344 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



CONTROL BY OHMIC CONDUCTION 



The carrier-injection shaping techniques work very well for n-typei 

 material. It is also possible to inject a significant number of holes intos 

 rather high resistivity p-type material. But what can be done about: 

 p-type material in general, short of developing cathodic etches? ] 



The ohmic resistivity of p-type material can be used as shown in Fig.!^ 

 10. More etching currect flows through surfaces near the small contact 

 than through more remote surfaces. A substantial dimpling effect is 

 observed when the semiconductor resistivity is equal to the electrolyte 

 resistivity, but improved dimpling is obtained on higher resistivity 

 semiconductor. This result is just what one might expect. But the math- 

 ematical solution for ohmic flow from a point source some distance from 

 a planar boundary between semi-infinite materials of different conduc- 

 tivities shows that the current density distribution does not depend on 

 the conductivities. An important factor omitted in the mathematical 

 solution is the small but significant barrier voltage, consisting largely of 

 electrochemical polarization in the electrolyte. The barrier voltage is; 

 approximately proportional to the logarithm of the current density; 

 while the ohmic voltage drops are proportional to current density. Thus,- 

 high current favors localization. 



ELECTROLYTES FOR ETCHING GERMANIUM AND SILICON » 



The electrolyte usually has two functions in the electrolytic etching 

 of an oxidizable substance. First, it must conduct the current necessary 

 for the oxidation. Second, it must somehow effect removal of the oxida- 

 tion product from the surface of the material being etched. 



The usefulness of an electrolytic etch depends upon one or both of: 



ANY CONTACT, 

 PREFERABLY OHMIC 



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Fig. 10 — Dimpling by ohmic conduction. 



