Characteristics of UHF transistors using autoregistered structures
The basis of a novel bipolar transistor structure was proposed by Dr R. Aubusson of Middlesex Polytechnic in 1977. The novelty lies in replacing the conventional overlay transistor's P+ base grid with a refractory metal grid, in order (a) to lower the base resistance and (b) to autoregister the emitter. It was claimed that the linearity of the transistor would also be improved. A number of questions raised by this idea have been investigated, the methods and conclusions of which are presented here. Plausible structures, using the metal base grid, are proposed and compared with conventional structures. Some advantages are seen to be possible. The current understanding of distortion analysis applied to transistors is reviewed. The main ideas are presented in a unified manner and are extended to higher order. A number of the transistor's second order effects are analysed in a novel fashion. The metal base grid transistor is analysed and compared with conventional transistors, with favourable results. Practical aspects of fabricating the metal base grid transistor were investigated. A procedure for deposition has been determined and is presented here along with the film physical and electrical characteristics. Analysis of the tungsten-silicon interface shows the suitability of the metallization as a base grid. Suitable means of delineating the tungsten film have been assessed and a working procedure determined. Subsequent deposition of various insulators has been investigated and the problems associated with the readily oxidized tungsten film have been overcome. Formation of the emitter, requiring further high temperature processing, has been assessed in view of the limitations imposed by the preformed base metallization. In summary, it has been shown that the novel structure can be constructed and that significant performance improvement is to be expected, although a full realization was not possible within the resource constraints of the project.