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We have investigated the controllability of the effective work function (phim,eff) of TiN as a work-function-determining metal (WFM) for various gate-electrode structures in HfSiON MOSFETs. phim,eff was controllable from 4.7 to 4.44 eV by changing the TiN thickness from 30 to 2 nm in poly-Si/TiN gate electrodes, without any distinct increase in EOT. Therefore, thin-TiN and thick-TiN WFMs are preferred for the reduction in threshold voltage in nMOSFETs and pMOSFETs with poly-Si/TiN gate electrodes, respectively. A similar controllability was not observed with W/TiN gate electrodes but was evident with W/TaSiN/TiN gate electrodes. This means that controllability is a characteristic of metal gate electrodes with a structure including a Si-rich layer (such as poly-Si and TaSiN)/TiN. It is considered that Ti suboxides, which increase phim,eff as a thin insulator with negative fixed charges, or interface dipoles in the TiN/HfSiON interface, are reduced by oxidation of the Si-rich layer, producing the required result of phim,eff decrease when the TiN thickness becomes as thin as 2 nm.