It is well known that point defects in high-k oxides act as fixed charge centers and trap centers. With respect to band gap of silicon (suppose n-type), how we will classify which point defects act as fixed charge centers and which ones will trap charges.
When impurity atoms are introduced at very low concentration such that band gap of the host material does not change and they act as electrically active center, then you can call them centers having fixed charges as in case of n or p type doping. Thouh they are localized states, they are not defect states in real sense. Defect states may be due to structural defects which act as traps for the charge carriers. If their energy is close to band edges we call them shallow traps and if they are far apart then we call them deep traps. These traps increase the response time of the devices. For example, in case of photo conductivity, rise and decay may be slowed down due to the presence of taps. In case of amorphous semiconductors, defect density is high and has continuous distribution with energy. These defect states may act as traps or recombination centers depending upon the position of Fermi level. Problem is, therefore, more serious in these semiconductors as compared to crystalline semiconductors.
In oxides, oxygen vacancies double plus charged trap electrons very effectively
from a purely theoretical viewpoint, all those point defects located in the bulk or near the surface of a dielectric film (SiO2 is the most common) will act as trapping centers for carriers if:
1. coulombic state allows carrier confinement (also neutral centers may trap charged carriers)
2. The defect realizes an energy well of empty or partially empty states (allowed by Pauli's priciple) whick penetrate the dielectrica gap from the conduction band edge for electrons to be trapped or from the valence band edge for holes to be trapped
3. the enrgy of the carrier is statistically low, within the projected defect cross section, to favor trapping.
From an experimental viewpoint, all those structural/chemical point defects wich possess a localized non-zero charge just as a simple consequence of material/device processing are considered as fixed charge defects WHE USUAL THERMAL/ELECTRICAL SOLICITATIONS DO NOT AFFECT THEIR CHARGE STATE. In other words, a fixed charged state is a local chemical or structural defect whose charge is due not to a trapped electron or hole, but to an asymmetry in the bound charge density in their premises.
Again, be aware that fixed charged defects may behave as free carrier traps if properly solicitated, for instance by allowing a tunnel current of energetic carriers in their vicinity.
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