SILICON CRYSTAL RECTIFIERS 5 



insert tended to give better performance as microwave converters while 

 the p-type, or positive insert, because of greater sensitivity at low voltages, 

 proved to be more useful in test equipment such as resonance indicators in 

 frequency meters. In certain instances also, it was advantageous for the 

 designer to be able to choose the polarity best suited to his circuit design. 

 In contrast, however, to the striking uniformity obtained with the silicon 

 processed in the laboratory, the pyrite inserts were very non-uniform. 

 Active rectification spots on these natural mineral specimens could be 

 found only by tediously searching the surface of the specimen. More- 

 over, rectifiers employing the pyrite inserts showed a greater variation in 

 properties with frequency than those in which silicon was used. 



In addition to providing a satisfactory semi-conductor, it was necessary 

 also to develop suitable materials for use as point contacts. For this use 

 metals were required which had satisfactory rectification characteristics 

 with respect to silicon or pyrites and sutBcient hardness so that excessive 

 contact areas were not obtained at the contact pressures employed in the 

 rectifier assembly. The metals finally chosen were a platinum-iridium 

 alloy and tungsten, which in some cases was coated with a gold alloy. 

 These were employed in the form of a fine wire spot welded to a suitable 

 spring member. The spring members themselves were usually of a wedge 

 shaped cantilever design and were made from coin silver to facilitate elec- 

 trical connection to the spring. Several contact springs of two typical 

 designs are shown in the photograph, Figs. 2b and 2c. 



A typical mounting block arranged for use with the inserts and points 



}: is shown in Fig. 1 (1940) and in Fig. 3. This block was so constructed that 



I it could be inserted in a 70 ohm coaxial line without introducing serious 



|l discontinuities in the line. The contact point of the rectifier was assembled 



1 in the block to be electrically connected to the central conductor of the 



I coaxial radio frequency input fitting, while the crystal insert screwed into 



I a tapered brass pin electrically connected to the central conductor of the 



} coaxial intermediate frequency and d-c output fitting. The tapered pin 



I fitted tightly into a tapered hole in a supporting brass cylinder, but was 



: insulated from the cylinder by a few turns of polystyrene tape several 



; thousandths of an inch thick. This central pin was thus one terminal of a 



i coaxial high-frequency by-pass condenser. The capacitance of this con- 



' denser depended upon the general nature of the circuits in which the block 



was to be used, and was generally about 15 mmfs. The arrangement of 



the point, the crystal insert and their respective supporting members was 



I such that the point contact could be made to engage the surface of the 



silicon at any spot and at the contact pressure desired and thereafter be 



clamped firmly in a fixed position by set screws. Typical direct current 



characteristics of the positive and negative silicon inserts and of pyrite 



inserts assembled and adjusted in this mounting block are shown in Fig. 4. 



