Semiconductor device and fabrication method thereof

A method for forming a semiconductor device, comprising:

• providing a substrate;
• forming a source and drain material layer over the substrate;
• forming a mask layer over the source and drain material layer, wherein the mask layer includes a first trench and the first trench exposes 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 by etching the source and drain material layer at a bottom of the first trench using the mask layer and the protective layer as a 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, the channel material layer being on the sidewalls and the bottom of the first trench and on sidewalls and bottom of the second trench;
• and
• removing the mask layer after the channel material layer and the gate structure are formed.