Semiconductor device comprising buried channel region

A semiconductor device, comprising:

• a semiconductor substrate;
• a pair of first diffusion layers formed within said semiconductor substrate;
• a gate insulating film formed on that portion of said semiconductor substrate which is positioned between said paired diffusion layers;
• a gate electrode including a first gate portion formed on said gate insulating film and a second gate portion formed on said first gate portion, a first width in a channel direction of said first gate portion being substantially equal to a width in said channel direction of said gate insulating film, and a second width in said channel direction of said second gate portion being larger than said first width;
• a gate side wall insulating film including a first side wall portion formed on a side surface of said first gate portion and on a side surface of said gate insulating film and a second side wall portion formed on a side surface of said second gate portion;
• a second diffusion layer formed apart from said first diffusion layers within that portion of said semiconductor substrate which is positioned below said gate insulating film;
• and
• a third diffusion layer including a first diffusion portion formed below said first diffusion layers under said first side wall portion and a second diffusion portion formed below said second diffusion layer,
• wherein a distance of a peak portion of a impurity concentration in said first diffusion portion from a surface of said semiconductor substrate is less than a distance of a peak portion of a impurity concentration in said second diffusion portion from the surface of said semiconductor substrate, and
• wherein a conductivity type of said third diffusion layer is equal to the conductivity type of said semiconductor substrate.