The influence of quantum mechanical size quantization effects and unintentional doping on narrow-width silicon on insulator (SOI) device structure, was investigated using computer simulation. The effective potential approach was used, and it was incorporated into the existing 3D Monte Carlo particle-based simulator. It was found that due to the size-quantization effect, a dopant ion trapped in the center of the channel produced the maximum fluctuations. Single impurity at the source end of the channel affected the drain current most, and produced the maximum shift in the threshold voltage. The largest effect was found in the subthreshold/weak-inversion regime where the density of the mobile carriers was very low.