(EN) A gate stack structure for field effect transistor (FET) devices includes a nitrogen rich first dielectric layer formed over a semiconductor substrate surface; a nitrogen deficient, oxygen rich second dielectric layer formed on the nitrogen rich first dielectric layer, the first and second dielectric layers forming, in combination, a bi-layer interfacial layer; a high-k dielectric layer formed over the bi-layer interfacial layer; a metal gate conductor layer formed over the high-k dielectric layer; and a work function adjusting dopant species diffused within the high-k dielectric layer and within the nitrogen deficient, oxygen rich second dielectric layer, and wherein the nitrogen rich first dielectric layer serves to separate the work function adjusting dopant species from the semiconductor substrate surface.