A metal oxide semiconductor (MOS) includes an isolation layer disposed in a semiconductor substrate to define an active region. A source region and a drain region are disposed on both sides of the active region such that a first direction is defined from the source region to the drain region. A channel recess is disposed in the active region between the source and drain regions. The channel recess has a convex surface when viewed from a cross-sectional view taken along a second direction orthogonal to the first direction. A gate electrode fills the channel recess and crosses the active region in the second direction. A gate insulating layer is interposed between the gate electrode and the active region.