Apparatus and circuits including transistors with different threshold voltages and methods of fabricating the same are disclosed. In one example, a semiconductor structure is disclosed. The semiconductor structure includes: a substrate; an active layer that is formed over the substrate and comprises a plurality of active portions; a polarization modulation layer comprising a plurality of polarization modulation portions each of which is disposed on a corresponding one of the plurality of active portions; and a plurality of transistors each of which comprises a source region, a drain region, and a gate structure formed on a corresponding one of the plurality of polarization modulation portions. The transistors have at least three different threshold voltages.