A semiconductor device is provided in accordance with some embodiments. The semiconductor device includes an interfacial layer disposed over a channel region, a gate dielectric structure disposed over the channel region, and a gate electrode disposed over the gate dielectric structure. The gate dielectric structure includes a first layer of an oxide of a first metal disposed over the interfacial layer and a second layer of an oxide or silicate of a second metal disposed over the first layer. The first layer has a first thickness, and the second layer has second a thickness that is at least three times greater than the first thickness. An oxygen areal density of the oxide of the first metal is greater than an oxygen areal density of the oxide of the second metal.