4.12 Basic mechanisms of breakdown

• Intrinsic breakdown

• Thermal breakdown

• Avalanche breakdown

• This mechanism is based on lattice ionization and subsequent increase in the electron temperature.

• Actual breakdown field is larger than critical breakdown field, E_c, needed to cause a critical breakdown electron temperature, T_c.

• This mechanism is field dependent and here appliedfield determines the electron temperature to reach critical level for breakdown

• The break time is very short, smaller than ms, suggesting that the process is electronic is nature.

• It is independent of sample or its geometry or waveform type.

• It is a purely material dependent process.

     4.12.2 Thermal Breakdown:

• It occurs due to heat dissipation in the sample due to current flowing through defective parts of the sample which in turn further increase the ionic defect concentration leading to subsequent increase in the conductivity and then failure.

• It is a very common process in most of the bulk materials.

• It depends on the speed of application of field.

• It is observed between room temperature and 300°C.

• Ambient temperature determines the electron temperature and not the electric field strength.

• Rate of application of field is an important factor.

• The process can be quite slow,  from minutes to ms, and is dependent on sample geometry

• The shorter the pulse time is, the higher is the breakdown voltage.

• Large electric field in the samples lead to energetic electrons which can further lead to a multiplication process i.e. a few electrons knock out more and more electrons leading to a large increase in the conductivity.

• There is a gradual build up of charge rather than sudden change in conductivity even through charge build up can be quite fast.

• Quite often it occurs in thin films.

• It occurs at low temperature and in short time.

      Other or Pseudo breakdown mechanisms are

• Dielectric discharge

• Electrochemical and/or mechanical breakdown

• In small pores which are always present in sintered dielectric ceramics, the field strength is higher than the average field and as a result, microscopic arc discharge in the pores may be initiated.

• Electrons and ions from the discharge bombard the inner surface and erode it. As the pores grow, the current in the arc increases leading to an increased sample temperature eventually leading to failure.

• This occurs due to transport of conducting material due to the presence of local electrochemical current paths or defect into the interior of the dielectric leading to overall increase in the sample conductivity and then failure.

• It is assisted by suitable atmospheric conditions such as humidity and pH.