A semiconductor device having a field effect transistor and a method of forming the same are provided. The semiconductor device preferably includes a device active pattern disposed on a predetermined region of the substrate. The gate electrode preferably crosses over the device active pattern, interposed by a gate insulation layer. A support pattern is preferably interposed between the device active pattern and the substrate. The support pattern can be disposed under the gate electrode. A filling insulation pattern is preferably disposed between the device active pattern and the filling insulation pattern. The filling insulation pattern may be disposed under the source/drain region. A device active pattern under the gate electrode is preferably formed of a strained silicon having a lattice width wider than a silicon lattice.