A tunnel field effect transistor includes a source region and a drain region, positioned on a substrate, a channel region positioned between the source region and the drain region and having a first length in a first direction, a gate electrode positioned on the channel region, and a gate insulating layer positioned between the channel region and the gate electrode, wherein the source region is doped with impurities of a first conductivity type and the drain region is doped with impurities of a second conductivity type that is different from the first conductivity type, and one of the source region and the drain region includes an extension region extending toward the other region, the extension region being positioned under the channel region to form a constant current independent of a gate voltage of the gate electrode.