A method for manufacturing a semiconductor device includes forming a metal oxide semiconductor layer and a first insulating layer on a substrate. A gate is formed on the first insulating layer. The first insulating layer is patterned by using the gate as an etching mask so as to expose the metal oxide semiconductor layer to serve as a source region and a drain region. A dielectric layer is formed on the substrate to cover the gate and the oxide semiconductor layer, where the dielectric layer has at least one of hydrogen group and hydroxyl group. A heating treatment is performed so that the at least one of hydrogen group and hydroxyl group reacts with the source region and the drain region. A source electrode and a drain electrode electrically connected to the source region and the drain region respectively are formed on the dielectric layer.