4.11 Dielectric Breakdown

Every material is bound to fail or breakdown under certain conditions. Basically, in case of a dielectric it means short circuiting across the dielectric.

Technically speaking, dielectric breakdown occurs when the electron density in the conduction band becomes very high during the application of an electric field such that conductivity increases rapidly resulting in a permanent damage to material. However, it is easier to measure and talk in terms of the electric current which is anyway a representation of electron density.

The most critical parameter is the field strength E in the dielectric. If it exceeds a critical limit, breakdown occurs. The (DC) current vs. field strength characteristic of a dielectric therefore may look like this:

After reaching a critical field or breakdown field, Ec, a sudden increase in the current may, within a few seconds or even quicker, completely destroy the dielectric resulting in something like a ‘burnt’ material.
However, Ec is not a well defined material property, it depends on many parameters such as material thickness (bulk or thin film), temperature, atmosphere, level of porosity, crystalline anisotropy, level of crystalinity and composition.

While electric field plays an important role, dielectric may also break in a gradual time dependent manner and in such cases we would rather call it as ‘failure’. In such situation, the field may be well below the nominal breakdown or critical field and material is destroyed in long time. In such cases, normally the breakdown field also decreases with time.

In such cases, the breakdown may not be sudden, rather a leakage current develops which increases over time, and it may develop until it suddenly increases leading to complete failure. You can do this measurement rather easily by letting a small current pass through the samples and then monitor the voltage needed as a function of time. You will notice that the voltage needed to pass this current reduces as time progresses indicating that materials is getting leakier.

A typical voltage-time curve may then look like this:

The values of breakdown fields for some materials are given below:

Example:

For example, in thin film memory devices, SiO₂ is used as a gate dielectric and has a thickness of a few nanometers say 5 nm. The voltages at which these devices operate are about 5 V which translates into a field of 10 MV/cm which is a very large field when compared to break down fields of most of the bulk materials. This explains the importace of material form on the breakdown field.