Semiconductor devices and methods of manufacture thereof are disclosed. In a preferred embodiment, a method of manufacturing a semiconductor device includes providing a workpiece, disposing a gate dielectric material over the workpiece, and disposing a gate material over the gate dielectric material. Cl or F is introduced to the gate material, wherein introducing the Cl or F to the gate material affects a work function of the gate material. The gate material and the gate dielectric material are patterned, forming at least one transistor.