A semiconductor device includes a gate structure, a source region and a drain region. The source region and the drain region are on opposite sides of the gate structure. The source region includes a first region of a first conductivity type and a second region of a second conductivity type. The second conductivity type is opposite to the first conductivity type. The first region is between the second region and the gate structure. The second region includes at least one projection protruding into the first region and toward the gate structure.