A semiconductor device includes a first transistor on a first region of a substrate, and a second transistor on a second region of the substrate. The first transistor includes a first gate insulating layer including a first interfacial insulating layer, a first lower high-κ dielectric layer, and a first composite dielectric layer, sequentially stacked on each of first semiconductor channel layers. The second transistor includes a second gate insulating layer including a second interfacial insulating layer, a second lower high-κ dielectric layer, a second composite dielectric layer, and a second upper high-κ dielectric layer, sequentially stacked on each of second semiconductor channel layers. The first and the second lower high-κ dielectric layers include a first metal element, the second upper high-κ dielectric layer includes a second metal element, and the first and the second composite dielectric layers include both of the first and the second metal elements.