Semiconductor devices with germanium-rich active layers and doped transition layers

A semiconductor device, comprising:

• a plurality of germanium (Ge)-rich nanowires aligned vertically over a silicon (Si) substrate;
• a SiGe transition layer disposed between the plurality of nanowires and the substrate;
• a p-type δ-doped SiGe layer disposed below the SiGe transition layer;
• an n-type doped SiGe layer disposed between the p-type δ-doped SiGe layer and the substrate;
• a gate stack disposed over the plurality of nanowires and completely surrounding a length of each of the plurality of nanowires;
• spacers disposed adjacent the gate stack;
• and
• source/drain regions disposed adjacent to the spacers on opposite sides of the gate stack, in contact with the plurality of nanowires.

A method of fabricating a nanowire semiconductor device, the method comprising:

• receiving a semiconductor device stack comprising:
• a germanium (Ge)-rich device layer disposed on a sacrificial semiconductor layer comprising more silicon (Si) than that in the device layer
• a p-type δ-doped SiGe layer buried below the sacrificial semiconductor layer;
• etching at least a partial thickness of the sacrificial semiconductor layer selectively to the device layer with a wet etchant to undercut the device layer and form a discrete channel region of a nanowire;
• and
• forming a gate stack completely surrounding the discrete channel region of the nanowire.