PURPOSE: To form source and drain of an LDD structure at the same time while decreasing the number of manufacturing steps by depositing an insulating film on a semiconductor substrate covered with a gate electrode, and implanting dopant ions with the insulating film used as a mask.

CONSTITUTION: A silicon substrate 1 is covered with gate oxide 2, on which a polysilicon film 3 is formed for a gate electrode. An oxide film 5 is deposited over the surface of the substrate by CVD. With the oxide film 5 as a mask, an n-type dopant is implanted. The dopant is introduced through the oxide film to source and drain regions in the substrate. According to the difference in oxide thickness, t1 and t2, shallow n- regions 4 and deep n+ regions 7 are alternately and continuously formed at the same time. The diffusion concentration is moderately controlled to reduce the concentration of electric field and increase breakdown voltage and reliability.