Semiconductor device having a metal gate

A semiconductor device having a metal gate, comprising:

• a substrate, having a first semiconductor device and a second semiconductor device disposed thereon, and having a first gate trench and a second trench in the first semiconductor device and in a second semiconductor device respectively;
• a bottom barrier layer disposed in the first gate trench and in the second trench, wherein the bottom barrier layer is a U-shaped structure;
• a first work function metal layer in the first gate trench, wherein the topmost portion of the first work function metal layer is lower than the opening of the first gate trench;
• and
• a second work function metal layer disposed on the first work function metal layer in the first gate trench and on the bottom barrier layer in the second trench respectively, wherein the topmost portion of the second work function metal layer, the opening of the first gate trench and the opening of the second trench are on the same level.