34S BELL S\yrEM TECIIMCAL JOURS AL 



transmission along a conductor being applicable to it. This high speed of 

 transmission does not, of course, imply a correspondingly high velocity of 

 motion of the current carriers. In fact the rapidity of signal transmission 

 has nothing to do with the speed of the carriers and comes about as follows: 

 If the ohmic material is an electronic conductor, then the withdrawal of a 

 few electrons by the emitter current produces a local positive charge. This 

 positive charge produces an electric field which progresses with the speed 

 of light and exerts a force on adjoining electrons so that they move in to 

 neutralize the space charge. The net result is that electrons in all parts of 

 the specimen start to drift practically instantaneously. They flow into the 

 specimen from the end terminals to replace the electrons flowing out at the 

 emitter point and no appreciable change in density of electrons occurs any- 

 where within the specimen.* 



The distinction between the process just described and that occurring 

 when holes are injected into germanium is of great importance in under- 

 standing many effects connected with transistor action. One way of sum- 

 marizing the situation is as follows: In a sample having carriers of one type 

 only, electrons for example, it is impossible to alter the density of carriers 

 by trying to inject or extract carriers of the same type. The reason. is, as 

 described above (or proved in the footnote), that such changes would be 

 accompanied by an unbalanced space charge in the sample and such an 

 unbalance is self-annihilating and does not occur.f 



When holes are injected into ;/-type germanium, they also tend to set 

 up a space charge. Once more this space charge is quickly neutralized by an 

 electron flow. In this case, however, the neutralized state is not the normal 

 thermal equilibrium state. Instead the number of current carriers present has 

 been increased by the injected holes and by an equal number of electrons 

 drawn in to neutralize the holes. The total number of electrons in the speci- 

 men will thus be increased, the extra electrons coming in from the metal 

 terminals which complete the circuit with the emitter point. The presence 

 of the holes and the neutralizing electrons near the emitter point modify the 

 conductivity. As we shall show below, this modification of conductivity may 

 be so great that it can be used to measure hole densities and also to give 

 power gain in modified forms of the transistor. We shall summarize this 

 situation as follows: /;/• a semiconductor conlainiug substantially only one type 

 of current carrier, it is impossible to increase the total carrier co)icentralion by 



* This is a descrii)tion in words of the result ordinarily expressed in terms of the dielectric 

 relaxation time obtained as follows: V-/ = —p,I = aE= —aV^, V"* = — 47rpA = 

 p /a so that p = po exp [- (47r<r/\)/], where / = current density, p = charge density, (r = 

 conductivity, E = electric field, ^ = electrostatic potential, k = dielectric constant. 



t In the'case of modulation of conductivity by surface charges," a net charge is jiro 

 duced by the field from the condenser plate. The changed charge density extends slightl\- 

 into thc'specimen but should not be confused with the true vokime effect of hole injection. 

 Such space charge layers are discussed in other articles in this issue.'' '- 



