A semiconductor device includes a substrate of first conductivity type; a first heavily doped region and a second heavily doped region of second conductivity type spaced apart from the first heavily doped region, located in the substrate; a channel region in the substrate and between the first heavily doped region and the second heavily doped region; a gate disposed on the channel region; a hard mask layer covering a top surface and a sidewall of the gate; and a spacer disposed on a sidewall of the hard mask layer.