A semiconductor device may include a plurality of first active fins protruding from a substrate, each of the first active fins extending in a first direction; a second active fin protruding from the substrate; and a plurality of respective first fin-field effect transistors (finFETs) on the first active fins. Each of the first finFETs includes a first gate structure extending in a second direction perpendicular to the first direction, and the first gate structure includes a first gate insulation layer and a first gate electrode. The first finFETs are formed on a first region of the substrate and have a first metal oxide layer as the first gate insulation layer, and a second finFET is formed on the second active fin on a second region of the substrate, and the second finFET does not include a metal oxide layer, but includes a second gate insulation layer that has a bottom surface at the same plane as a bottom surface of the first metal oxide layer.