In a method of manufacturing a semiconductor device, a first selective epitaxial growth (SEG) process is performed on a substrate to form a first channel. A first etching process is performed to form a first recess through the first channel and an upper portion of the substrate. A sidewall of the first channel exposed by the first recess is slanted with respect to an upper surface of the substrate. A second SEG process is performed to form a second channel on a surface of the substrate and the sidewall of the first channel exposed by the first recess. A gate structure is formed to fill the first recess. An impurity region is formed at an upper portion of the substrate adjacent to the gate structure.