POINT-CONTACT TRANSISTOR SURFACE EFFECTS 773 



form an 7i-type inversion layer under the point (p-n hook) which, when 

 the point is under reverse bias, acts as an electron emitter. Such a situa- 

 tion might arise as a result of diffusion of impurities from the collector 

 point at the high temperature reached during the forming pulse. An 

 acceptor element, such as copper, with a high diffusion coefficient 

 might penetrate substantially farther into the germanium than donor 

 elements such as phosphorous or antimony^^ with lower diffusion con- 

 stants. Thus, the donor concentration near the point might be substan- 

 tially higher than the acceptor concentration if the solubility of the 

 acceptor element is low. 



On the other hand, an appreciable number of donor atoms may pene- 

 trate the germanium as far as do the acceptors. Thus, the equilibrium 

 value of Wp may be increased simply by decreasing the effective concentra- 

 tion of acceptors in the p-layer. Such a case might arise when a collector 

 point such as copper is doped with a suitable amount of a donor element 

 with a large diffusion coefficient and limited solubility. 



The observation of regions of melted germanium^ under heavily 

 formed points gives evidence for a somewhat different interpretation of 

 the forming process. It has been suggested that forming is essentially 

 a remelt process. For example, forming of a phosphor-bronze point may 

 produce a copper-germanium eutectic, allowing the introduction of a 

 sizeable phosphorus concentration in the remelt region which is main- 

 tained after freezing. Thus the depth of penetration of the donor ele- 

 ment depends upon the size of the remelt region, and the penetration of 

 the acceptor element depends upon its solid state diffusion coefficient. 

 This mechanism can lead either to the formation of a p-n hook, or at 

 least to a Iyer of p-germanium with a high equilibrium electron concen- 

 tration. 



Whatever the reason for the decrease in resistance of the collector 

 barrier, if it is sufficient, the magnitude of E(r) for r > r^ can be increased 

 by forming to sufficient value to ensure efficient collection of holes and 

 a transport factor /3 close to unity. 



It would then be expected that for a formed donor-free point, such 

 as the beryllium-copper alloy points often used as unformed emitters, 

 the formed p-region would have a high acceptor concentration, n^ would 

 be small, and under reverse bias, the magnitude of V j would be large, 

 with I /co I , I V{r^ I , and average a small, [solid curve. Fig. 1(b). On 

 the other hand, a formed phosphor bronze point of the kind conven- 

 tionally used to make transistor collectors, should exhibit under reverse 

 bias, a lesser magnitude of V j , with | /eo | , | Vir^) \ , and a as much as 

 an order of magnitude larger, (dashed line in Fig. 1(b)]. 



