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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



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fully used in the fabrication of devices involving n-type semiconductors. 

 Pitting can be reduced relative to "normal" uniform etching by any 

 agency that increases the concentration of holes in the semiconductor. 

 Thus, elevated temperatures, flooding with light, and injection of holes 

 by an emitter all favor smooth etching. 



SHAPING BY MEANS OF INJECTED CARRIERS 



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Hole-electron pairs are produced when light is absorbed by semi- 

 conductors. Light of short wavelength is absorbed in a short distance, 

 while long wavelength light causes generation at considerable depths. 

 The holes created by the light move by diffusion and drift and increase 

 the current flow through an anodic electrolyte-germanium barrier at 

 whatever point they happen to encounter the barrier. In general, more 

 holes will diffuse to a barrier, the nearer the barrier is to the point at 

 which the holes are created. For n-type semiconductors, the current 

 due to the light can be orders of magnitude greater than the dark cur- 

 rent, so that the shape resulting from etching is almost entirely deter- 

 mined by the light. As shown in Fig. 3, the dark current can be made 

 very small by lowering the temperature. 



An example of the shaping that can be done with light is shown in 

 Fig. 8. A spot of light impinges on one side of a wafer of n-type germanium 

 or silicon. The semiconductor is made anodic with respect to an etching 

 electrolyte. Accurately concentric dimples are produced on both sides of 

 the wafer. Two mechanisms operate to transmit the effect to the oppo- 

 site side. One is that some of the light may penetrate deeply before 

 generating a hole-electron pair. The other is that a fraction of the car- 

 riers generated near the first surface will diffuse to the opposite side. 

 By varying the spectral content of the light and the depth within the \ 



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-n-TYPE SEMICONDUCTOR 



LIGHT 



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Fig. 8 — Double dimpling with light. 



