Integrated circuits typically consist of thousands of transistors which are built into and interconnected on large substrates. In their fabrication, consisting of many operations, cumulative structural and locational tolerances limit precision and result in some loss of electrical performance of the transistors. This is particularly true in the case of bipolar transistors where, for example, the collector electrode is in the substrate crystal, the base and emitter electrodes being epitaxially arranged above it. In such a structure, one must consider not only lithographic limitations in positioning but also as to how relative areas of electrodes affect device parameters.
In the prior art, some consideration has been given to the relative size of the base, which, in turn, directly affects base-to-collector capacitance. The ideal relative sizes would be that the base-to-collector area be the same as the base-to-emitter area.
In U.S. Pat. No. 4,380,774, the collector is implanted with ions so there remains only an active region that coincides with the emitter area.
In U.S. Pat. No. 4,428,111, there is grown a base and emitter on a collector region, the base being very thin. Side layers of the same conductivity as the base serve as contacts and control relative areas.
In U.S. Pat. No. 4,593,305, control is achieved by control of doping adjacent to the junctions.
In Asbeck et al, IEEE Electron Device Letters Vol. EDL-5, No. 8, Aug. 1984, p.310, 02 is implanted under the extrinsic base to keep the base area to the emitter size.