18 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



0.1 



0.2 



0.3 



0.4 



0.5 



0.6 



0.7 



Fig. 12 — Dependence of base layer sheet condiictivitj^ on diffusant surface 

 concentrations and diffusion lengths. The lines of constant Ii are essentiallj' lines 

 of constant base sheet conductivity. The ordinate is the surface concentration 

 (in reduced units) of the diffusant which determines the conductivity type of the 

 base layer and the abscissa is the ratio of the diffusion lengths of the two difi'using 

 impurities. 



4.4 Transit Time 



With a knowledge of where the minimum value, ^', of (4.3) occurs, 

 it is possible to calculate over what fraction of the base width the fields 

 are retarding. The interesting quantity here is 



13 - a 



^ is a function of ri/r2 and X and varies only very slowly with ri/r2 . 

 a is also a function of ri/r2 and X and varies only slowly with ri/r2 . 

 The most rapidly changing part of bJi is l^ which depends primarily on 

 r2 as noted above. Fig. 13 is a plot of the constant LR contours in the 

 r2 — X plane for values of A/2 in the range 0.1 to 0.3. This graph is 



