To solve this problem, after the trench is buried with an insulating material, a large dose quantity (8E12/cm2) of a p-type dopant such as boron (B) is field-ion-implanted on the entire surface of the trench and in an active region at an energy level of 100 keV. Thus, a channel stop impurity region is formed in the lower portion of the isolation region and the active region, so that the threshold