Colbhaidh
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Scotland
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Breakdown across a reverse biased junction behaves as follows:
VBV ≈ E2crit.εsi(Na+Nd)/(2.q.Na.Nd)
where εsi=1.04E-12 F/cm Na is acceptor doping atoms/cm3 Nd is donor doping atoms/cm3 q is electronic charge and the critical electric field across the junction Ecrit is between 1E5 and 1E6 v/cm in Silicon.
It is really just a way of defining when a reverse biased junction fails catastrophically. However the breakdown value itself is not so important as any design that allows the electric field across a junction at or beyond Ecrit is doomed to fail anyway. A more important concern is what happens before reaching Ecrit when there is significant leakage current across the junction. If we define the breakdown voltage as that voltage where the junction leakes 10 nAmps, then there are voltages below this where the junction leaks say 1nAmp or 100pAmps. This may seem low but this can have two effects: One is, over time, such a stressed junction will deteriorate and eventually (prematurely) fail. An example of this would be ringing when switching, when the junction could be stressed for a very short time. But over time.... But second may be that two matched devices, one currently more stressed than the other, will no longer be matched as expected because of this leakage current.
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