.UO BELL SYSTEM TECH MCA L JOI'RXAL 



appear and the bond structure becomes comj^letc, the excess energy being 

 given up to the atoms in the form of thermal vibrations.' 



If the temperature is sufficiently elevated, spontaneous breaking of some 

 fraction of the covalent bonds by agitation will occur producing electrons 

 and holes in equal numbers. In a diamond this effect would occur at such 

 high temperatures that it would not be observed. However, it plays a major 

 role in silicon and germanium at temperatures well within the range of in- 

 vestigation in the laboratory. 



On the basis of quantum mechanical theory, it is found that a very high 

 degree of symmetry exists between the behavior of electrons and the behavior 



Fig. 4. 



of holes. One may think of the hole as moving through the crystal as a posi- 

 tively charged particle with much the same attributes as a free electron ex- 

 cept for the sign of its charge. 



Impurity Semiconductors: Donors .\nd Acceptors 



If the only cases of conductivity open lo investigation were like those dis- 

 cussed above, for which electrons and holes are present in equal numb rs, 

 the problem of interpreting the data would be very difficult. Fortunately, 

 in the semiconductors silicon and germanium, there are cases in which con- 

 ductivity is due to excess electrons only or to holes only.^ 



^ The process of recombination ma\' actually be much more complicated and ma>- 

 involve intermediate stages in which the hole or the electron is trapped. 



' The behavior of silicon with impurities of the sorts discussed here was investigated 

 by Scaff, Theurer, and Schumacher. Their work was stimulated by the development of 

 silicon detectors for microwave use by R. S. Ohl, also of Bell Telephone Laboratories, 

 in the prewar years. 



