346 



BELL SYSTEM TECHNICAL JOCKS A L 



observed on the oscilloscope, has the wave form shown in part (b) of the 

 figure. 



These data are interpreted as follows: When the emitter circuit is closed, 

 the electrons in the emitter wire start to flow away from the germanium 

 (i.e. positive current flows into the germanium). These electrons are furnished 

 by an electron flow in tlie germanium towards the point of contact. The 

 flow in the germanium may be either by the excess electron process or by 

 the hole process. In Fig. 2 we illustrate these two possibilities. At first 



-^83 



B, 



1 



T 



■xxKWsVS^VS^VVH^yXyCAVl - 



u 



J 



n-TYPE 

 GERMANIUM 



TO 



CATHODE-RAY 

 OSCILLOSCOPE 



(b) 



ti ta "t.3 t4 



TIME,t >■ 



Fig. 1 — Experiment to investigate the behavior of holes injected into «-type germanium 



(a) Experimental arrangement. 



(b) Signal on oscilloscope showing delay in hole arrival at h in respect to closing S 

 at /i and delay in hole departure at ti in respect to opening S at /s . 



glance it might appear that the difference between these two processes is 

 unimportant since the net result in both cases is a transfer of electrons from 

 the germanium to the emitter point. There is, however, an important differ- 

 ence, one which makes several forms of transistor action possible. In the case 

 of the hole process an electron is transferred from the valence band struc- 

 ture to the metal. After this the hole moves deeper into the germanium. As 

 a result the electronic structure of the germanium is modified in the neigh- 

 borhood of tlie emitter point by the presence of the injected holes. 



Under the influence of the electric field !>, the injected holes drift toward 



