[Elecraft] OT: Diode snubbers

[David Gilbert]

I worked in the discrete semiconductor industry for 30 years 
(engineering, manufacturing, management) and that statement is simply 
not true at all.

Zener diodes are designed to break down uniformly across the entire 
junction, but most other semiconductors are not.  A reverse voltage 
breakdown of the collector-base junction will in most cases force 
current through an extremely small region of the junction (often 
referred to as a "puncture") and fuse the silicon at that point, 
rendering the device useless.  The current required to do so is much 
less than would be required to heat the device even a few degrees, and 
the time required to destroy the device is short.  Junctions with 
steeper diffusion gradients (RF devices, switching transistors, etc) 
will fail more easily (sometimes in microseconds), while sloppier 
junctions (power devices, etc) will take considerably more abuse.  It is 
not possible to predict the current at which catastrophic failure will 
occur.

Emitter-base junctions are typically more graded and won't fail 
catastrophically as quickly, but repeated reverse bias conditions will 
degrade the transfer gain of the device by creating defect centers that 
kill the carrier lifetimes in the base region.

The inductive kick from even a small relay is sufficient to puncture the 
junction of many commonly used transistors or driver ICs, and there are 
reams of failure analysis reports documenting that fact.  There isn't a 
manufacturer on this earth that will honor the warranty for a device 
used as you describe unless the device was specifically designed to 
survive it.

Dave   AB7E



Rick Shindley wrote:
> What kills solid state PN junctions is excessive power dissipation (heat).
> You can abuse the breakdown voltage of a PN junction as long as the power
> dissipated there is within the power dissipation range of the PN junction.
> That's how Zener diodes work and it is why they have a power dissipation
> rating. All diodes will function as low-current Zener diodes if you manage
> to provide a high enough reverse voltage across them to break them down and
> limit the current so the power they must dissipate won't destroy the PN
> junction.