The realization of on-chip optical interconnects requires the integration of active micro-optical devices with microelectronics. However, it is not clear yet how silicon photonics could be integrated within CMOS chips. In this context the non-crystalline forms of silicon, such as laser-annealed polycrystalline and hydrogenated amorphous silicon (a-Si:H), can deserve some advantages as they can be included almost harmlessly everywhere in a CMOS typical run-sheet, yielding low-cost and flexible fabrication. In particular, a-Si:H can be deposited using the CMOS-compatible low temperature plasma enhanced chemical vapour deposition (PECVD) technique, which brings clear advantages particularly for a back-end photonic integrated circuit (PIC) integration. However, till now a-Si:H has been mainly considered for the objective of passive optical elements within a photonic layer at λ=1.55 μm. Only a small number of examples have been reported, in fact, on waveguide integrated active devices. In this paper we detail about an effective refractive index variation obtained through an electrically induced carrier depletion in an as-deposited a-Si:H-based p-i-n waveguiding device. For this device switch-on and switch-off times of ~2 ns were measured allowing a modulation rate higher than 150 MHz.ÿÿ