As the channel length of conventional metal-oxide-semiconductor devices is entering the decananometer scale, control over doping profile in the channel region becomes problematic and an unavoidable variation of various transistor parameters takes place due to the finite number of doping atoms contained in the channel region. Here we study the influence of discrete doping atoms on the electrical characteristics of n-channel multigate silicon nanowire FETs in both ballistic and dissipative regimes of quantum transport. A three-dimensional quantum mechanical device simulator based on the non-equilibrium Green's function formalism in the coupled-mode space approach that can handle electron-phonon interactions has been developed to extract the physical parameters of the devices. We show that introducing discrete doping atoms in the channel region of silicon nanowire creates resonance energy levels in the channel region which increase considerably the subthreshold current, even in the presence of electron-phonon interaction mechanism. (C) 2011 Elsevier Ltd. All rights reserved.