The present application discloses a semiconductor device and a method for fabricating the semiconductor device. The semiconductor device includes 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. 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.