In an embodiment, a semiconductor device includes a first channel region disposed in a first device region over a substrate; a first gate dielectric layer disposed over the first channel region; and a gate electrode disposed over the first gate dielectric layer. The first gate dielectric layer includes a first dipole dopant and a second dipole dopant. The first dipole dopant along a thickness direction of the first gate dielectric layer has a first concentration peak, and the second dipole dopant along the thickness direction of the first gate dielectric layer has a second concentration peak. The second concentration peak is located between the first concentration peak and an upper surface of the first gate dielectric layer. The second concentration peak is offset from the upper surface of the first gate dielectric layer.