A semiconductor device is disclosed comprising an improved gate dielectric layer formed of a high dielectric alloy-like composite together with a method for fabricating the same. The semiconductor device comprises a semiconductor substrate and a gate dielectric layer consisting essentially of a high-k alloy-like composite containing a first element, a second element, and oxygen (O). The first element is at least one member selected from a first group consisting of Al, La, Y, Ga, and In. The second element is at least one member selected from a second group consisting of Hf, Zr, and Ti. A diffusion barrier is formed on the gate dielectric layer, and a gate is formed on the diffusion barrier.