808 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



the formability of a particular germanium surface. It is shown that poor 

 diode characteristics are usually associated with poor forming yields. 

 One convenient way of controlling the diode characteristics to ensure 

 successful forming is to etch electrolytically. High current density results 

 in the most desirable surface characteristics. Electro-etched germanium 

 which has been subsequently treated in hydrofluoric acid shows little 

 tendency to oxidize either in room air or dry nitrogen ambient, while 

 superoxol-etched germanium, given the same HF treatment, changes 

 quite rapidly in room air presumably due to oxidation of surface. Sulli- 

 van^^ has also observed differences in the stability of electro-etched and 

 chemically-treated surfaces. 



Different surfaces can be prepared chemically which show more than 

 the amount of variation normally found in pilot and manufacturing 

 process lines. However, extreme variations in storage ambients have 

 relatively little significant effects on any of these surfaces. It is therefore 

 concluded that although certain chemical treatments may affect forming, 

 the variations in process yields are not attributable to interaction 

 between the germanium surface and storage ambients. 



The results of Sections 4.2.2 and 4.2.3 suggest the possibility of passi- 

 vation of the germanium surface. An electro-etched surface followed 

 by an HF treatment exhibits a higher degree of stability to ambient 

 than does a superoxol-etched surface treated in the same way. Treat- 

 ment of a lapped germanium surface with two components of CP4 

 (HF -f HNO3) will inhibit subsequent etching in superoxol. 



The possibility that contamination before etching may affect the char- 

 acteristics of the germanium surface after etching is considered. Experi- 

 ments show that contamination of the germanium with corrosive zinc 

 chloride-ammonium chloride flux before etching significantly affects the 

 rectification properties of the germanium surface obtained after etching. 

 The surface recombination velocity (in so far as it is determinative of the 

 turn-off time of the transistor) is also significantly affected. However, 

 on the basis of the results quoted here, it is not possible to conclude 

 that such contamination can account for an appreciable amount of the 

 unassignable variability in forming yields experienced in pilot and manu- 

 facturing process lines involving soldered base-wafer connections. 



5. GENERAL CONCLUDING REMARKS 



The experiments which have been described have implications which 

 are important in both design and processing of point-contact transistors. 

 These are summarized below: 



