Source electrodes (3) and drain electrodes (4) are each constituted of an alternation of first layers (5) and second layers (6) made of a germanium and silicon composite. The first layers (5) have a concentration of germanium ranging from 0 % to 10 %, and the second layers (6) have a concentration of germanium ranging from 10 % to 50 %. At least one channel (1) connects two second layers (6a, 6b), respectively, of the source electrodes (3) and drain electrodes (4). The invention involves the etching of source and drain regions connected by a narrow area, in a stack of layers (5, 6). Next, a superficial thermal oxidation of said stack is effected whereby oxidizing the silicon of the germanium and silicon composite having a concentration of germanium ranging from 10 % to 50 % and condensing the germanium Ge. The silicon oxide of the narrow area is eliminated and a gate dielectric (7) and a gate (2) are deposited on the condensed germanium of the narrow area.