A High-Frequency Diffused Base 

 Gernianiuni Transistor 



By CHARLES A. LEE 



(Manuscript received November 15, 1955) 



Techniques of impurity diffusion and alloying have been developed which 

 make possible the construction of p-n-p junction transistors utilizing a 

 diffused surface layer as a base region. An important Jeature is the high 

 degree of dimensional control obtainable. Diffusion has the advantages of 

 being able to produce uniform large area junctions which may be utilized in 

 high power devices, and very thin surface layers which may be utilized in 

 high-frequency devices. 



Transistors have been made in germanium which typically have alphas 

 of 0.98 and alpha-cutoff frequencies of 500 mcls. The fabrication, electrical 

 characterization, and design considerations of these transistors are dis- 

 cussed. 



INTRODUCTION 



Recent work ■ concerning diffusion of impurities into germanium 

 and silicon prompted the suggestion that the dimensional control in- 

 herent in these processes be utilized to make high-frecjuency transistors. 



One of the critical dimensions of junction transistors, which in many 

 cases seriously restricts their upper freciuency limit of operation, is the 

 thickness of the base region. A considerable advance in transistor proper- 

 ties 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 tempera- 

 tures and times, the depth of diffused surface layers may be as small as 

 10" cm. Already in the present works layers slightly less than 1 micron 

 (10~ cm) thick have been made and utilized in transistors. Moreover, 

 the times and temperatures required to produce 1 micron surface laj^ers 

 permit good control of the depth of penetration and the concentration 

 of the diffusant in the surface layer with techniciues described below. 



If one considers making a transistor whose base region consists of such 



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