In the prior art, French patent application No. 2 626 406 proposes a bipolar transistor which is compatible with MOS technology. As can be seen more clearly in FIG. 1, the bipolar transistor T is made on a semiconductor substrate 1 having a first conductivity type, e.g. P type. Starting from a main face of the substrate 1, the following are formed: firstly lateral insulating zones, thick partitions, or thick oxide zones 2 by the conventional local oxidation of silicon (LOCOS) technique, and secondly an active transistor junction region 3 constituted by a retrograde well making it possible to obtain high concentrations at depth by deep implantation in conventional manner. The retrograde well 3 which acts as a collector possesses a second conductivity type opposite to the first, namely N type in the chosen preferred example.
A base active region 4 having P-type conductivity and disposed between base contact regions 5 on either side thereof is installed in a surface plane of the collector region 3 between two insulating zones 2. The regions 5 are highly doped on the surface after the emitter and spacers have been formed as described below so as to form base contact regions of very low resistivity referred to as extrinsic base regions 9.
An active emitter region 6 of the N.sup.+ type is formed to cover substantially the same face of the substrate over the base region 4. The emitter region 6 is covered by an emitter contact region 7 made of tungsten silicide WS1.sub.2. An insulating layer 10 of silicon oxide is then deposited on the main face of the substrate and windows are formed therein by local photo-etching so as to give access to the base contact regions 5 and so as to implant a collector contact region 11 of the N.sup.+ type formed between two insulating zones 2. Metallization is then performed to obtain metallic contacts 12 and 14 through the windows, respectively for the collector (contact 12) and for the base (contact 14).
Such a transistor whose structure is self-aligned at emitter-base level serves to improve the emitter-base system at high current densities. Although such a structure provides undeniable advantages over the state of the art, it should nevertheless be understood that this transistor has high collector resistance. In addition, forming a surface collector contact gives rise to additional unwanted resistance.
The present invention therefore seeks to propose a bipolar transistor which is compatible with MOS technology and whose collector resistance is considerably improved compared with the known solutions in the state of the art.