Embodiments of the present disclosure relate to methods for forming a source/drain extension. In one embodiment, a method for forming an nMOS device includes forming a gate electrode and a gate spacer over a first portion of a semiconductor fin, removing a second portion of the semiconductor fin to expose a side wall and a bottom, forming a silicon arsenide (Si:As) layer on the side wall and the bottom, and forming a source/drain region on the Si:As layer. During the deposition of the Si:As layer and the formation of the source/drain region, the arsenic dopant diffuses from the Si:As layer into a third portion of the semiconductor fin located below the gate spacer, and the third portion becomes a doped source/drain extension region. By utilizing the Si:As layer, the doping of the source/drain extension region is controlled, leading to reduced contact resistance while reducing dopants diffusing into the channel region.