Gate structures and methods of forming the gate structures are described. In some embodiments, a method includes forming source/drain regions in a substrate, and forming a gate structure between the source/drain regions. The gate structure includes a gate dielectric layer over the substrate, a work function tuning layer over the gate dielectric layer, a metal-containing compound over the work function tuning layer, and a metal over the metal-containing compound, wherein the metal-containing compound comprises the metal as an element of the compound.