A semiconductor device includes a substrate with first and second areas, a first trench in the first area, and first and second PMOS transistors in the first area and the second area, respectively. The first transistor includes a first gate insulating layer, a first TiN layer on and contacting the first gate insulating layer, and a first gate electrode on and contacting the first TiN layer. The second transistor includes a second gate insulating layer, a second TiN layer on and contacting the second gate insulating layer, and a first TiAlC layer on and contacting the second TiN layer. The first gate insulating layer, the first TiN layer, and the first gate electrode are within the first trench. The first gate electrode does not include aluminum. A threshold voltage of the first transistor is smaller than a threshold voltage of the second transistor.