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Vbe stuff

Discussion in 'Electronic Design' started by Robert Baer, Oct 15, 2006.

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  1. Robert Baer

    Robert Baer Guest

    Following what Mr Pease had to say about Vbe, ie read, at 27C that it
    measures 799mV at 100uA, 640mV at 10uA, and 580mV at 1uA for 60mV per
    decade.
    But.
    Measuring a 1N4006 at 21C i get 474mV at 100uA and 387mV at 10uA for
    87mV per decade.
    Measuring a MPSA42 in DCT mode at 21C i get 564mV at 100uA and 505mV
    at 10uA for 59mV per decade.
    Seems to make sense; the larger die area will have a lower current
    density than a small die area, given the same current.
    Comments?
     
  2. Guest

    Ebers-Moll, Gummel-Poon - have you looked at what the classical
    transistor models predict?

    As far as I know, the Gummel-Poon 64-parameter model of the bipolar
    transistor is the one normally realised in Spice simulators, while the
    simpler (and less accurate) Ebers-Moll model is the one taught by lazy
    academics to undemanding students.

    The evil behaviour of the 1N4006 is well known - I haven't a clue why
    rectifier diodes behave oddly, but I am well aware that they do.
     
  3. Something to do with being PIN diodes, perhaps?
     
  4. Jim Thompson

    Jim Thompson Guest

    Probably

    ...Jim Thompson
     
  5. Jim Thompson

    Jim Thompson Guest

    Density has nothing to do with vbe change per decade _provided_ you're
    not into where resistance is affecting the measurement.

    ...Jim Thompson
     
  6. Eeyore

    Eeyore Guest

    Bulk resistance ?

    Graham
     
  7. Fred Bloggs

    Fred Bloggs Guest

    I suppose it has more to do with a design goal minimizing conduction
    losses in the high current diode thereby prohibiting the use of very
    large ratios of majority doping material density as with the transistor.
    This will cause a departure from the simplified minority carrier density
    at the transition region boundaries as a function of forward bias
    because the minority carrier density on both sides of the transition
    region change significantly with a coupled dependence.
     
  8. Jim Thompson

    Jim Thompson Guest

    As someone else pointed out, the 1N4xxx devices are also very lightly
    doped (almost PIN-diode-like) to get the "high-voltage" performance.

    But, Fred, your dissertation sounds just like that... dissertation out
    of the mouth of some _twisted_ PhD ;-)

    ...Jim Thompson
     
  9. All the same, it makes some sense to me. Shockley's equation develops
    from an assumption that the forward current is entirely due to
    minority carrier diffusion in neutral regions, I think. I need to
    think more about it, but on first cut it sounds like the right
    direction.

    Jon
     
  10. Guest

    The diode equation (Shockley, "Theory of p-n Junctions", Bell System
    Technical Journal
    #28, p 435-489 , 1949)
    applies to planar semiconductor junctions.

    I = Is * (exp( (Qe*V) / (k * T) ) - 1)

    Is is dependent on the current-conducting area, T is the junction
    temperature

    The base-emitter junction of a modern transistor (MPS-A42) is a good
    fit.

    A 1N4006 is intended for high current, and there's a fault condition
    called
    thermal runaway that could funnel ALL the current in the diode through
    one small
    hot spot. To keep that fault from occurring, this kind of diode has
    some
    distributed electrical resistance added to the diode; the diode
    equation might
    still fit well, but you have to consider that you're measuring both the
    diode and
    the series resistor that's built-in.

    At the high end of diode-equation compliance, low noise transistors can
    follow
    the equation for about eight decades in current (the deviations from
    the equation
    behavior are one part in a hundred million). At the low end, items
    like point-contact
    diodes have (literally) different diode area at different biases. Your
    crystal radio
    diode might not follow the formula well at all.

    In addition to 1N4006 diodes not being good logarithmic-response
    elements, high
    current transistors are similarly treated. The 1N4006 has a surge
    rating of 10A,
    and the highest current rating I could find for a near-ideal transistor
    was about
    the 2A current rating (TIP 31).
     
  11. Jim Thompson

    Jim Thompson Guest

    Large ratios of doping level are precluded by (primarily) breakdown
    voltage and forward resistance considerations.

    ...Jim Thompson
     
  12. Ban

    Ban Guest

    Well, in the theory there is a factor I=Is e^(Vf/mVt) with m being 1...2 for
    diodes, which predicts this result. On the upper current range there comes a
    point where the density of the injected minority carriers equals the
    impurity concentration, so only majority carriers can flow more. Higher
    blocking voltage has lower doping and this effect starts early.
    For very low currents the recombination current(long charge carrier
    lifetime) becomes higher than the diffusion current and forces m again to 2.
     
  13. But isn't forward resistance a valid part of this measurement being
    discussed?

    Jon
     
  14. Jim Thompson

    Jim Thompson Guest

    Certainly. I believe someone (was it you, Jon?) mentioned PIN diodes.
    I believe that the 1N4xxx series is very PIN-like in doping. I'll ask
    someone at ON Semi.

    ...Jim Thompson
     
  15. Eeyore

    Eeyore Guest

    I'm confident of it. I've never seen rbb specified for devices like the MPSAs
    but I have a vague recollection of seeing a figure of ~ 1k somewhere once for
    small signal parts. If so, the 100uA would account for 100mV in the resisitve
    part of Vbe.

    Graham
     
  16. Eeyore wrote...
    Whoa, 100mV is a serious error! Let's talk 10mV; that'd be 10uA
    of base current, which would correspond to about 2mA of collector
    current. Since the measured characteristics don't deviate from
    the ideal by that much by 2mA, I'd imagine rbb' must be under 1k.
    But we're probably in the right ballpark. A 1k rbb' corresponds
    to 4nV of lowest en noise at high collector currents. I don't
    know the lowest en value for mppa parts, but I imagine they're
    in the 2 to 5nV region; certainly not below 2nV (250 ohms).
     
  17. No, that was me. I learned it here, during a discussion
    I started about Tom McEwan avalanching them (at LLNL) to
    get 100ps pulses peaking around 100KW. He used a 1000V
    step of a couple of ns into a pulse-forming network, and
    the MOSFET step generator was also extensively discussed.
     
  18. Eeyore

    Eeyore Guest

    Look at the numbers. The OP did mention 100uA btw.

    One of the few parts I've seen En for was a Rohm part ( 2SB737) which was ~
    600pV. That had an rbb' of ~ 10 ohms IIRC.

    Graham
     
  19. Robert Baer

    Robert Baer Guest

    OK; scratch the 1N4006; the MPSA42 did not conformeither; it just was
    "closer".
     
  20. Robert Baer

    Robert Baer Guest

    The MPSA42 has a multi-milliamp rating,and should follow the log
    relationship to at least 10mA or more; i measured at 100uA max - so
    internal resistance is not significant.
     
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