A semiconductor device can include a semiconductor substrate and an active region in the semiconductor substrate, where the active region can include an oxide semiconductor material having a variable atomic concentration of oxygen. A first source/drain region can be in the active region, where the first source/drain region can have a first atomic concentration of oxygen in the oxide semiconductor material. A second source/drain region can be in the active region spaced apart from first source/drain region and a channel region can be in the active region between the first source/drain region and the second source/drain region, where the channel region can have a second atomic concentration of oxygen in the oxide semiconductor material that is less than the first atomic concentration of oxygen. A gate electrode can be on the channel region and extend between the first source/drain region and the second source/drain region.