A semiconductor device and a method of manufacturing a gate stack for such a semiconductor device. The device includes a gate stack that has a gate insulation layer provided over a channel region of the device, and a metal layer that is insulated from the channel region by the gate insulation layer. The metal layer contains work function modulating impurities which have a concentration profile that varies along a length of the metal layer from the source region to the drain region. The gate stack has a first effective work function in the vicinity of a source region and/or the drain region of the device and a second, different effective work function toward a center of the channel region.