8 BELL SYSTEM TECHNICAL JOURNAL 



the block structure required considerable skill. With recognition of the 

 military importance of silicon crystal rectifiers, effort was intensified in 

 the development of standardized structures suitable for commercial pro- 

 duction. 



In the 1940-1941 period, contributions to the design of silicon crystal 

 rectifiers were made by British workers as a part of their development of 

 new military implements. For these projected military' uses, the problem 

 of replacement and interchangeability assumed added importance. The 

 design trend was, therefore, towards the development of a cartridge type 

 structure with the electrical adjustment fixed during manufacture, so that 

 the unit could be replaced easily in the same manner as vacuum tubes. 



In the latter part of 1941 preliminary information was received in this 

 country through National Defense Research Committee channels on a 

 rectifier design originating in the laboratories of the British Thomson- 

 Houston Co., Ltd. A parallel development of a similar device was begun 

 in various American laboratories, including the Radiation Laboratory at 

 the Massachusetts Institute of Technology, and Bell Telephone Labora- 

 tories. In the work at Bell Laboratories, emphasis was placed both on 

 development of a structure similar to the British design and on explora- 

 tion and test of various new structures which retained the features of 

 socket interchangeability but which were improved mechanicalh- and 

 electrically. 



In the work on the ceramic cartridge, the external features of the British 

 design were retained for reasons of mechanical standardization but a number 

 of changes in process and design were made both to improve performance 

 and to simplify manufacture. To mention a few, the position of the silicon 

 wafer and the contact point were interchanged because measurements 

 indicated that an improvement in performance could thereby be obtained. 

 To obviate the necessity for searching for active spots on the surface of 

 the silicon and to improve performance, fused high purity silicon was 

 substituted for the "commercial" silicon then employed by the British. 

 The rectifying element was cut from the ingots by diamond saws, and 

 carefully polished and etched to develop optimum rectification character- 

 istics. Similar improvements were made in the prej^aration of the point 

 or "cats whisker", replacing hand operations l:)y machine techniques. To 

 protect the unit from mechanical shock and the ingress of moisture, a sjiecial 

 imjjregnating comjjound was de\'eloped which was completely satisfactory 

 even under conditions of rapid changes in temperature from —40° to 4-70°C. 

 All such improvements were directed towards ini]iro\ing quality and 

 establishing techniques for mass production. 



In this early work time was at a jircmium because of the need for prompt 

 standardization of the design in order that radar system designs might in 



