Semiconductor device having LDD structure and method for producing the same

A semiconductor device having an LDD structure, comprising:

• a first conductive type semiconductor substrate;
• and
• a field effect MOS transistor formed at a surface of said semiconductor substrate;
• said field effect MOS transistor having:
• a gate electrode formed on said semiconductor substrate;
• a drain region, said drain region including:
• a second conductive type first diffusion layer formed at the surface of said semiconductor substrate, said second conductive type being opposite to said first conductive type;
• and
• a second conductive type second diffusion layer formed closer to said gate electrode than said first diffusion layer at the surface of said semiconductor substrate and having impurity density lower than that of said first diffusion layer;
• and
• a source region, said source region including:
• a second conductive type third diffusion layer formed at the surface of said semiconductor substrate, and
• a second conductive type fourth diffusion layer formed closer to said gate electrode than said third diffusion layer at the surface of said semiconductor substrate and having impurity density lower than that of said third diffusion layer, a diffusion coefficient of impurity in said fourth diffusion layer being smaller than that in said second diffusion layer.