G8WRB said:
>the rate of metal migration would decrease
>exponentially with increasing thickness
There is a limit to how thick you can deposit metal. The Unitrode process,
which uses the end of the lead in direct contact has some advantages here.
At least in diodes, you don't tend to get the step coverage problem that
you get in ICs. However, metal migration is also a temperature related
phenomena, so depending on the diode construction, and the temperature and
the operating time, it might or might not be a problem.
Operating in avalanche mode is not desirable for a semiconductor junction
that hasn't been designed to so do, at least if you want reliability. It
used to be said that a cheap zener could be obtained by using the reverse
base emitter breakdown of a transistor: in practice, it tends to drift with
time.
73
Peter G3RZP
"Dr. David
Kirkby"
<david.kirkby@one To
tel.net> Dave Haupt <emailw8nf@yahoo.com>
Sent by: cc
amps-bounces@cont amps@contesting.com
esting.com Subject
Re: [Amps] Re: Equalizing resistors
with HV diodes
22/09/2004 08:46
Dave Haupt wrote:
> My employer makes lots of things, among those things
> are diodes. Our diodes are hot carrier, microwave
> switch, GaAs detector, PIN, switching, etc - no power
> diodes. However, in the past we have made power
> diodes. I brought this topic up to one of our diode
> designers. He responded at some small length, and
> ultimately said this: a diode should not be routinely
> operated at avalanche threshold unless it was
> specifically designed for that operation. High
> voltage diodes, he advised, are designed with the
> expectation that the circuit designer will take steps
> to prevent the diode from reaching avalanche voltage.
> It is usually the case that Is at avalanche voltage
> will cause metal migration, which will substantially
> reduce the mean time between failures (MTBF).
> 73,
>
> Dave W8NF
I was thinking about this before you mentioned it, but I suspect it will
be far less of an issue with high-voltage devices, than with small
devices such as microwave diodes or high speed CPUs.
Whereas a few tens of nm metal migration might well be an issue in the
latest Pentium IV CPUs, or microwave diodes, with their very fine
structures, I doubt the same depth of metal migration will be an issue
with high-voltage power diodes, which must have a much thicker layer of
silicon to avoid breakdown in the first place.
Breakdown voltage is proportional to the thickness of the silicon,
whereas (I suspect) the rate of metal migration would decrease
exponentially with increasing thickness, since metal migration is a
diffusion process.
So whilst a 1000V diode must have 10x the junction thickness of a 100 V
diode, the rate of diffusion on the 1000V diode will be only
Exp(-10)=0.0000453999 times as great (all other things being equal of
course).
I'm probably making some gross oversimplifications there, but I think
you get the point I'm driving at.
David Kirkby G8WRB
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