Semiconductor device having contact plugs with different interfacial layers

A semiconductor device, comprising:

• a substrate having a first region and a second region;
• a first contact plug on the first region, wherein the first contact plug comprises:
• a first interfacial layer having a first conductive type;
• a first work function metal layer having the first conductive type on the first interfacial layer;
• and
• a third work function metal layer having a second conductive type on the first work function metal layer;
• a second contact plug on the second region, wherein the second contact plug comprises:
• a second interfacial layer having the second conductive type, wherein the first conductive type and the second conductive type are different conductive types;
• and
• a second work function metal layer having the second conductive type on the second interfacial layer.