A semiconductor integrated circuit device is disclosed which has an MOSFET with a lightly doped drain or LLD structure. A gate electrode layer is insulatively provided above a semiconductor substrate of p conductivity type. Source and drain layers of n conductivity type are formed in the substrate in such a manner as to be substantially self-aligned with the gate electrode. Each of these source and drain layers is comprised of a heavily doped diffusion layer and a lightly doped diffusion layer. The n- diffusion layer is deep enough to fully surround the heavily doped layer in the substrate. The n- diffusion layer has a step-like cross-section, whereby the effective channel length of MOSFET is increased inside the substrate to increase the punch-through voltage level.