Abstract: Formation of well-regions of a conductivity type opposite to that of a substrate is achieved in such a manner to determine a first threshold voltage level. Ion implantation is effected on desirably selected gates in the respective channels formed on the substrate and the well-regions. Two channels on the ion implanted substrate and on the well-region in which the ion implantation is not effected, are coupled to form a complementary-MOS transistor pair having a first threshold voltage level. The channels on the substrate in which the ion implantation is not effected and on the ion implanted well-region are coupled to form another complementary-MOS transistor pair having a second threshold voltage level.