1. A method of fabricating a self-aligned metal-semiconductor field effect transistor, the transistor being formed on a semi-insulating semiconductor substrate and comprising a current channel and associated source, gate and drain electrodes, characterized in that it comprises the following steps: forming an active channel layer at the surface of said semiconductor substrate, depositing a refractory metal gate layer over said substrate surface, selectively etching said gate layer to form said gate electrode, depositing a thin dielectric layer over said gate electrode and over said substrate surface, etching said dielectric layer so as to form an insulating sidewall layer on the vertical ediges of said gate electrode, forming a highly doped continuous semiconductor contact layer over said substrate surface, over said gate electrode and over said insulating sidewall layer, said contact layer being mono-crystalline material over said substrate surface and poly-crystalline material over said gate electrode and over said insulating sidewall layer, removing said poly-crystalline material, and depositing and selectively etching a metal layer over said mono-crystalline material to form said source and said drain electrodes.