2 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



that it is possible to envision transistors with base layer thicknesses of a 

 micron and frequency response of several thousand megacycles per 

 second. 



A major deterent to the application of diffusion to silicon transistor 

 fabrication in the past was the drastic decrease in lifetime which generally 

 occurs when silicon is heated to the high temperatures required for dif- 

 fusion. There was also insufficient knowledge of the diffusion parameters 

 to permit the preparation of structures with controlled layer thicknesses 

 and desired dopings. Recently the investigations of C. S. Fuller and co- 

 workers have produced detailed information concerning the diffusion of 

 Group III and Group V elements in silicon. This information has made 

 possible the controlled fabrication of transistors with base layers suffi- 

 ciently thin that high alphas are obtained even though the lifetime has 

 been reduced to a fraction of a microsecond. In a cooperative program 

 with Fuller, diffusion structures were produced which have permitted 

 the fabrication of transistors whose electrical behavior closely approxi- 

 mates the behavior anticipated from the design. This paper describes 

 these techniques which have resulted in high alpha silicon transistors 

 with alpha-cutoff of over 100 mc/sec. 



1.0 FABRICATION OF THE TRANSISTORS 



Fuller's work has shown that in silicon the diffusion coefficient of a 

 Group III acceptor is usually 10 to 100 times larger than that of the 

 Group V donor in the same row in the periodic table at the same tem- 

 peratures. These experiments were performed in evacuated silica tubes 

 using the Group III and Group V elements as the source of diffusant. 

 Under these conditions a particular steady state surface concentration 

 of the diffusant is produced and the depth of diffusion is sensitive to 

 this concentration as well as to the diffusion coefficient. The experiments 

 show that the effective steady state surface concentration of the donor 

 impurities produced under these conditions is ten to one hundred times 

 greater than that of the acceptor impurities. Thus, by the simultaneous 

 diffusion of selected donor and acceptor impurities into n-type silicon 

 an n-p-n structure will result. The first n-la,yer forms because the surface 

 concentration of the donor is greater than that of the acceptor. The 

 p-laycr is protluced because the acceptor diffuses faster than the donor 

 and gets ahead of it. The final n-region is simply the original background 

 doping of the n-type silicon sample. It has been possible to produce n-p-n 

 structures by the simultaneous diffusion of several combinations of 

 donors and acceptors. Often, however, the diffusion coefficients and 

 surface concentrations of the donors and acceptors are such that opti- 



1 C. S. Fuller, private communication. 



