Aspects of the present inventive concept provide a semiconductor device capable of enhancing performance and reliability through source/drain engineering in a transistor including an oxide semiconductor layer. The semiconductor device includes a substrate, a metal oxide layer disposed on the substrate, a source/drain pattern being in contact with the metal oxide layer and including a portion protruding from a top surface of the metal oxide layer, a plurality of gate structures disposed on the metal oxide layer with the source/drain pattern interposed therebetween and each including gate spacers and an insulating material layer, the insulating material layer being in contact with the metal oxide layer, and not extending along a top surface of the source/drain pattern, and a contact disposed on the source/drain pattern, the contact being connected to the source/drain pattern.