Pseudochemical equilibrium

Many nonchemical reactions can be treated by using similar techniques. One such is the creation of mobile electrons and holes in an intrinsic semiconductor. In this case, thermal energy excites some electrons from a filled valence band to the energetically nearby empty conduction band, the result of which is a population of mobile holes in the valence band and a population of mobile electrons in the conduction band. At ordinary temperatures, this is a dynamic equilibrium. There is a continuous excitation of electrons, and these are continuously falling back to the valence band and recombining with holes. The appearance of stasis is illusory. 

This situation can be represented by a pseudochemical equation. As the electrons and holes are generated by thermal energy, the equation can be written  

K will depend on temperature, in the expected way, but not on pressure or how much semiconductor is in the sample. 

In an intrinsic semiconductor, the number of holes win equal the number of electrons. However, the equilibrium equation also applies to doped semiconductors, and the equilibrium constant derived for a pure intrinsic semiconductor is valid for a doped sample of the same semiconductor. This is an extremely useful finding because it means that as the concentration of electrons in a semiconductor is increased by doping the concentration of holes decreases, and vice versa. Thus an n-type semiconductor can be changed to a p-type semiconductor simply by increasing the number of holes present, by appropriate doping, and vice versa. This possibility underlies the fabrication of semiconductor devices. This information is used in Sections 13.2.2 and 13.2.4. 

The equilibrium population of point defects is another example of a pseudochemical equUibrium. For the creation of a Frenkel defect on a cation array  

---

Because the configurational entropy of interstitial defects has the same form as that of vacancies, a population of self-interstitial atoms is also thermodynamically stable. The creation of these defects can then also be treated as a pseudochemical equilibrium, and an equation for the relationship between the number of self-interstitials and the appropriate equilibrium constant for interstitial generation, Kv is readily... 

---