PRINCIPLES OF TRANSISTOR ACTION 275 



volts/cm. Multiplying by the mobility gives a drift velocity, Vd of 10^ 

 cm/sec, which is approximately thermal velocity.''^ The hole current is 



Ik = nneVdAc (5.4) 



where Ac is the area of the collector contact, and «a the concentration of 

 holes in the barrier. Solving for the latter, we get 



Uk = h/eVdAc (5.5) 



For h = .001 amps Vd = 10^ cm/sec, and A c = 10~® cm, iih is about .6 

 X 10^^, which is of the same order as the concentration of donors. Thus 

 the hole current can be expected to alter the space charge in the barrier by 

 a significant amount, and correspondingly alter the fiow of electrons from 

 the collector. It is believed that current multiplication (values of a > 1) 

 can be accounted for along these lines. 



As discussed in Section II, there is an influence of collector current on 

 emitter current of the nature of a positive feedback. The collector current 

 lowers the potential of the surface in the vicinity of the emitter by an 

 amount 



V = pIJl-KS (5.6) 



The feedback resistance Rp as used in Eq. (2.2) is 



Rr = p/lirs (5.7) 



For p = 10 ohm cm and s = .005 cm, the value of Rp is about 300 ohms, 

 which is of the observed order of magnitude. It may be expected that Rp 

 will decrease as p decreases with increase in emitter current. 



The calculations made in this section confirm the general picture which 

 has been given of the way the transistor operates. 



\T — Conclusions 



Our discussion has been confined to the transistor in which two point 

 contacts are placed in close proximity on one face of a germanium block. 

 It is apparent that the principles can be applied to other, geometrical designs 

 and to other semi-conductors. Some prehminary work has shown that tran- 

 sistor action can be obtained with silicon and undoubtedly other semi-con- 

 ductors can be used. 



Since the initial discovery, many groups in the Bell Laboratories have 

 contributed to the progress that has been made. This work includes 

 investigation of the physical phenomena involved and the properties of 

 the materials used, transistor design, and measurements of characteristics 

 and circuit applications. A number of transistors have been made for ex- 

 perimental use in a pilot production. Obviously no attempt has been made 



