The disclosed technology generally relates a semiconductor device comprising transistors, and more particularly to a semiconductor device comprising transistors each having a gate stack with a different effective work function, and methods of fabricating such a device. In one aspect, the method of fabricating the semiconductor comprises providing at least two channel regions in the substrate and providing a dielectric layer on the substrate. The method additionally includes forming a plurality of gate regions by providing openings in the dielectric layer. The method further includes providing a gate dielectric layer in the openings and providing on the gate dielectric layer of each of the gate regions a barrier layer stack having different thickness along the different gate regions.