Hydrogenated amorphous silicon (a-Si:H) has recently emerged as a promising material to provide microchips with passive and active photonic functions through a back-end and CMOS-compatible technological process. In this paper, we discuss the performance achieved with different configurations of a-Si:H-based electro-optical amplitude modulators integrated into passive waveguides. All of the analysed devices are based on the plasma dispersion effect, a phenomenon that allows us to reach useful performance at the communication wavelength of λ ∼ 1.55 μm. The behaviour of the various proposed modulation approaches has been tested by ad hoc interferometric structures, such as Fabry─Perot integrated resonators or integrated Mach─Zehnder interferometer, as well as by multistack devices to enhance the static modulation efficiency. The performance of each modulator has been analysed through several figures of merit.