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Beta degradation quantified?

Discussion in 'Electronic Design' started by Spehro Pefhany, Sep 18, 2007.

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  1. Hi, all:-

    What kind of degradation can I expect in an ordinary BJT with
    transient reverse breakdown on the E-B junction? Is it a reduction
    from say 350 to 200, which I can design for, or something more
    catastrophic? Say the emitter was exposed to handing etc. with 10K in
    series... or perhaps a few mA in reverse breakdown now and then..

    Tbanks for any info. I'm sure someone has looked into this, but I
    don't see anything with a quick google.

    Best regards,
    Spehro Pefhany
  2. Jim Thompson

    Jim Thompson Guest

    I don't know of any specifics. I just know that repetitive breakdown
    DOES degrade the beta.

    Can't you put a diode there to prevent it?

    ...Jim Thompson
  3. Yes, I could use a diode (not directly across) but would prefer to
    avoid it if it won't affect the reliability. I only need a beta of
    10 or 20.

    Best regards,
    Spehro Pefhany
  4. Jim Thompson

    Jim Thompson Guest

    For what period of time, and what temperature? Eventually the beta
    does go away. When running breadboard tests under the hood of a car
    the transistors died in about two weeks.

    ...Jim Thompson
  5. Benign environment, but it is intended to last 'forever'. Maybe I'll
    check it out if no-one else has done any tests. Whatever the problem
    extent is, it doesn't seem to be severe enough that people worry too
    much about ESD on regular BJTs and bipolar transistors. Do you happen
    to know if there is a diode structure in an SN7400 input?

    Best regards,
    Spehro Pefhany
  6. Jim Thompson

    Jim Thompson Guest

    There ARE diodes protecting the inputs of an SN7400.

    Good to see that you are using such modern parts ;-)

    ...Jim Thompson
  7. Joerg

    Joerg Guest

    AFAIK beta degradation is cumulative, like a bank account similar to
    MOVs. But I know of only one paper that discusses it and I am not a
    member of the IEEE COMSOC society, so can't get it easily. Jim, are you?

    Long story short I would not do it.
  8. Hmm.. it's a lateral PNP, right? So it should have a relatively high
    E-B breakdown according to Camenzind. No, I'm not using 7400s (though
    you can still buy them) just a faint echo of the internal circuit.

    Best regards,
    Spehro Pefhany
  9. Cumulative is fine. I bet it's not an issue in this case.

    Here's some info:

    The effect seems to be worse at very low operating collector currents
    and high temperatures. I don't have any idea what the square micron
    measurement of a typical discrete transistor EB junction would be.

    Best regards,
    Spehro Pefhany
  10. Fred Bartoli

    Fred Bartoli Guest

    Le Tue, 18 Sep 2007 13:39:59 -0400, Spehro Pefhany a écrit:
    I did take some quick measurements once:

  11. Thanks, Fred. Two hours at a punishing 30mA! That's a lifetime here,
    and only a 16% drop. I can stand a 90% drop.

    Best regards,
    Spehro Pefhany
  12. linnix

    linnix Guest

    You mean very low temperature, right? Not too many of us need to deal
    with 100K.
  13. That's a nice spring day here.

    Best regards,
    Spehro Pefhany
  14. Fred Bartoli

    Fred Bartoli Guest

    Le Tue, 18 Sep 2007 15:11:40 -0400, Spehro Pefhany a écrit:
    From a quick read I understand that the effect is worse at low
    temperature, which goes well with an HP/Agilent paper I remember
    indicating that annealing (high direct current through junction, for ex.)
    somewhat restores beta (but not totally).

    Old National discrete databook have drawings of their processed dies.
    This heavily depends on the transistor, but for ex. a 2N3904 (process 23)
    show 7 fingers (4 base+3emitter) in about a 150um square.
    Assuming equal finger width, that'll be about 8000 sq um emitter area.
    About 2000 times the one in the paper you found.
  15. linnix

    linnix Guest

    My thanks also, Fred. Let's call the 50% drop at 180mAHr the Fred
    Rate. It the Fred Rate linear over time and current? Is it
    consistent with different devices?
  16. Joerg

    Joerg Guest

    Fine? But that means your beta will slowly shrivel up. Well, it's your
    circuit ;-)

    Figure 5 looks kinda spooky.

    You could probably find out for older devices, or maybe Jim knows this
    off the top of the head.
  17. Have you tried a beta test? ;-P

  18. Joerg

    Joerg Guest

    So now there is a Fed Rate and also a Fred Rate :)
  19. <groan> No, I'm more of an alpha geek.

    Best regards,
    Spehro Pefhany
  20. John Larkin

    John Larkin Guest

    Base Fredgradation.

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