Tunneling field effect transistors and fabrication methods thereof are provided, which include: obtaining a gate structure disposed over a substrate structure; and providing a source region and a drain region within the substrate structure separated by a channel region, the channel region underlying, at least partially, the gate structure, and the providing including: modifying the source region to attain a narrowed source region bandgap; and modifying the drain region to attain a narrowed drain region bandgap, the narrowed source region bandgap and the narrowed drain region bandgap facilitating quantum tunneling of charge carriers from the source region or the drain region to the channel region. Devices including digital modulation circuits with one or more tunneling field effect transistor(s) are also provided.