In this paper, we report the first systematic study of quantum transport simulation of the impact of precisely positioned dopants on the performance of ultimately scaled gate-all-around silicon nanowire transistors (NWTs) designed for digital circuit applications. Due to strong inhomogeneity of the selfconsistent electrostatic potential, a full 3-D real-space nonequilibrium Green function formalism is used. The simulations are carried out for an n-channel NWT with 2.2 × 2.2 nm2 cross section and 6-nm channel length, where the locations of the precisely arranged dopants in the source-drain extensions and in the channel region have been varied. The individual dopants act as localized scatters, and hence, impact of the electron transport is directly correlated to the position of the single dopants. As a result, a large variation in the ON-current and a modest variation of the subthreshold slope are observed in the ID-VG characteristics when comparing devices with microscopically different discrete dopant configurations. The variations of the current-voltage characteristics are analyzed with reference to the behavior of the transmission coefficients.