Electrically induced phase modulation is characterized for the first time in a waveguide-integrated Fabry–Perot resonating cavity based both on an index- and conductivity- high-contrast amorphous silicon/amorphous silicon carbide (a-Si:H/a-SiC:H) multistack. The device consists of a single mode a-Si:H rib waveguide containing three insulating thin layers of a-SiC:H embedded within the core thickness. The effective refractive index change, Δneff, at the wavelength of λ=1.55 μm is achieved through the application of an electric field across the stack which induces carrier accumulation at all the a-Si:H/a-SiC:H interfaces, resulting in turn in a high interaction between the optical beam and the accumulation layers. This configuration allows to obtain a Vπ×Lπ product of about 5.9 V cm, not far from what observed in high performing electro-optical modulators in crystalline silicon.