III-V MOSFET with self-aligned diffusion barrier

A method for forming a field effect transistor with an implanted source, the method comprising the steps of:

• forming a gate stack on a channel layer of the field effect transistor;
• forming a first spacer on a sidewall of the gate stack, the implanted source being self-aligned with the gate stack and the first spacer;
• forming a raised source at least partially on the implanted source after formation of the implanted source, the raised source comprising III-V material;
• forming an interfacial layer at least partially on the raised source, the interfacial layer comprising at least one of silicon and germanium;
• forming a metal layer at least partially on the interfacial layer, the metal layer comprising transition metal;
• and
• forming a diffusion barrier at least partially on the raised source, the diffusion barrier comprising transition metal from the metal layer bonded to the at least one of silicon and germanium from the interfacial layer.