POINT-CONTACT TRANSISTOR SURFACE EFFECTS 



801 



4.2.2 Relation of Unformed Diode Characteristics to Transistor "Forma- 

 bility" 



From the results of the previous sections, it appears that superoxol- 

 etched germanium surfaces treated with reagents in which germanium 

 dioxide is soluble provide point contact diode characteristics unsuited 

 to electrical pulse forming. Part of this difficulty, manifested in the in- 

 ability to reach a specified value of average a without a prohibitive in- 

 crease in I CO , probably results from a lower injection efficiency, 7, for 

 the emitter on such a surface. This seems reasonable in view of the lower 

 forward and higher reverse currents indicated in Table III produced by 

 an HF soak. In Section 4.2.3 evidence will be shown that surface recom- 

 bination is greater on n-type germanium surfaces treated with HF. This 

 effect can also lead to difficulty in forming to high a without increase 

 in Ico , since, for the same drift field, one would expect more minority 

 carriers to die at the surface during their transit to the collector. 



On the other hand, there is evidence for believing that the nature of 

 the forming process itself may be quite different on an HF treated sur- 

 face. Fig. 14(a) shows the time dependence of the collector voltage dur- 

 ing a typical condenser discharge forming pulse. 



The envelope of the voltage pulse follows roughly an exponential de- 

 cay of a condenser-resistor series combination. However, inspection 

 shows that during the discharge time, the resistance of the combination 



400 



300 



_l 

 o 



> 200 



z 



> 



100 



I— ( tri 



..LU 



Z liJ 

 uj Q- 



q: < 



D 



o z 



50 100 150 200 250 300 350 



TIME IN MICROSECONDS 



400 



450 



500 



Fig. 14 — Collector current and voltage versus time for a condenser discharge 

 forming pulse. 



