Semiconductor transistors providing various operating voltages

A semiconductor device comprising:

• a substrate comprising a first region and a second region;
• a first gate structure in the first region and comprising a first gate dielectric layer, a first conductive layer, a second conductive layer, and a third conductive layer, stacked in sequence;
• and
• a second gate structure in the second region and comprising a second gate dielectric layer, the second conductive layer, and the third conductive layer, stacked in sequence,
• wherein the first gate dielectric layer and the first conductive layer comprise a first element, and wherein the second conductive layer and the second gate dielectric layer are free of the first element,
• wherein the second gate dielectric layer is positioned on at least a first surface and a second surface of the second conductive layer, and
• wherein a first transistor comprising the first gate structure and a second transistor comprising the second gate structure define respective p-type MOSFETs.