24 BELL SYSTEM TECHNICAL JOVRXAL 



In actual practice the i)roperties of the rectifier are governed by the 

 resistivity of the silicon material, the contact area, and the degree of oxida- 

 tion of the surface. By the controlled alteration of these factors units 

 may be engineered for specific applications. The body resistance of the 

 silicon is controlled by the kind and quantity of the impurities present. 

 Aluminum, beryllium or boron may be added to purified silicon to reduce 

 its resistivity to the desired level. Boron is especially effective for this 

 purpose, the quantity added usually being less than 0.01 per cent. As little 

 as 0.001 per cent has a very pronounced effect upon the electrical properties. 

 The contact area is determined by the design of contact spring employed 

 and the deflection applied to it in the adjustment of the rectifier. The 

 degree of oxidation is controlled by the time and temperature of the treat- 

 ment and the atmosphere employed. 



In the development of the present rectifier processes, certain experimental 

 relationships were obtained between the performance and the contact area 

 on the one hand, and the power handling ability and contact area on the 

 other. These show the manner in which the processes should be changed 

 to produce a desired change in properties. For example. Fig. 12 shows the 

 relationship between the spring deflection applied to a unit and the conver- 

 sion loss at a given frequency. The apparent contact area, (i.e., the area of 

 the flattened tip of the spring in contact with the silicon surface, as measured 

 microscopically) also increases with increasing spring deflection. It will be 

 seen in Fig. 12 that for a given silicon material, the conversion loss at 10,000 

 megacycles increases rapidly with the contact area. The curves tend to 

 reach constant loss values at the higher spring deflections. It is believed 

 that this may be ascribed to the fact that for a given spring size and form, 

 the increment in contact area obtained by successive increments in spring 

 deflection would diminish and finally become zero after the elastic limit of 

 the spring is exceeded. 



The losses plotted in Fig. 12 were measured on a tuned basis, that is, the 

 converter was adjusted for maximum intermediate frequency output at a 

 fixed beating oscillator drive for each measurement. Were these measure- 

 ments made on a fixed tuned basis, that is, with the converter initially ad- 

 justed for maximum intermediate frequency output for a unit to which the 

 minimum spring deflection is applied, and the units with larger deflections 

 then measured without modification of the converter adjustment, even 

 greater degradation in conversion loss than that shown in Fig. 12 would be 

 observed. This results from the dependence of the radio frequency imped- 

 ance upon the contact area. In loss measurements made on the tuned basis, 

 changes in the radio frequency impedance occasioned by the changes in the 

 contact area do not affect the values of mismatch loss obtained, while on the 



