374 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 60 



of as conduction by electrons. The apparent motion of the seat left 

 vacant in the stalls corresponds to conduction by "positive holes." 



It was pointed out earlier that carbon, silicon, germanium, tin, and 

 lead all belong in the same column of the periodic table. The first 

 three have identical crystal structures, but behave very differently 

 electrically. Diamond is the insulator. It has an energy gap of about 

 6 eV. Silicon has a gap of 1.1 eV, in gennanium the gap is 0.72 eV, in 

 gray tin 0.1 eV, and in white tin and lead the energy bands overlap so 

 that there is no forbidden energy region and they are good conductoi-s. 



The different conducting properties of graphite and diamond are 

 now capable of interpretation. The largest atomic separation for 

 graphite between nearest neighbors is that between the platelike layers 

 which are shown in figure 3. This separation is 1.54 A for diamond, 

 and for graphite the separation is 3.35 A. The average spacing in 

 graphite is larger and corresponds to a region in which the energy 

 bands overlap and the graphite behaves like a metal. Thinking in 

 terms of the electron bonding, it is observed that in graphite the four 

 valence electrons are shared by three nearest neighbors in one of the 

 sheets of atoms. On the average, each atom can thus contribute part of 

 an electron which is free to wander through the crystal and participate 

 in electrical conduction. 



As will be shown presently, though most diamonds are insulators, 

 others are conductors and, by suitable treatment, all of them can be 

 made conductors. To explain this, we inquire now into some of the 

 possible ways in which electrons can be moved from the lower filled 

 band into the upper empty one so that conduction becomes possible. 



First there is the possibility that by heating the material an electron 

 may be given sufficient energy to jump the gap. On substituting 

 values for h^ Boltzmann's constant, in the relation E=hT, it is found 

 that to impart 1 electron volt would require temperatures of 10,000° 

 centigrade. This method is clearly not applicable in the case of pure 

 diamonds, but it does play an important part in semiconductors where 

 the gap between the energy bands is much less than is the case for 

 diamonds. 



The second possibility is that light waves or photons could impart 

 the required energy. Wlien this happens, it is spoken of as photo- 

 conductivity. Using the Einstein relation E=1ig/\^ the photon energy 

 of visible light is found to lie between 1.6 eV at the red end of the 

 spectrum and 3.2 eV in the blue. For the energy gap of diamond, 

 light of wavelength 2,200 A or less is required. This wavelength is 

 in the ultraviolet region. The electron ejected by this means is now 

 in the conduction band. If an electric field is applied to the crystal, 

 the electron drifts in the direction of the applied field and carries a 

 current. The vacant space from which the electron came constitutes 



