The present disclosure provides in various aspects methods of forming a semiconductor device, methods for forming a semiconductor device structure, a semiconductor device and a semiconductor device structure. In some illustrative embodiments herein, a gate structure is formed over a non-planar surface portion of a semiconductor material provided on a surface of a substrate. A doped spacer-forming material is formed over the gate structure and the semiconductor material and dopants incorporated in the doped spacer-forming material are diffused into the semiconductor material close to a surface of the semiconductor material so as to form source/drain extension regions. The fabricated semiconductor devices may be multi-gate devices and, for example, comprise finFETs and/or wireFETs.