A complementary metal-oxide-semiconductor (CMOS) semiconductor device includes a substrate. The CMOS semiconductor device further includes an isolation region in the substrate. The CMOS semiconductor device further includes a P-metal gate electrode extending over the isolation region, wherein the P-metal gate electrode includes a first function metal and a TiN layer doped with a first material. The CMOS semiconductor device further includes an N-metal gate electrode extending over the isolation region, wherein the N-metal gate electrode includes a second function metal and a TiN layer doped with a second material different from the first material, a portion of the P-metal gate electrode is between a portion of the N-metal gate electrode and the substrate, and a portion of the TiN layer doped with the second material is between the portion of the P-metal gate electrode and the substrate.