BJT emitter-base voltage VBEO violation

Hi,

by operating a NPN BJT with a positive VBE voltage the corresponding emitter-base track will be operated as zener diode. So for example if I'm exceeding the maximum VBEO e.g. 6 V for a 2N2222 [1] the voltage across emitter to base will be limit to around 6 V. I know this zener voltage is some how random, therefor BJTs are misused in this way as zener diodes in noise generators.

I'm more interested in a possible and maybe slowing ongoing permanent damage of the BJT and possible (unintended) power handling capabilities.
For example, if there is a base resistor installed (to limit the base input current in ordinary opertion), this restistor will limit the current from emitter to base, as well when the VBEO voltage is exceeded e.g. by a fault condition. The resulting emitter to base current as well as the emitter to base (zener) voltage will define the power loss. Is it correct to consider a save operation if this power loss is below the stated total device power dissipation e.g. 625 mW @ TA = 25°C for the 2N2222 [1]. Hornestly, I have doubts about that as this would allow a large current (~ 100 mA @ 6 VBEO) and only the emitter to base track is involved here.

Maybe someone can clarify my thoughts, or can suggest some further reading covering this topic. I already had a look at a couple of text books, including The Art of Electronics which is covering BJTs well (in my opinion), but I wasn't able to get to the bottom of this issue.

BR

[1] https://www.onsemi.com/pub/Collateral/P2N2222A-D.PDF

Other than with dedicated zener diodes or avalanche rectifiers, BE- junction breakdown must be expected to occur only in a region of the junction, e.g. in the boundary zone. Respectively it doesn't necessarily withstand the full rated power.

It's known that BE reverse breakdown operation deteriorates the parameters of small signal transistors, e.g. current gain and noise performance.

You will induce hot-carrier charging in the
junction-adjacent oxide and this will drift
Vbe as well as likely degrading low current
beta. In integrated op amps you'd see it
ruin Vio and Iib / Iio.

BVebo is not a well controlled parameter.

Hi and thank you for your replies.

FvM said:

Respectively it doesn't necessarily withstand the full rated power.

Click to expand...

Ok, I already thought so as there is onley the base emitter region involved.

Can anyone suggest further readings on this topics e.g. scientific papers, application notes or books. Or maybe a report including measurement results highlighting the effects. It seems the adverse effects are not addressed in my textbooks and I was not able to find an app note from a semiconductor manifacturer.

BR

From Googling

"BJT breakdown emitter base degradation"

comes this pretty good one:


and many more below it, which I did not
look at.

Thank you,

the linked article includes measurement results, exactly what I have looked for!

BR

It is worth noting that a BJT base metallization
is liable to be much narrower than emitter
(the collector, is the whole metallized backside
of the die) and the E-B breakdown path is much
less current-capable than the expected C-E
current path (betting on beta-active or at
least forced-beta-saturated (~ 10) factor of
expected current @ max rated Ic, and layout
would have divided the available die area by
about that ratio as allocated to base and emitter
metal. So upshot is, you might encounter a fuse
current in the 10-100mA range rather than the
100mA - 1A Ic(max) the device might advertise.

A designed zener diode would have (a) the
expectation of equal and rated terminal currents
and (b) often built as a mesa diode with full face
metallization (no routing at all, just a slab)
and the leads coined directly to the die for
a much higher current withstand. The tri-metal
endcaps and large vertical extent will preclude
the aluminum-spiking "zener zap programming"
which can also be an outcome of pushing too
much reverse current on an aluminum-interconnect
junction, if you heat the contact region to the Si-Al
eutectic you will entrain aluminum and short the
junction. Though it's probably a foot-race between
interconnect fusing and junction "programming"
that depends on the E, B contacts' proximity and
layout-induced current crowding (optimized for
"easy damage" for a zap-trim element, the
opposite for a should-be-rugged discrete BJT).

It's difficult to know from the usual docs, what the
frontside metal stack of a discrete BJT actually is.
Old timey types stand a good chance of being
aluminum-only, and even high power ones may
use Al-Ti-Ni-Au (which could still deliver the
aluminum) or Ti-Ni-Au (which would form a
Ti-silicide barrier and eliminate this specific
failure mode).