption spectroscopy system is also described.