SILICON CRYSTAL RECTIFIERS 23 



proper, that is the total ohmic resistance of the siHcon to current through 

 the point. The capacitance Cb being shunted across the rectifying bound- 

 ary, decreases the efficiency of the device by its by-pass action because the 

 current through it would be dissipated as heat in the resistance Rs. Losses 

 from this source increase rapidly with increased frequency because of the 

 enhanced by-pass action. It would appear, therefore, that to improve effi- 

 ciency it would be important to minimize both Rs and Cb by some method 

 such as reducing the area of the rectifying contact and lowering the body 

 resistance of the silicon employed. For a given silicon material, the imped- 

 ances desired for reasons of circuitry and considerations of mechanical stabiUty 

 place a limit on the extent to which performance may be improved by 

 reducing the contact area. Rs may be reduced by using silicon of lower 

 resistivity, but this generally results in poorer rectification. This impair- 

 ment is due apparently to some subtle change in the properties of the 

 rectifying junction resulting from decreasing the specific resistance of the 

 silicon material. 



Rg (NON-LINEAR 

 BARRIER RESISTANCE) 



Rs I WV 



(SPREADING RESISTANCE) 



vw 



Cb 

 (barrier capacity) 



Fig. 11 — Simplified equivalent circuit of crystal rectifier. 



The answer to this apparent dilemma lies in the application of an oxidizing 

 heat treatment to the surface of the semi-conductor. This process derives 

 from researches conducted independently in this country and in Britain, 

 though there was considerable interchange of information between various 

 interested laboratories. In the oxidizing treatment, apparently the im- 

 purities in the silicon which contribute to its conductivity diffuse into the 

 adhering silica film, thereby depleting impurities from the surface of the 

 silicon. When the oxide layer is then removed by solution in dilute hydro- 

 fluoric acid, the underlying silicon layer is exposed and remains intact as 

 the acid does not readily attack the silicon itself. 



Since decreasing the impurity content of a semi-conductor increases its 

 resistivity, the silicon surface has higher resistivity after the oxidizing 

 treatment than before. Thus by oxidation of the surface of low resistance 

 silicon it is possible to secure the enhanced rectification associated with 

 the high resistance surface layer, while by virtue of the lower resistivity 

 of the underlying material the PR losses through Rs are reduced. 



