1212 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



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Fig. 2 — Apparatus for locating and marking p-n junctions. 



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until the needle rests on the p-n junction as seen by the electrical detec- 

 tor. By moving the sample in the y-direction the same needle scrapes a 

 line through the aquadag. In this line, the reflecting sample surface is 

 bared and thus, a reflecting line is produced within a non-reflecting sur- 

 rounding and can be seen in the fringe pattern. 



If the ridge of the sample is exactly lined up with the y-direction, the 

 needle moves along the junction and the line in the aquadag indicates 

 the position of the junction exactly. To minimize an error due to poor 

 alignment, it is advisable to locate the jimction close to the edge of the 

 aquadag. By doing this on two different sides of the coating, the average 



