A semiconductor device including a conductive line on a substrate, a first gate electrode on the conductive line, a second gate electrode separated by a gate isolation insulating layer on the first gate electrode, a first channel layer on a side surface of the first gate electrode, with a first gate insulating layer therebetween, a first source/drain region on another side surface of the first gate electrode, a second channel layer on another side surface of the second gate electrode on a side that is opposite to the first channel layer, with a second gate insulating layer therebetween, a second source/drain region on the second channel layer, and a third source/drain region on the first channel layer and on a side surface of the second gate electrode on a same side as the first channel layer may be provided.