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As you know, the
first law of Materials
science is "Everything can be
broken". Dielectrics are no exception to this rule. If you increase
the voltage applied to a capacitor, eventually you will produce a big bang and
a lot of smoke - the dielectric material inside the capacitor will have
experienced "electrical
breakdown" or electrical break-through, an irreversible and
practically always destructive sudden flow of current. |
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The critical parameter is the field strength E
in the dielectric. If it is too large, breakdown occurs. The (DC) current vs. field
strength characteristic of a dielectric therefore may look look this: |
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After reaching Ecrit, a sudden flow
of current may, within very short times (10–8 s)
completely destroys the dielectric to a smoking hot mass of undefinable
structure. |
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Unfortunately, Ecrit is not a well defined material property, it depends on
many parameters, the most notable (besides the basic material itself) being the
production process, the thickness, the temperature, the internal structure
(defects and the like), the age, the environment where it is used (especially
humidity) and the time it experienced field stress. |
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In the cases where time plays an essential role,
the expression "failure" is
used. Here we have a dielectric being used at nominal field strength well below
its breakdown field-strength for some time (usually many years) when it more or
less suddenly "goes up in smoke". Obviously the breakdown field
strength decreases with operating time - we observe a failure of the
material. |
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In this case the breakdown may not be explosive; but a leakage
current may develop which grows over time until a sudden increase leads to
total failure of the dielectric. |
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The effect can be most easily tested or simulated, by
impressing a constant (very small) current
in the dielectric and monitoring the voltage needed as a function of time.
Remember that by definition you cannot have a large current flowing through an
insulator = dielectric; but "ein bißchen was geht immer" - a
tiny little current is always possible if you have enough voltage at your
disposal. A typical voltage-time curve may then look like this: |
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The voltage needed to press your tiny test
current through the dielectric starts to decrease rapidly after some time -
hours, days, weeks, ..., and this is a clear indication that you dielectric
becomes increasingly leaky, and will go up in smoke soon. |
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A typical result is that breakdown of a
"good" dielectric occurs after - very roughly - 1 C of charge
has been passed. |
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The following table gives a rough
idea of critical field strengths for certain dielectric materials |
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| Material |
Critical Field Strength
[kV/cm] |
| Oil |
200 |
| Glass, ceramics |
200...400 |
| Mica |
200...700 |
| Oiled paper |
1800 |
| Polymers |
50...900 |
| SiO2 in ICs |
> 10 000 |
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The last examples serves to remind you that
field strength is something totally different from voltage! Lets look at typical
data from an integrated memory circuit, a so- called
DRAM, short for
Dynamic Random Access Memory.
It contains a capacitor as the central storage device (no charge = 1;
charge = 0). This capacitor has the following typical values: |
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Capacity » 30 fF (femtofarad)
Dielectric:
ONO, short for three
layers composed of Oxide (SiO2), Nitride
(Si3N4) and Oxide again - together about 8
nm thick!
Voltage: 5 V, and consequently
Field strength E = 5/8 V/nm
» 6 · 106 V/cm. |
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This is far above the critical field
strength for practically all bulk materials! We see very graphically that high
field strength and voltage have nothing to do with each other. We also see for
the first time that materials in the form of a thin
film may have properties quite different from their bulk behavior -
fortunately they are usually much "better". |
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Last, lets just note in passing, that electrical
breakdown is not limited to insulators
proper. Devices made from "bad "
conductors - i.e. semiconductors or ionic conductors - may contain regions
completely depleted of mobile carriers - space charge regions at junctions are
one example. |
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These insulating regions can only take so much field strength
before they break down, and this may severely limit their usage in
products |
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© H. Föll (Electronic Materials - Script)
