Semiconductor device and method for fabricating the same

A semiconductor device, comprising:

• a substrate;
• a first semiconductor stack having a first threshold voltage and comprising a first insulating stack positioned on the substrate;
• a second semiconductor stack having a second threshold voltage and comprising a second insulating stack positioned on the substrate;
• and
• a third semiconductor stack having a third threshold voltage and comprising a third insulating stack positioned on the substrate;
• wherein the first threshold voltage, the second threshold voltage, and the third threshold voltage are different from each other;
• a thickness of the first insulating stack is different from a thickness of the second insulating stack and a thickness of the third insulating stack;
• and the thickness of the second insulating stack is different from the thickness of the third insulating stack;
• wherein the first insulating stack comprises a first bottom insulating layer positioned on the substrate, and the third insulating stack comprises a third bottom insulating layer positioned on the substrate and a third top insulating layer positioned on the third bottom insulating layer;
• wherein the second insulating stack comprises a second bottom insulating layer positioned on the substrate, a second middle insulating layer positioned on the second bottom insulating layer, and a second top insulating layer positioned on the second middle insulating layer;
• wherein the second semiconductor stack further comprises a second bottom conductive layer positioned on the second top insulating layer;
• and
• wherein the second semiconductor stack further comprises a second functional layer positioned between the second top insulating layer and the second bottom conductive layer, and the second functional layer has a thickness between about 10 angstroms and about 15 angstroms.