In this paper, we investigate random doping fluctuation effects in trigate SOI MOSFETs by solving the three-dimensional (3D) Poisson, drift-diffusion and continuity equations numerically. A single doping impurity atom is introduced in the undoped channel region of the device and the resulting shift of threshold voltage is measured from the simulated I-V characteristics. This enables the derivation of the threshold voltage shift (Delta V(TH)) for any arbitrary location of the doping atom in the transistor. Based on an analysis of a sub-20 nm trigate MOSFET device, we find that the typical variation of V(TH) per doping atom is a few tens of mV. Inversion-mode (IM) trigate devices are more sensitive to the doping fluctuation effects than accumulation-mode (AM) devices. The threshold voltage shift arising from doping fluctuations is maximum when the doping atom is near the center of the channel region, which means the original Sol film doping, the random contamination effects or any other impurity doping in the channel region is more important than atoms introduced in the channel by the S/D implantation process for sub-20 nm transistors. (C) 2008 Elsevier Ltd. All rights reserved.