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We propose new methods to control the work function (WF) of nickel-fully-silicided (Ni-FUSI) gates. We clarified the amounts of segregated dopants, the status at the dielectric surface, and the composition of NiSi to determine the WF of the gates. We demonstrated the segregation mechanism for n- and p-type dopants during the silicidation processes. The amount of dopants used is effective for achieving wide-range WF. Nitridation of the gate oxide surface impacts both the WF of p-type gates and the oxide reliability. The Ni content of NiSi also changes the WF. These methods can be used to simultaneously control the WFs. Based on these three origins, we should be able to achieve wide-range WF differences (0.91 eV), suitable for use in 45-nm node CMOS devices.