Field effect transistors are provided. According to the field effect transistor, a source region and a drain region are provided on a substrate and a fin portion is provided to protrude from the substrate. The fin portion connects the source region and the drain region to each other. A gate electrode pattern is disposed on the fin portion and extends to cross over the fin portion. A gate dielectric layer is disposed between the fin portion and the gate electrode pattern. A semiconductor layer is disposed between the fin portion and the gate dielectric layer. The semiconductor layer and the fin portion have dopant-concentrations different from each other, respectively.