2.19.2 Role of Donor and Acceptors
In semiconductors such as Si, donors such as As and P are used used for n-type behavior and acceptor atoms such as B and Al are used for p-type behavior. These donor and acceptor atoms basically create donor and acceptor energy levels very close to conduction and valence band respectively, such that the difference with the band edges is approximately equal to kT at room temperature.
In Ionic solids, all the ionic defects with non-zero effective charge can be viewed as either a donor or acceptor. Obviously, defects with positive charge act as donors while those with negative charge act as acceptors.
| Figure 2.9 Figure showing the positions of impurity energy levels in the band diagram of MgO |
For example, oxygen vacancy can be viewed as donor according to the following reaction
The ionization energies are
These energy levels are situated with respect to the conduction and valence band edges in the band gap.
For example in MgO, Al acts as a donor while Na acts as an acceptor according to the following reactions:
Similarly, Cl acts as a donor while N acts as an acceptor.
In case of BaTiO3, substitution of Ba by La leads to an electron i.e. La acts as a donor whereas Al and Fe substitution on Ti sites leads to creation of holes and hence these are termed as acceptor impurities. Y atom can replace either Ba or Ti due to its intermediate size.
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