A semiconductor device and/or gate structure having a composite dielectric layer and methods of manufacturing the same is provided. In the semiconductor device, gate structure, and methods provided, a first conductive layer may be formed on a substrate. A native oxide layer formed on the first conductive layer may be removed. A surface of the first conductive layer may be nitrided so that the surface may be altered into a nitride layer. A composite dielectric layer including the first and/or second dielectric layers may be formed on the nitride layer. A second conductive layer may be formed on the composite dielectric layer. The first dielectric layer may include a material having a higher dielectric constant. The second dielectric layer may be capable of suppressing crystallization of the first dielectric layer.