SILICON CRYSTAL RECTIFIERS 11 



stage in the process the rectification action is developed, and to a considerable 

 degree, controlled. If the point is brought into contact with the silicon 

 surface and a small compressional deflection applied to the spring, direct- 

 current measurements will show a moderate rectification represented by 

 the passage of more current at a given voltage in the forward direction than 

 in the reverse. If the side of the unit is now tapped sharply by means of 

 a small hammer, the forward current will be increased, and, at the same 

 time, the reverse current decreased.^ With successive blows the reverse 

 current is reduced rapidly to a constant low value while the forward current 

 increases, but at a diminishing rate, until it also becomes relatively constant. 

 The magnitude of the changes produced by this simple operation is rather 

 surprising. The reverse current at one volt seldom decreases by less than a 

 factor of 10 and frequently decreases by as much as a factor of 100, while 

 the forward current at one volt increases by a factor of 10. Paralleling 

 these changes are improvements in the high-frequency properties, the 

 conversion loss and noise both being reduced. The tapping operation is 

 not a haphazard searching for better rectifying spots, for with a given 

 silicon material and mechanical assembly the reaction of each unit to tapping 

 is regular, systematic and reproducible. The condition of the sihcon surface 

 also has a pronounced bearing on "tappability" for by modifications of 

 the surface it is possible to produce, at will, materials sensitive or insensitive 

 in their reaction to the tapping blows. 



In the development of the compounds for filling the rectifier, special 

 problems were met. For example, storage of the units for long periods 

 of time under either arctic or tropical conditions was to be expected. Also, 

 for use in air-borne radars operating at high altitudes, where equipment 

 might be operated after a long idle period, it was necessary that the units 

 be capable of withstanding rapid heating from very low temperatures. 

 The temperature range specified was from —40° to -|-70°C. Most organic 

 materials normally solid at room temperature, as the hydrocarbon waxes, 

 are completely unsuitable, as the excessive contraction which occurs at 

 i low temperatures is sufficient to shift the contact point and upset the precise 

 adjustment of the spring. Nor are liquids satisfactory because of their 

 tendency to seep from the unit. However, special gel fillers, consisting 

 of a wax dispersed in a hydrocarbon oil, were devised in Bell Telephone 

 Laboratories to meet the requirements, and were successfully applied by 

 the leading manufacturers of crj^stal rectifiers in this country-. Materials 

 of a similar nature, though somewhat different in composition, were also 

 used subsequently in Britain. Further improvements in these compounds 

 have been made recently, extending the temperature range 10°C at low 



' Southworth and Kin^; loc. cit. 



