Frochlich’s Theory
Electronic Breakdown/ High Temperature Breakdown
- On a pure dielectric the valance band is separated from conduction band by a very large energy gap. Electron’s can’t acquire enough energy to jump to the conduction band. (to remove an electrons from the atomic structure a field of the order of 10’v/cm is required. But normally breakdown occur much below it.
- Therefore in a perfect dielectric there are no free electrons and conductivity is almost zero.
In general all crystals contain imperfection due to
- Vacancies
- Extra atom-which can take interstitial positron or lattice may rearrange to make vacancies.
- Imperfection due to foreign atoms.
- An imperfect crystal or amorphous crystal will have some free electrons in the conducting band. At room temperature energy gap between trapping band and conduction band is small.
- When a field is applied to an imperfect crystal electrons in the conduction band gain energy from it and due to collisions between them, the energy is stored by all electron. In the imperfect crystal collision between electron, predominates, the electronic gain energy more randomly than they lose it, therefore electronic temperature will be relatively higher than the lattice.
- The effect of increased temperature is to eject more electrons to the conduction band increasing conductively leading to breakdown.