HOLE INJECTION IN GERMANIUM 



347 



Ihc collector i)oinl with velocity y.,>E, where Mp is the mobility of a hole, and 

 thus tra\-erse the distance /. to the collector point in a time L/npE. When 

 they arrive at the collector point, they increase its reverse current and pro- 

 duce the si^Mial shown at /n . 



There are two important differences between the signal produced at k 

 and that produced at k . The signal at /i , which is in a sense a pickup signal, 

 would l)e produced even if no hole injection occurred. We shall illustrate this 

 by considering the case of a piece of ohmic material substituted for the 



METAL 



SEMICONDUCTOR 



ELECTRON 

 GAS 



MOTION OF HOLE 



Fig. 2 — Electron flow to the metal may be jjroduced by an excess electron moving 



toward the metal or by bonding electrons jumping (dashed arrows) successively 



into a hole thus displacing the hole deeper into the semiconductor. 



germanium. Conventional circuit theory applies to such a case; however, in 

 order to contrast this purely ohmic case with that of hole injection, we shall 

 also give a description of the conventional theory of signal transmission in 

 terms of the motion of the carriers. According to conventional circuit theory, 

 the addition of the current I, would simply produce an added IR drop due 

 to current flow in the segment of the specimen to the right of the collector. 

 This voltage drop is denoted as hRa in part (b), Ra representing the proper 

 combination of resistances to take into account the way in which /< divides 

 in the two branches. This signal will be transmitted from the emitter to the 

 collector with practically the speed of light — the ordinary theory of signal 



