A method for fabricating a field-effect transistor includes the following steps. A gate structure layer in a line shape including a first region and a second region abutting to the first region is formed on a silicon layer. A first implanting process is performed to implant first-type dopants at least into a second portion of the second region of the gate structure layer. A second implanting region is performed to implant second-type dopants into the silicon layer to form a source region and a second region corresponding to the first region of the gate structure layer. The gate structure layer has a conductive-type junction at an interface between the first and second portions of the second region. A width of the silicon layer under the second region of the gate structure layer is smaller than a width of the gate structure layer.