Semiconductor device having various threshold voltages and manufacturing same

Claim 1

A semiconductor device comprising: a first transistor comprised of first diffusion layers which are exclusively comprised of a first conductivity type; a second transistor comprised of second diffusion layers of said first conductivity type in contact with a channel portion of said second transistor and third diffusion layers of said first conductivity type in contact with said second diffusion layers on a side opposing said channel portion of said second transistor; a third transistor comprised of fourth diffusion layers of a second conductivity type in contact with a channel portion of said third transistor and fifth diffusion layers of said second conductivity type in contact with said fourth diffusion layers on a side opposing said channel portion of said third transistor; wherein a concentration of said first diffusion layers is substantially the same as a concentration of said second diffusion layers and concentration of said third diffusion layers is higher than said first and second diffusion layers, and a concentration of said fifth diffusion layers is higher than said fourth diffusion layers; and further wherein said first, second and third transistors each have a different first conductivity type impurity concentration on the surface of the channel portions of said first, second and third transistors.