Apparatus and circuits with dual threshold voltage transistors and methods of fabricating the same

A circuit, comprising:

• a first transistor including a first gate, a first source and a first drain;
• a second transistor including a second gate, a second source and a second drain;
• a substrate,
• a first layer comprising a first III-V semiconductor material disposed over the substrate;
• a polarization modulation layer disposed above the first layer;
• and
• a passivation layer disposed at least partially on the polarization modulation layer, wherein:
• the first gate is disposed partially on the polarization modulation layer and partially on the passivation layer,
• the second gate is disposed partially on the polarization modulation layer and partially on the passivation layer, and
• the second gate comprises a material that is different than a material of the first gate such that the first and second transistors have different threshold voltages.

A method for forming a semiconductor structure, comprising:

• forming a first layer comprising a first III-V semiconductor material over a substrate;
• forming a polarization modulation layer above the first layer;
• forming a passivation layer at least partially on the polarization modulation layer;
• forming a first transistor over the substrate, wherein the first transistor comprises a first gate structure comprising a first material, a first source region and a first drain region;
• and
• forming a second transistor next to the first transistor over the substrate, wherein the second transistor comprises a second gate structure comprising a second material, a second source region and a second drain region, wherein:
• the first gate structure is formed partially on the polarization modulation layer and partially on the passivation layer;
• the second gate structure is formed partially on the polarization modulation layer and partially on the passivation layer;
• and
• the first material is a different material than that of the second material such that the first and second transistors have different threshold voltages.