828 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



2. DEVICE CHARACTERISTICS 



2.1. Junction Rectifiers 



A junction rectifier is made from a wafer cut from a single crystal of 

 semiconductor material. The materials now being used for this purpose 

 are germanium and silicon, but to date the use of germanium is more 

 common than silicon. Pure germanium in its undisturbed or intrinsic 

 state is a poor conductor; but its conductivity can be increased by dis- 

 turbances such as cosmic rays, photons of light, external potentials, or 

 by the addition of very small amounts of selected impurities. We are 

 concerned here only with the addition of impurities. There are two classes 

 of these impurities, called "donors" and "acceptors." The physical 

 mechanism by which pure germanium becomes conductive depends on 

 which of these two classes of impurities are present. Donor impurities 

 result in a surplus of free electrons which can conduct current by nega- 

 tive charges passing through the germanium crystal. Thus the addition 

 of donor impurities to pure germanium creates "n" type material. 

 Presence of acceptor impurities results in a shortage of electrons creating 

 "holes," which have positive charges. These holes are mobile and they 

 can conduct current through the crystal.'^ Thus the addition of acceptor 

 impurities to pure germanium creates "p" material. 



When an abrupt change is made from p to n type material inside the 

 crystal a rectifying junction exists at the boundary between the two 

 materials. This p-n junction exhibits rectifier action in that it \\\\\ con- 

 duct current every easily from p toward n; but, in its rectifier operating 

 range, only minute currents can be made to flow from n toward p. We 

 say that this junction has a low forward resistance and a high reverse 

 resistance. All rectifiers have these characteristics to a greater or lesser 

 degree and the p-n junction rectifier characteristics have been compared 

 elsewhere to other rectifier de vices. ^ 



There are two methods of producing the junction inside the crystal. 

 It can be obtained by growing part of the crystal from p type material 

 and part from n type. This is called a "grown" junction. It can also be 

 obtained by diffusing impurities into the crystal after it has been grown. 

 This has been called an "alloy" process, a "fused junction" process, or 

 a "diffused junction" process. 



2.11. Junction Rectifier Terminology 



Before discussing the characteristics of junction diodes, it may be 

 helpful for the reader to consider the terminology employed. As in other 



