Provided are field effect transistors and methods of fabricating the same. The transistor may include a substrate with an active pattern, the active pattern having a top surface and two sidewalls, a gate electrode proximal to the top surface and the sidewalls of the active pattern and crossing the active pattern, a gate spacer covering a sidewall of the gate electrode, a gate dielectric pattern at a bottom surface of the gate electrode, a source electrode on the active pattern at one side of the gate electrode, a drain electrode on the active pattern at another side of the gate electrode, and silicide patterns on surfaces of the source and drain electrodes, respectively. The gate dielectric pattern includes at least one high-k layer and the gate spacer has a dielectric constant that is smaller than that of the gate dielectric pattern.