In this paper we report results on a field-effect induced light modulation at λ= 1.55 um in a high-index-contrast waveguide based on a multisilicon-on-insulator (MSOI) platform. The device is realized with the hydrogenated amorphous silicon (α-Si: H) technology and it is suitable for monolithic integration in a CMOS Integrated Circuit. The device exploits the free carrier optical absorption electrically induced in the semiconductor core waveguide. The dynamic behaviour of the device was experimentally and theoretically analyzed in presence of a visible illumination showing a link between the photogeneration and the free carriers provided by doped α-Si: H layers.