4 BELL SYSTEM TECHNICAL JOURNAL 



As the properties of various semi-conductors were known to vary widely, 

 an essential part of the early work was a survey of the properties of a number 

 of minerals and metalloids potentially useful as rectifier materials. There 

 were examined and tested approximately 100 materials, including zincite, 

 molybdenite, galena, iron pyrites, silicon carbide, and silicon. Of the 

 materials investigated most were found to be unsuitable for one reason 

 or another, and iron pyrites and silicon were selected as having the best 

 overall characteristics. The subsequent studies were then directed toward 

 improving the rectifying material, the rectifying surface, the j^oint contact 

 and the mounting structure. 



Fig. 2 — Rectilicr inserts untl contact jxjints lor use in early 3(K)t) megacycle converters. 

 Overall length of insert ^-inch approximately. 



i'"()r use at freciuencies in the region of .-^OOO megac}-cles standard demount- 

 able elements, consisting of rectitier "inserts" and contact points, were 

 develojied for use in various housings or mounting blocks, depending upon 

 the j)articular circuit requirements. The rectitier "inserts" consisted of 

 small wafers of iron pyrite or silicon, soldered to hexagonal brass studs as 

 shown in Fig. 2a. In these devices the surface of the semi-conductor was 

 prei)ared by grinding, polishing, and etching to develop good rectification 

 characteristics. Our knowledge of the metallurgy of silicon had acKanced 

 by this time to the stage where a uniformly acti\e rcctilier surface could 

 be j)roduccd and searching for active spots was not nccessar\'. l'\irther- 

 more, it was jiossible to ])repare inserts of a jiositive or negative \ariety, 

 signifying that the easy direction of current llow was obtained with the 

 silicon i)ositive with respect to the point or \ice \ersa. Owing to a greater 

 noiilincarity of the current \-oUage characteristic, the n-t)"pe or negative 



