SEMICONDUCTOR DEVICE

A semiconductor device includes a substrate, a first gate structure, and a second gate structure. The first gate structure is disposed on the substrate. The first gate structure includes a first capping layer and a first underlying layer below the first capping layer. The second gate structure is disposed on the substrate. The second gate structure includes a second capping layer and a second underlying layer below the second capping layer. The material of the first capping layer and the second capping layer have a material having higher resistant to oxygen or fluorine than materials of the first underlying layer and the second underlying layer. The first capping layer, the second capping layer and the second underlying layer include a same metal element.