A semiconductor device includes a substrate including a first area and a second area, and first and second transistors formed in the first area and the second area, respectively. The first transistor includes a first gate insulating layer on the substrate, a first TiN layer on the first gate insulating layer contacting the first gate insulating layer, and a first filling layer on the first TiN layer. The second transistor includes a second gate insulating layer on the substrate, a second TiN layer on the second gate insulating layer contacting the second gate insulating layer, and a second filling layer on the second TiN layer. A threshold voltage of the first transistor is less than that of the second transistor, the second gate insulating layer does not comprise lanthanum, and an oxygen content of a portion of the first TiN layer is greater than that of the second TiN layer.