A semiconductor device includes a substrate having an active region, and first to third transistors on the active region of the substrate, each of the first to third transistors including a dielectric layer on the substrate, a metal layer on the dielectric layer, a barrier layer between the dielectric layer and the metal layer, and a work function layer between the dielectric layer and the barrier layer, wherein the barrier layer of the third transistor is in contact with the dielectric layer of the third transistor, and wherein a threshold voltage of the second transistor is greater than a threshold voltage of the first transistor and less than a threshold voltage of the third transistor.