Integrated circuits with varying gate structures and fabrication methods

A device comprising:

• an integrated circuit, the integrated circuit comprising:
• a varying gate structure disposed over a substrate structure, the varying gate structure comprising a first gate stack in a first region of the substrate structure, a second gate stack in a second region of the substrate structure, a third gate stack in a third region of the substrate structure, and a fourth gate stack in a fourth region of the substrate structure, wherein the first gate stack, second gate stack, third gate stack and fourth gate stack are different gate stacks;
• a first field-effect transistor in the first region of the substrate structure, the first field-effect transistor comprising the first gate stack of the varying gate structure and having a first threshold voltage;
• and
• a second field-effect transistor in the second region of the substrate structure, the second field-effect transistor comprising the second gate stack of the varying gate structure and having a second threshold voltage, wherein the first threshold voltage is different from the second threshold voltage,
• a third field-effect transistor in the third region of the substrate structure, the third field-effect transistor comprising the first gate stack of the varying gate structure and having a third threshold voltage different than the first and second threshold voltages;
• and
• a fourth field-effect transistor in the fourth region of the substrate structure, the fourth field-effect transistor comprising the fourth gate stack of the varying gate structure and having a fourth threshold voltage that is different from the first, second and third threshold voltages,
• wherein the first gate stack and the second gate stack of the varying gate structure comprises a gate dielectric layer, a first cap layer overlying the gate dielectric layer, a second cap layer overlying the first cap layer, a first work-function layer overlying the second cap layer, and a metal layer overlying the second work function layer,
• wherein the second cap layer has a first thickness in the first gate stack and a second thickness in the second gate stack that is different from the first thickness,
• wherein the first gate stack and the second gate stack of the varying gate structure comprises the layers of the first and second gate stacks and a second work function layer positioned between the second conductive cap layer and the first work function layer, and
• wherein the second cap layer has the first thickness in the third gate stack and the second thickness at the fourth gate stack, and
• wherein the second work function layer has a first thickness in the third gate stack and a second thickness in the fourth gate stack that is different from the first thickness.