Semiconductor devices and fabrication methods are provided. In an exemplary method, a semiconductor layer including a first opening can be provided. The first opening can be filled with a stress material. The stress material can then be etched to form a second opening having a width less than a width of the first opening to leave a stress material layer in the semiconductor layer and on each sidewall of the second opening. The semiconductor layer can be etched to form a fin structure on a sidewall surface of the stress material layer. A main gate structure can be formed on the sidewall surface of the fin structure. A back gate structure can be formed on the sidewall surface of the stress material layer.