1. Field of the Invention
The present invention is in the field of producing bipolar transistor structures with self-adjusted emitter-base zones wherein both the emitter zone and the base zone in the substrate are produced by diffusion out of structures consisting of doped polysilicon layers which are directly deposited on the substrate and which form the later terminals. Using SiO.sub.2 layers as masking and insulating layers, the base zone is first produced and then the emitter zone is produced centrally therein in such a manner that the active base zone is formed beneath the emitter zone and an inactive base zone is formed symmetrically therewith. Dry etching procedures which produce vertical etching profiles are used in order to structure the SiO.sub.2 and polysilicon layers.
2. Description of the Prior Art
A process of the type herein described is disclosed in German OS No. 32 43 059 Al. The process described therein permits the production of bipolar transistor circuits in a high packing density which have high switching speeds and gates which permit greatly reduced gate transit times. In these bipolar circuits, the base terminal material consists of boron-doped polysilicon which, due to its excessively small grain size, results in undesirably high layer resistances which control the extrinsic base resistance of the self-adjusted bipolar transistors.
A reduction in the p.sup.+ (boron)-doped polysilicon layer resistance may be achieved by a process described in German OS No. 34 02 188.A1. In the process described in that patent, the boron-doped polycrystalline silicon layers are brought from the vaporous phase into the amorphous state by a chemical deposition process and are converted into the polycrystalline state in a later thermal process. These procedures serve to produce a larger grain size together with a smooth layer surface which results in the reduction of the layer resistance to approximately one-third of its normal value.
Another possibility of reducing the extrinsic base resistance consists in modifying the specific geometry. Thus, for example, the article by A. Wieder in the Siemens Research and Development Report, Volume 13 (1984), pp. 246 to 252, describes a reduction of extrinsic base resistance by use of two base terminals.