A method of manufacturing a method of manufacturing a thin film transistor. An aluminum gate electrode is formed on a substrate. A protective layer is formed on the top surface and the sidewall of the aluminum gate electrode. A gate dielectric layer is formed on the substrate and the protective layer. An intrinsic amorphous-silicon thin film is formed on the gate dielectric layer. A heavily doped amorphous-silicon thin film is formed on the intrinsic amorphous-silicon thin film. A patterned source/drain conductive layer is formed on the heavily doped amorphous-silicon thin film to expose a portion of the heavily doped amorphous-silicon thin film. The portion of the heavily doped amorphous-silicon thin film exposed by the patterned source/drain conductive layer is removed to expose a portion of the intrinsic amorphous-silicon thin film.