30 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



A plot of this function for various values of rj is shown in Fig. 3. For ?? = 0, 

 the above expression reduces to the well known case of a uniformly doped 

 base region. The important feature to be noted in Fig. 3 is that relatively 

 small gradients of the impurity distribution in the base layer can produce 

 a considerable enhancement of the frequency response. 



It is instructive to calculate what the alpha-cutoff f recjuency would be 

 for a base region with a uniform distribution of impurity. The effective 

 thickness of the base layer may be estimated by decreasing the measured 

 thickness of the surface layer by the penetration of the space charge 

 region of the collector and the depth of the alloyed emitter structure. 

 Using a value for the diffusion constant of holes in the base region appro- 

 priate to a donor density of about 10 Vcc, 



300 mc/s ^fa^ 800 mc/s 



This result implies that the frecjuency enhancement due to "built-in" 

 fields is at most a factor of two. In addition it was observed that the 

 alpha-cutoff frequency was a function of the emitter current as shown 

 in Fig. 4. This variation indicates that at least intermediate injection 



<Si 



0.1 



0.2 



0.4 0.6 0.8 1 



6 8 10 



20 



40 60 80 100 



w2 

 <^-U} -g- , (RADIANS) 



Fig. .3 — The variation of | i3 | ver.sii.s frequency for various values of a uniform 

 drift field in the base region. 



