Source and drain electrodes are each formed by an alternation of first and second layers made from a germanium and silicon compound. The first layers have a germanium concentration comprised between 0% and 10% and the second layers have a germanium concentration comprised between 10% and 50%. At least one channel connects two second layers respectively of the source electrode and drain electrode. The method comprises etching of source and drain zones, connected by a narrow zone, in a stack of layers. Then superficial thermal oxidation of said stack is performed so a to oxidize the silicon of the germanium and silicon compound having a germanium concentration comprised between 10% and 50% and to condense the germanium Ge. The oxidized silicon of the narrow zone is removed and a gate dielectric and a gate are deposited on the condensed germanium of the narrow zone.