354 BELL SYSTEM TECHNICAL JOURNAL 



emitter current is holes. For the opposite case in which electrons are injected 

 into />-type material,^' the corresponding value of /„,'/t extrapolates to 0.6 

 indicating that for this case 60% of the current is carried by electrons and 

 40% by holes. For these particular specimens the lifetimes are found to be 

 0.9 and 0.41 microseconds respectively. There is a body of evidence, some 

 of which we discuss below, that holes combine with electrons chiefly on the 

 surface of the filament. 



3. The Influenxe of Hole Density ox Point Contacts 



The presence of holes near a collector point causes an increase in its 

 reverse current ; in fact the amplification in a type-A transistor is due to the 

 modulation of the collector current by the holes in the emitter current. The 

 influence of hole density upon collector current has been studied in con- 

 nection with experiments similar to those of Fig. 3. After the hole current 

 and the hole density are measured, a reverse bias of 20 to 40 volts is applied. 

 The reverse current is found to be a linear function of the hole density. 

 Figure 5 shows typical plots of such data. Different collector points, as 

 shown, have quite different resistances. However, once data like that of 

 Fig. 5 have been obtained for a given point, the currents can then be used as a 

 measure of hole density. This experimental procedure for determining hole 

 density is simpler than that involved in using the two points and much 

 better adapted to studies of transient phenomena. It is necessary in em- 

 ploying this technique to keep the current drawn by the collector point 

 somewhat smaller than /b + /« ; otherwise the disturbance in the current 

 flow due to the collector current is too great and the sample of the hole 

 current is not representative. Experiments have shown, however, that this 

 condition is readily achieved and that the collector current may be satis- 

 factorily used as a measure of hole density. 



The hole density also affects the resistance of a point at low voltage. 

 Studies of this effect have also been made in connection with the experi- 

 ment of Fig. 3. After the hole density has been determined from measure- 

 ments of AV and /& -f- /« , a small additional voltage (0.015 volts) was ap- 

 plied between Pi and P2 and the current flowing externally between Pi and P2 

 was measured. From these data a differential conductance, for small cur- 

 rents, is obtained for the two points Pi and P2 in series. As is shown in 

 Fig. 6, this conductance is seen to be a linear function of the hole concen- 

 tration. The conductance of a point contact arises in part from electron flow 

 and in part from hole flow. l""rom experiments using magnetic fields^, it has 

 been estimated that under equilibrium conditions the two contributions to 

 the conductance may be comparable. In connection with Fig. 6 it should 

 be noted that the hole concentration on the abscissa is the average hole 



