Numerical simulations have been performed to study the single-charge-induced ON current fluctuations (random telegraphic noise) in conventional (MOSFET) and non-conventional (silicon nanowire) nanoscale field-effect transistors. A semi-classical three-dimensional particle-based Monte Carlo device simulator (MCDS 3-D) has been integrated and used in this work. Quantum mechanical space-quantization effects have been accounted for via a parameter-free effective potential scheme that has been proved quite successful in describing charge set back from the interface and quantization of the energy (bandgap widening) within the channel region of the device. The effective potential is based on a perturbation theory around thermodynamic equilibrium and leads to a quantum field formalism in which the size of the electron depends upon its energy. To treat full Coulomb (electron-ion and electron-electron) interactions properly, the simulator implements two different real-space molecular dynamics (MD) schemes: the particle-particle-particle-mesh (P3M) method and the corrected Coulomb approach. For better accuracy, particularly in case of nanowire FETs, bandstructure parameters (bandgap, effective masses, and density of states) have been computed via a 20-band nearest-neighbor sp3d5s* tight-binding scheme. Also, since the presence of single impurities in the channel region represents a rare event in the carrier transport process, necessary event-biasing algorithms have been implemented in the simulator that, while enhancing the statistics, results in a faster convergence in the chan-nel current. The study confirms that, due to the presence of single channel charges, both the electrostatics (carrier density) and dynamics (mobility) are modified and, therefore, simultaneously play important roles in determining the magnitude of the current fluctuations. The relative impact (percentage change in the ON current) depends on an intricate interplay of device size, geometry, crystal direction, gate bias, temperature, and energetics and spatial location of the trap.