23 



NON-THERMIONIC DIODES 



Non-thermionic diodes rely for their action on the rectifying properties of 

 junctions between certain pairs of materials. The important pairs are : 



Copper-Copper oxide 

 Selenium-Iron 

 P Germanium-N Germanium 

 P Sihcon-N Silicon 



The letters P and N associated with germanium and silicon denote the presence 

 of minute quantities of specially introduced contaminants which make rectifi- 

 cation possible. In each case the direction of easy current flow is from the 

 first to the second material. Terminals are labelled either by colours or signs, 

 as shown in Figure 23.1. 



+ Black Red 



-W ° or c ^1 — ^ 



Half -wave 



+ Black Red 



-M-r-H- or ° M I M ° 



Green 





Green Green 



0' •»! I M- 



T 



Red 



Full wave 



Voltage doubter 



^ or 



Bridge 



Red 

 Green ^^Oir Green 



Black 



Figure 23.1 



Each of these four junctions has associated with it, at a particular tempera- 

 ture, two important ratings : 



(1) A maximum mean forward current, such that the heat produced shall 

 not damage the junction. This current is roughly proportional to the junction 

 area. 



(2) A maximum backward voltage or 'Peak Inverse Voltage', such that 

 the electric field produced shall not damage the junction. Diodes are often 

 used in applications where the P.I.V. for a single junction would be exceeded. 

 Hence a complete diode unit consists in general of a number of junctions in 

 series. Consequently complete rectifying assembUes are a 'pile' or 'stack' 

 structure whose length is an indication of the voltages, and whose thickness 

 is a measure of the current with which they are intended to be used. 



Diodes intended for signal rectification generally have only one junction 

 and are usually referred to as 'diodes'. Diodes intended for a.c. power 



315 



