Novel device structures such as dual gate SOI, Ultra thin body SOI, FinFETs, etc. have emerged as a solution to the ultimate scaling limits of conventional bulk MOSFETs. In this paper, we use a semi-classical 3D Monte Carlo device simulator to investigate important issues in the operation of FinFETs. Fast Multipole Method (FMM) has been integrated with the EMC transport kernel to enhance the simulation time. Fin extension length on each side of the gate plays an important role in controlling the device behavior. It is found from the simulation that the presence of single unintentional dopant in the lightly doped or undoped channel has significant effects on device performance particularly near subthreshold regime. Also impurities at the source end of the channel are found to have most significant impact on the device performance.