A process for realizing TFT devices on a substrate comprises the steps of: forming on the substrate, in cascade, an amorphous silicon layer and a heavily doped amorphous silicon layer, forming a photolithographic mask on the heavily doped amorphous silicon layer provided with an opening, removing the heavily doped amorphous silicon layer through the opening for realizing opposite portions of the heavily doped amorphous silicon layer whose cross dimensions decrease as long as they depart from the amorphous silicon layer, removing the photolithographic mask, carrying out a diffusion and activation step of the dopant contained in the portions of the heavily doped amorphous silicon layer inside the amorphous silicon layer, for realizing source/drain regions of said TFT device.