372 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 60 



Figure 6. 



bidden" energy gaps. As the atoms are brought together to form the 

 crystal, the discrete energy levels in the atom spread out into energy 

 bands, and the electrons are distributed among the possible energy 

 levels in accordance with the exclusion principle and according to 

 the statistical distribution of Fermi. The uppermost band is called 

 the conduction band, while the next lower is called the valence band. 

 For the energy of an electron to be changed, there must be an un- 

 occupied level into which it may go. In the case of the diamond, 

 the conduction band is empty and all the electrons are in the valence 

 band and occupy all the possible states in that band. There are no 

 unoccupied states in the lower band and consequently there is no 

 possibility of small changes of energy being imparted to an electron. 

 The closest unoccupied state is at the bottom to the conduction band 

 and an energy of at least Eg must be imparted to the electron to 

 effect tliis transition. The situation in this respect may be likened to 

 a theater with a gallery. The seats represent the possible energy 

 states and these are at two levels. The audience represents the elec- 

 trons. If a seat has been reserved in the stalls, then quite a lot of 

 energy is needed to obtain a transfer to the circle. In the cjise of the 

 diamond, all the stall seats are full and all the circle seats are empty. 



