Re: [Amps] Re: Equalizing resistors with HV diodes

["Dr. David Kirkby" <david.kirkby@onetel.net>]

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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