540 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



Figure 11 introduces a new quantity, a, the current amplification factor 

 of the transistor. This quantity is defined by the equation 



Tm + n 



Tc + n 



(3) 



Since rm and Tc are both very large compared to rj, , a is approximately equal 

 to the ratio of fm, to Tc . It will be shown in a later section that this quantity 

 is important in determining some of the circuit properties of the transistor 

 and that many of the circuit properties become more desirable as a ap- 

 proaches unity. 



800 



700 



600 



500 



400 



300 



200 



-0.2 -0,4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0 

 le IN MILLIAMPERES 



Fig. 10 — The emitter resistance is inversely proportional to emitter current. 



It can be seen from Fig. 1 1 that in this transistor a is approximately equal 

 to 0.98 and that it increases slightly with increasing emitter current. The 

 highest value of a so far achieved is 0.9965. 



Those units which have been made in the laboratory so far show consid- 

 erable variation in some of the properties, but this is partly due to the 

 fact that changes have been made deliberately to test one aspect or 

 another of Shockley's theory. The data in table I are presented to indicate 

 what properties have been achieved to date. The collector capacitance Cc 

 will be discussed in a later section. 



General Considerations and Formulae 



It is a consequence of the fact that a is always less than unity in this 

 structure that these transistors are unconditionally stable with all termina- 



