DIFFUSED EMITTER AND BASE SILICON TRANSISTORS 



19 



lor data with ri/r2 = 10. As ri/r2 increases at constant r2 and X, AR 

 decreases slightly. At ri/r2 = 10\ the average change in AR is a decrease 

 of about 25 per cent for constant r2 and X when AR ^ 0.3. The error is 

 larger for values of AR greater than 0.3. It was noted above that when 

 AR becomes greater than 0.3, the retarding fields become dominant. 

 Therefore, this region is of slight interest in the design of a high frequency 

 transistor. 



4.5 A Sample Design 



By superimposing Figs. 11, 12 and 13 the ranges of r2 , ri/r2 and X 

 which are consistent with desired values of y, gt and AR can be deter- 



0.7 



Fig. 1.3 — Dependence of the built-in field distribution on concentrations and 

 diffusion lengths. The lines of constant aR indicate the fraction of the base layer 

 thickness over which built-in fields are retarding. The ordinate is the surface 

 concentration (in reduced units) of the diffusant which determines the conductiv- 

 ity type of the base layer and the abscissa is the ratio of the diffusion lengths of 

 the two diffusing impurities. 



