A semiconductor device and a fabrication method thereof are disclosed. A silicon nitride film is formed over a silicon semiconductor substrate. Impurity ions are then implanted into desired areas of the silicon semiconductor substrate, so that nitrogen atoms and silicon atoms from the silicon nitride film are incorporated into the surface of the silicon semiconductor substrate together with introduction of impurity ions. The silicon semiconductor substrate has a minimized content of oxygen mixed thereinto and restored crystal defects filled by nitrogen atoms upon implanting of impurity ions. The fabricated semiconductor device is free from a trade-off relation between gate-electrode depletion and junction current leakage, and short-channel effects.