Extrinsic region

Diffusion in the extrinsic region can readily be modified by doping, although knowledge of the mechanism by which the diffusion takes place is important if this is to be immediately successful. For example, sodium chloride structure materials that conduct by a vacancy mechanism can have the cation conductivity enhanced by doping with divalent cations, as these generate compensating cation vacancies. The inclusion of cadmium chloride into sodium chloride can be written ... 

The explanation for the two slopes in the plot lies in the fact that even a very pure crystal of NaCl contains some impurities, and the line corresponding to low temperatures (on the right of the plot) is due to the extrinsic vacancies. At low temperatures, the concentration of intrinsic vacancies is so small that it can be ignored because it is dominated by the defects created by the impurity. For a particular amount of impurity, the number of vacancies present will be essentially constant, jj in this extrinsic region thus depends only on the cation mobility due to these extrinsic defects, whose temperature dependence is given by Equation (5.9) ... 

Increasing the impurity levels does not affect the intrinsic (left-hand side) of the graph. It does, however, increase the value of o (and thus of Ino) in the extrinsic region. As the activation energy, a, for cation movement stays the same, the slope... 

For the case of semiconductors doped with donors ( -type) and acceptors (p-type), three regions can be distinguished. In the low-temperature extrinsic region kT Eg), the conductivity is given by... 

Further indirect information about the topology of the grain boundary phases can be obtained from the temperature dependence of the quantities and r i,. In crystalline ionic conductors in the extrinsic region, the conductivity is thermally activated and described by ... 

For the limiting case (Cd g) > we find that (VAg) — (CdAg). That is, the vacancy concentration is completely fixed by the addition of CdBr2. This is called the region of exclusively extrinsic disorder, as opposed to the region of intrinsic disorder. In the extrinsic region, those physical properties of the crystal which depend upon the point defect disorder are functions only of the concentration of dopant. However, in deriving eq. (4-26), it has been tacitly assumed that point defects do not form complexes. This assumption, as shown later, must eventually be modified. 

Addition of higher valent cations which dissolve substitutionally in the oxide will in the extrinsic region result in an increase in the concentration of electrons and a decrease in the concentration of oxygen vacancies. This may be shown by setting up the appropriate electroneutrahty condition combined with the defect equilibria in the same manner as shown above. 

Because this material is extrinsic and p-type (i.e.,p n), the electrical conductivity is a function of both hole concentration and hole mobility according to Equation 18.17. In addition, it is assumed that at room temperature, all the acceptor dopant atoms have accepted electrons to form holes (i.e., that we are in the extrinsic region of Figure 18.17), which is to say that the number of holes is approximately equal to the number of acceptor impurities N -... 

For extrinsic semiconductors, on a plot of majority carrier concentration versus temperature, carrier concentration is independent of temperature in the extrinsic region... 

---