forming a pad oxide layer and a nitride layer sequentially over a first-conductivity-type semiconductor substrate defined by an active area and a device isolation area;

selectively etching the nitride layer to form an opening through which a part of the pad oxide layer is exposed;

forming a first-conductivity-type doping area using the selectively etched nitride layer as a mask;

forming spacers on sidewalls of the opening;

selectively etching the exposed part of the pad oxide layer and underlying substrate using the etched nitride layer and the spacers as a mask to form a trench;

forming a dielectric layer in the trench;

forming an isolation layer on the dielectric layer to fill the trench;

removing the spacers, the nitride layer and the pad oxide layer; and

forming a second-conductivity-type diffusion area in the active area of the substrate in such a way that between the second-conductivity-type diffusion area and the isolation layer is a space corresponding to the first-conductivity-type doping area and the dielectric layer.