DIAMONDS — LOGIE 373 



In addition, the height of the circle above the grornid floor is large in 

 comparison with most other materials which we nonnally encounter. 

 Interpreted in another way, it means that all the electrons are firmly 

 held in the covalent bonds and considerable energy is required to 

 release them. This aspect of the diamond structure has been dwelt on 

 at some length, but it is fundamental to an understanding of many 

 processes in diamonds. 



"VVliile the general picture which has been given here of the band 

 structure of solids is generally followed by all materials, the details 

 for various substances differ, and in particular the energy separation 

 of the two bands is different and this profoundly affects the proper- 

 ties of the substance. In what follows we shall be mainly concerned 

 with the passage of electrical currents and it is important to look into 

 this aspect further. 



The conduction of electricity in most of the processes which are en- 

 countered daily is due to the motion of electrons. Not all materials 

 have the ability to conduct electricity, and solids have been divided 

 into three categories. Wlien there are electrons free to move about in 

 the crystal, we have a conductor. Metals fall into this categoiy. 

 Wlien there is no freedom of movement for the electrons, then the sub- 

 stance is called an insulator. There is an intermediate group of ma- 

 terials known as semiconductors, which become conductors only mider 

 certain conditions of temperature and purity. In the case of dia- 

 monds, one anticipates that they will behave as an insulator because 

 all the available electrons are closely bound to the atoms and none can 

 move about under the influence of an applied electric field. In order 

 that a current should flow, some of the electrons must be made to move 

 from their association with a given pair of atoms and be permitted to 

 wander through the ci-ystal. The question at once arises. How firmly 

 are these electrons held? The answer is to be found in the band 

 energy diagram of figure 6, which shows a forbidden energy gap of 

 about 6 eV. If we think of the theater analogy again, the situation 

 is that all the stall seats are full and for a person to move from one 

 position to another (which would correspond to a flow of current) is 

 not possible. The only way that freedom of movement can be acliieved 

 is by transferring one or more people to the gallery. These people in 

 the almost empty gallery can now move about freely and at the same 

 time the vacancies which now exist in the stalls enable an exchange of 

 seats to take place there also. If one seat is vacant, for example, then 

 as a general exchange of seats takes place, the appearance would be 

 that of a vacant seat moving about from one position to another in 

 the auditorium. Both these processes have their counterj^art in what 

 happens in crystals of the diamond type. Events in the gallery where 

 an electron is thought to be moving in the conduction band are spoken 



