Generally, the present disclosure provides example embodiments relating to tuning threshold voltages in transistor devices and the transistor devices formed thereby. Various examples implementing various mechanisms for tuning threshold voltages are described. In an example method, a gate dielectric layer is deposited over an active area in a device region of a substrate. A dipole layer is deposited over the gate dielectric layer in the device region. A dipole dopant species is diffused from the dipole layer into the gate dielectric layer in the device region.