The field-effect transistors are one of the building blocks of modern electronics. In the last 50 years, transistor technology had a noticeable development. According to Moore's law, the transistor dimensions became and continue to become very small: from the first MOSFET having a channel length of 20 um in 1960 to the actual multigate devices (FinFET and GAAFET) with a channel length of a few nanometers. This continuos race is due to the increasing demand in electronic systems, not only for high integration density, but also for high performance and low power consumption. In this context, this thesis aims to investigates, by means of physical simulations, multigates FETs.