In an embodiment, a method includes: forming a gate dielectric layer on an interface layer; forming a doping layer on the gate dielectric layer, the doping layer including a dipole-inducing element; annealing the doping layer to drive the dipole-inducing element through the gate dielectric layer to a first side of the gate dielectric layer adjacent the interface layer; removing the doping layer; forming a sacrificial layer on the gate dielectric layer, a material of the sacrificial layer reacting with residual dipole-inducing elements at a second side of the gate dielectric layer adjacent the sacrificial layer; removing the sacrificial layer; forming a capping layer on the gate dielectric layer; and forming a gate electrode layer on the capping layer.