The continued scaling of complementary metal oxide semiconductor (CMOS) devices into the sub-70 nm regime will require a fundamental change in transistor gate stack materials. Refractory metals and their metallic derivatives are among the few materials other than poly-silicon suitable for use as metal oxide semiconductor field effect transistor gate electrodes. In this paper we report the dependence of the Mo gate work function on nitrogen implant dose and energy. By implanting nitrogen into Mo/SiO2/Si stacks, the interfacial Mo work function can be controllably lowered from an initial (unimplanted) value of ∼5 eV. The ability to engineer the Mo gate work function over the range of the Si energy bandgap makes it an attractive candidate for future bulk-Si and ultrathin body silicon-on-insulator-CMOS gate electrodes.