Apparatus and circuits including transistors with different threshold voltages and methods of fabricating the same

A circuit, comprising:

• a plurality of transistors each of which comprises:
• a source, a drain, a gate and a polarization modulation portion, wherein:
• the plurality of transistors are formed on a same semiconductor wafer including a plurality of active portions,
• the polarization modulation portion of each of the plurality of transistors is disposed on a corresponding one of the plurality of active portions,
• a threshold voltage of each of the plurality of transistors has a fixed value, and
• threshold voltages of the plurality of transistors have in total at least three different fixed values.

A method, comprising:

• forming a plurality of active portions over a substrate;
• forming a plurality of polarization modulation portions each of which is disposed on a corresponding one of the plurality of active portions;
• and
• forming a plurality of transistors each of which comprises a source region, a drain region, and a gate structure disposed on a corresponding one of the plurality of polarization modulation portions, wherein
• a threshold voltage of each of the plurality of transistors has a fixed value, and
• threshold voltages of the plurality of transistors have in total at least three different fixed values.