A method is provided for fabricating a semiconductor device structure with a short channel and long channel component having different gate dielectric layers without using lithography processes or masks. The method includes forming first and second openings having sidewalls and bottom surfaces in a dielectric layer, the first opening being narrower than the second opening. A first material layer is formed in the first and second openings. A protective layer is formed over the first material layer, wherein the protective layer covers the sidewalls and the bottom surface of the second opening. A block layer is formed to fill the second opening and cover the protective layer therein. The method further includes removing side portions of the protective layer to expose upper portions of the first material layer in the second opening. The block layer is removed from the second opening to expose the protective layer remaining in the second opening. A second material layer is formed over the first material layer on the exposed upper portions of the first material layer in the second opening. An intermix layer is formed in the second opening using the first and second material layers. The protective layer from the second opening is removed to expose the first material layer.