In a dielectric, the charge displacement increases with increasing field strength. Beyond a critical value of the field strength, there is an electric breakdown due to the physical deterioration of the dielectric material. The dielectric strength is defined as the breakdown voltage per unit thickness of the material. When the applied electric field is large, some of the electrons in the valance bond cross over the large conduction band across the band gap giving rise to large conduction currents. Under this condition, the strength of the local field is of the order of 1 mega volt/cm. Thus the liberation or movement of electrons from valence band is called internal field emission of electrons and this breakdown is called the Instrinsic breakdown or Zener breakdown.
The electrons moving under the high accelerating electric force collide with the atoms or molecules. They release more electrons and holes by breaking covalent bonds between them. Thus the current density increases. The covalent bonds between atoms or molecules are broken continuously and finally dielectric breakdown occurs. This type of breakdown is called Avalanche breakdown. Impurities in the dielectric material create additional energy levels in the energy gap and so they help the intrinsic breakdown to occur at lower applied voltage.
1. This can occur even at lower temperatures.
2. This requries relatively large electric fields.
3. Mostly this kind of breakdown occurs in thin samples.
4. This does not depend on the electrodes configuration and shape of the material.
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