A High-Frequency Diffused Base Germanium Transistor

Recent work x - 2 concerning diffusion of impurities into germanium and silicon prompted the suggestion 3 t h a t the dimensional control inherent in these processes be utilized to make high-frequency transistors. One of the critical dimensions of junction transistors, which in many cases seriously restricts their upper frequency limit of operation, is the thickness of the base region. A considerable advance in transistor properties can be accomplished if it is possible to reduce this dimension one or two orders of magnitude. The diffusion constants of ordinary donors and acceptors in germanium are such that, with n realizable temperatures and times, the depth of diffused surface layers may be as small as 10"6 cm. Already in the present works layers slightly less than 1 micron (10 - 4 cm) thick have been made and utilized in transistors. Moreover, the times and temperatures required to produce 1 micron surface layers permit good control of the depth of penetration and the concentration of the diffusant in the surface layer with techniques described below. If one considers making a transistor whose base region consists of such 23