DESIGN THEORY OF JUNCTION TRANSISTORS 1287 



carrier admittances are many orders of magnitude smaller than the 

 input and forward transfer terms. The output and feedback admittances 

 decrease ^\dth increase of collector reverse voltage in much the same way 

 that collector capacitance decreases, while the input and forward trans- 

 fer admittances are but little affected. 



This simple picture of the junction transistor is incomplete in that it 

 ignores the important effects of majority carrier resistance in the base 

 region. These effects are discussed in the next section. 



4.0 EFFECTS OF BASE REGION RESISTANCE 



4.1 General 



The resistance of the base region to the flow of currents parallel to 

 the emitter and collector junctions is important primarily because of 

 voltage developed between the base contact and the emitter junction. 

 Necessarily associated with this is power dissipation which is usually, 

 however, of less importance than the feedback effect of the voltage. 



The primary factors in determination of base region feedback voltages 

 are the materials and geometry of the base region and the flow paths for 

 the transverse currents moving through the base region to the base 

 contact. Reduction of the base region resistance effects to equivalent 

 circuit elements permits them to be incorporated in the equivalent cir- 

 cuit obtained in the previous section. This circuit is then an essentially 

 complete model for the electronic mechanisms or functions in junction 

 transistor triodes. 



4.2 Base Region Currents 



Two of the three principal components of the base current have nearly 

 identical origin and flow paths, while the third component differs in 

 origin and may differ greatly in flow paths to the base contact. The dc 

 component of the base current arises principally from recombination of 

 injected holes in the portion of the base between emitter and collector,* 

 as does also the ac component associated with the diffusion admittances. 

 The ac component of base current required to charge the collector barrier 

 capacitance, however, is introduced into the base uniformly over the 

 surface of the collector. The ac component required to charge the emitter 

 capacitance is both small and similar in flow paths to the first compo- 

 nents discussed. 



* In some transistors, much of the recombination occurs on the exposed surface 

 of the base region. This surface recombination may be replaced by a reduction of 

 volume lifetime for a one-dimensional analysis. This is not exact, but is a fair 

 approximation. 



