A semiconductor device includes a gate insulator layer above a semiconductor substrate, a gate electrode above the gate insulating layer, a sidewall insulator layer on sidewalls of the gate electrode and above the substrate, source and drain regions within the substrate on both sides of the gate electrode, a first region within the substrate below a part of the sidewall insulator layer closer to the source region and having an impurity concentration lower than the source region, a second region provided within the substrate below a part of the sidewall insulator layer closer to the drain region and having an impurity concentration lower than the drain region, a channel region provided within the substrate between the first and second regions, and a third region within the substrate below the channel region and including impurities of a different type and having an impurity concentration higher than the channel region.