A 250nm focused-ion-beam MOSFET (FIBMOS) has been simulated using a two-dimensional coupled Monte Carlo-Poisson particle-based solver, in which quantum effects have been taken into account by incorporating an effective potential scheme into a classical particle simulator. Inclusion of quantum effects in the analysis of FIBMOS operation is crucial because the high doping density of the p + -implant leads to strong quantum confinement of carriers at the implant/oxide interface. We show that the device drive current, threshold voltage and transconductance are indeed extremely sensitive to the proper treatment of quantization.