A field effect transistor includes a substrate, an isolation layer, a gate, a channel, drain and a source. The substrate has an active region having a rectangular area and at least one protrusion protruded from the rectangular area. The isolation layer is formed on the substrate and encircling the active region. The gate crosses the active region and is formed above a middle portion of the active region. The channel is formed in the active region directly under the gate, extends to the at least one protrusion, and divides the active region into a first section and a second section. The drain formed in the first section and the source formed in the second section.