418 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



of 2?-type germanium as part of the same single crystal. Ohmic non- 

 rectifying contacts are securely fastened to the three regions as shown, 

 one being labelled emitter, one base and one collector. In many simple 

 respects, except for change in conductivity type from jp-n-'p in the point- 

 contact (see Fig. 2) to n-p-n in the junction type, the essential behavior 

 is similar. 



As shown in Fig. 6, if the collector junction is biased in the reverse 

 direction, i.e., electrode C biased positively with respect to electrode B, 

 only a small residual back current of holes and electrons will diffuse 

 across the collector barrier as indicated. However, unlike the point- 

 contact device, this reverse current will be very much smaller and rela- 

 tively independent of the collector voltage because the reverse impedance 

 of such bulk barriers is so many times higher than that of the barriers 

 produced near the surface in point-contact transistors. Now again, if 

 the emitter barrier is biased in the forward direction, a few tenths of a 

 volt negative with respect to the base is adequate, then a relatively large 

 forward current of electrons will diffuse from the electron-rich n-type 

 emitter body across the reduced. emitter barrier into the base region. If 

 the base region is adequately thin so that the injected electrons do not 

 recombine in the p-type base region (either in bulk or on the surface), 

 practically all of the injected emitter current can diffuse to the collector 

 barrier; there they are swept through the collector barrier field and 

 collected as an increment of controlled collector current. Hence, again, 

 since the electrons were injected through the low forward impedance 

 and collected through the very high reverse impedance of bulk type p-n 

 barriers, very high voltage amplification will result. No current gain is 

 possible in such a simple bulk structure and the maximum attainable 

 value of alpha is unity. However, because the bulk barriers are so much 

 better rectifiers than the point surface barriers, the ratio of collector 

 reverse impedance to emitter forward impedance is many times greater, 

 more than enough to offset the point-contact higher alpha; thus, the 

 junction unit may have much larger gain per stage. ' ' Fig. 7 is a photo- 

 graph of a developmental model of such a junction transistor called 

 the M1752. 



The upper part of Fig. 8 is a collector family of static characteristics 

 for the M1752 n-'p-n junction transistor. By way of comparison to those 

 of the point contact family, note the much higher reverse impedance 

 of the collector barrier (relatively independent of collector voltage) and 

 the correspondingly smaller collector currents when the emitter current 

 is zero. In fact. Fig. 9 is an expanded plot of the lower left rectangle of 

 the collector family of Fig. 8. The almost ideal straight-line character 



