A very simple and fast MachZehnder electro-optic modulator based on a p-i-n configuration, operating at λ = 1.55 μm, has been fabricated at 170°C using the low cost technology of hydrogenated amorphous silicon (a-Si:H). In spite of the device simplicity, refractive index modulation was achieved through the free carrier dispersion effect resulting in characteristic rise and fall times of ~2.5 ns. By reverse biasing the p-i-n device, the voltage-length product was estimated to be V_π∙L_π = 40 V⋅cm both from static and dynamic measurements. Such bandwidth performance in as-deposited a-Si:H demonstrates the potential of this material for the fabrication of fast active photonic devices integrated on standard microelectronic substrates.