

810 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



current flow and a substantial drift field near the emitter. Some of these 

 units show an a vahie at all emitter currents quite comparable in magni- 

 tude to that of conventionally formed collectors, and surface treatment 

 alone can also introduce in these unformed collector characteristics 

 anomalies similar to those found in some formed units. It is difficult to 

 visualize a p-n hook structure arising at the germanium surface as a re- 

 sult of the chemical treatments discussed. If such a possibility is pre- 

 cluded, the p-n hook mechanism does not seem necessary to the attain- 

 ing of high a values at low emitter currents, or an a emitter current 

 dependence of the kind normally observed in anomaly-free units. To 

 account for values of a obtained with unformed collectors at low emitter 

 currents, other mechanisms, such as the suggestion of Shockley, involv- 

 ing hole trapping in the germanium under the collector point *"' ^ or the 

 suggestion of Van Roosbroeck, involving conductivity modulation, 

 might in this case be more suitable. 



Further, unformed transistors made by appropriate chemical treat- 

 ments can duplicate qualitatively the electrical characteristics of con- 

 ventionally formed units, including alpha-emitter cvuTent dependence 

 and output characteristic anomalies of types (1), (2) and (3). These 

 phenomena can thus occur under circumstances where a well-defined 

 hook structure is improbable. 



5.3 Surface Properties and Transistor Fornmig 



It has been found that a major factor in determining the forming yield 

 of point-contact transistors is the chemical history of the surface. Thus 

 in processing of point-contact transistors, major attention should be 

 paid to ensuring chemical control of the base wafer surface if the forming 

 yield is to be kept high. On the other hand, considerable variation may 

 apparently be tolerated in storage ambients. Of course it has not been 

 shown that such variations in storage conditions do not have an efl'ect 

 on subsequent reliability of the product. Processes which permit expos- 

 ure of surfaces to solder fumes either before or after etching are to be 

 regarded with suspicion. Monitoring of the reverse emitter diode char- 

 acteristics should prove useful as a means of securing proper control of 

 the pre-forming surface. 



ACKNOWLEDGEMENT 



The authors wish to acknowledge the help of M. S. Jones, who carried 

 out many of the experiments mentioned here, and N. Carthage who did 

 the electroetching work. The continued support and encouragement of 

 N. J. Herbert has been greatly appreciated. 



1- 



