A method for fabricating a semiconductor device to enlarge a channel region is provided. The channel region is enlarged due to having pillar shaped sidewalls of a transistor. The transistor includes a fin active region vertically protruding on a substrate, an isolation layer enclosing a lower portion of the fin active region, and a gate electrode crossing the fin active region and covering a portion of the fin active region. An isolation layer is formed enclosing a lower portion of the fin active region and the isolation layer under the spacers is partially removed to expose a portion of the sidewalls of the fin active region. Subsequently, dry etching is performed to form the sidewalls having a pillar/neck.