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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



hypothesis discussed in the previous section to account for the effect of 

 HF on the unformed transistors. 



Such evidence, however, is at best only indirect evidence for the build- 

 up of an oxide layer on prolonged exposure to room air. In experiments 

 Avith grown p-n junction diodes, the authors have found great variations 

 in the length of time required for the electrical properties of the diodes 

 to recover after short wash periods in low conductivity water. Thus the 

 slow changes mentioned above may at this point result from simply a 

 longer time required for the surface to "dry out" after the washing treat- 

 ment. However, a substantial difference in the physical properties of the 

 oxide layer left by the two etches concerned is still implied. In this con- 

 nection it is also worth noting that hysteresis effects appear primarily in 

 unformed units made on HF treated surfaces. 



The results of these experiments have important implications in the 

 technology of point contact transistors. The results of an application of 

 these results to transistor forming procedures are given in the following 

 section. 



4. RELATION OF GERMANIUM SURFACE PROPERTIES TO TRANSISTOR 

 FORMING 



4.1 Pilot Production Problems 



The pilot production and early manufacturing stages of cartridge- 

 type point-contact transistors has generally been characterized by peri- 

 ods during which the forming yields have been very high and similar 

 periods of very low yield. Often these alternate periods occurred during 

 the use of germanium taken from the same rod-grown or zone-leveled 

 crystal. Considerable effort has been expended in attempting to corre- 

 late these variations in yield to variations, from crystal to crystal, or 

 in different portions of the same crystal, or such bulk properties as re- 

 sistivity or minority carrier lifetime. Although these properties of ger- 

 manium do have some effect on device parameters such as average alpha, 

 reverse emitter current, and Ico , there has not been any positive indica- 

 tion that variations in yield are attributable to the amount of variation 

 of bulk properties normally found in the germanium which meets the 

 specifications of the particular device concerned. 



This problem was compounded during the early stages of the develop- 

 ment of the process for hermetically sealing the point-contact transistor. 

 It was found that although reasonable yields were obtained in the car- 

 tridge process, equivalent transistors in the hermetically sealed structure 

 were made only with greatly reduced yield. Further, although micro- 



