338 



BELL SYSTEM TECIIMCAL JOl'RXAL 



Thus, if higli-eiit'rg>- particles or quanta of radiation fall upon the crystal, 

 they can break the bonds. Conductivity in diamond induced by bombard- 

 ment in this way has recently received considerable prominence in connec- 

 tion with "crystal counters" which have been used to detect nuclear parti- 

 cles and in experiments on conductivity induced by electron bombardment. 

 An electron ejected from a bond constitutes a localized negative charge in 

 the crystal and, since initially the bond structure was electrically neutral, 

 the electron as it departs from its point of liberation leaves behind an equal, 

 localized positive charge. Such a migratory electron, because it represents 



5^~V 



Fig. 2. 



an excess over and above that required to complete the bond structure in 

 its neighborhood, is called an "excess electron." Since it cannot enter any 

 of the completed bonds in the lattice, it moves about in the crystal in a ran- 

 dom manner under the influence of thermal agitation. If an electric field is 

 applied, it tends to drift in the direction of the applied force and to carry a 

 current. This illustrates conduction by excess electrons (referred to simply 

 as conduction by electrons) and, as we shall see, is to be distinguished from 

 the other process whereby an electron deficit enables conduction to be ef- 

 fected by "holes." 



Such a hole, constituting a net, localized, positive charge in the crystal, 

 moves from place to place by a reciprocal motion of electrons in the valence 

 bonds; and, under the influence of an electric field, its random motion ac- 



