An embodiment of a semiconductor device includes a semiconductor substrate having a principal surface, spaced-apart source and drain regions separated by a channel region at the principal surface, and a multilayered gate structure located over the channel region. The multilayered gate structure includes a gate dielectric layer in contact with the channel region, a first conductor comprising a metal oxide overlying the gate dielectric layer, a second conductor overlying the first conductor, and an impurity migration inhibiting layer between the gate dielectric layer and the first conductor or between the first conductor and the second conductor.