In one embodiment, a semiconductor device includes a substrate, a gate insulator on the substrate, and a gate electrode on the gate insulator. The device further includes a source diffusion layer of a first conductivity type and a drain diffusion layer of a second conductivity type disposed on a surface of the substrate so as to sandwich the gate electrode. The device further includes a junction forming region disposed between the source diffusion layer and the drain diffusion layer so as to contact the source diffusion layer. The junction forming region includes a source extension layer of the first conductivity type, a pocket layer of the second conductivity type above the source extension layer, and a diffusion suppressing layer disposed between the source extension layer and the pocket layer and containing carbon so as to suppress diffusion of impurities between the source extension layer and the pocket layer.