Bipolar transistors and MOS transistors are formed in a common process. A semiconductor layer is arranged on an insulating layer. On a side of the bipolar transistors: an insulating region including the insulating layer is formed; openings are etched through the insulating region to delimit insulating walls; the openings are filled with first epitaxial portions; and the first epitaxial portions and a first region extending under the first epitaxial portions and under the insulating walls are doped. On the side of the bipolar transistors and on a side of the MOS transistors: gate structures are formed; second epitaxial portions are made; and the second epitaxial portions covering the first epitaxial portions are doped.