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The first law of materials science
obtains: At field strengths larger than some critical value, dielectrics will
experience (destructive) electrical breakdown |
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This might happen suddenly (then calls
break-down) , with a bang and smoke, or |
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it may take time - months or years - then called
failure. |
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Critical field strength may vary from < 100
kV/cm to > 10 MV / cm. |
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Highest field strengths in practical
applications do not necessarily occur at high voltages, but e.g. in integrated
circuits for very thin (a few nm) dielectric layers |
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Example 1: TV set, 20 kV cable, thickness of insulation =
2 mm. Þ E = 100 kV/cm
Example 2: Gate dielectric in transistor, 3.3 nm thick, 3.3
V operating voltage. Þ E
= 10 MV/cm |
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Properties of thin films may be quite different
(better!) than bulk properties! |
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Electrical breakdown is a major
source for failure of electronic products (i.e. one of the reasons why things
go "kaputt" (= broke)), but there is no simple mechanism following
some straight-forward theory. We have: |
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Thermal
breakdown; due to small (field dependent) currents flowing through
"weak" parts of the dielectric. |
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Avalanche
breakdown due to occasional free electrons being accelerated in the
field; eventually gaining enough energy to ionize atoms, producing more free
electrons in a runaway avalanche. |
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Local
discharge producing micro-plasmas in small cavities, leading to slow
erosion of the material. |
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Electrolytic
breakdown due to some ionic micro conduction leading to structural
changes by, e.g., metal deposition. |
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© H. Föll
