A semiconductor device includes a substrate. A planar insulating layer is disposed on an upper surface of the substrate. A channel region is disposed above the planar insulating layer. A gate electrode is disposed on the channel region. The semiconductor device includes a source region and a drain region. Each of the source region and the drain region is disposed on the substrate and is connected to the channel region. The planar insulating layer has a length equal to or greater than a length of the channel region, and the planar insulating layer includes first and second insulating layers having different permittivities.