Lightly doped drain forming method and semiconductor device

The invention provides a lightly doped drain forming method and a semiconductor device. The method comprises the following steps of: providing a semiconductor substrate, wherein the semiconductor substrate comprises a core device area and an input/output device area, and the side of a grid is provided with a first side wall; forming a masking layer on the semiconductor substrate to cover the core device area; removing the first side wall outside the side wall of the grid in the input/output device area; removing the masking layer covered on the core device area; forming lightly doped drains in the core device area and the input/output device area by a ion implantation method, wherein the first side wall outside the side wall of the grid in the core device area blocks the ions from being implanted into the lower area. The lightly doped drain forming method can improve the overlapping capacitance of the grid and source/drain, avoid generating hot carrier effect, and improve the property and reliability of the input/output device.