Performance and/or uniformity of sophisticated transistors may be enhanced by incorporating a carbon species in the active regions of the transistors prior to forming complex high-k metal gate electrode structures. For example, a carbon species may be incorporated by ion implantation into the active region of a P-channel transistor and an N-channel transistor after selectively forming a threshold adjusted semiconductor material for the P-channel transistor, while the active region of the N-channel transistor is still masked.