Semiconductor devices and fabrication methods are provided. A fabrication method includes: forming a source and drain material layer over a substrate; forming a mask layer over the source and drain material layer and including a first trench exposing a portion of the source and drain material layer; forming a protective layer on sidewalls of the first trench; forming a second trench in the source and drain material layer using the mask layer and the protective layer as an etch mask; removing the protective layer after the second trench is formed; forming a channel material layer and a gate structure on the channel material layer after the protective layer is removed; and removing the mask layer after the channel material layer and the gate structure are formed. The channel material layer is on the sidewalls and the bottom of the first trench and the second trench.