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Transport Saturation Current (Is) in a BJT

Discussion in 'Electronic Basics' started by Paul_M, Apr 16, 2004.

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  1. Paul_M

    Paul_M Guest

    I'm trying to find out exactly what the Transport Saturation Current
    (Is) is that the Ebers Moll equation relates to. Typical values are
    10e-16, this value is so small that it can't possibly be the Collector
    or Emitter current when the transistor is in saturation. All materials
    I've read give hardly any explanation and I've searched the internet
    for some days now. The Art of electronics book just says that it's the
    Saturation current of the particular transistor (depends on T) and in
    a different paragraph it says that it represents the reverse leakage
    I find this very confusing. Can anyone enlighten me?


  2. Jim Thompson

    Jim Thompson Guest


    Ie = Is*(e^(q*vbe/k/T)-1)

    *Roughly*, Is is the reverse *leakage* current.

    ...Jim Thompson
  3. Paul_M

    Paul_M Guest

    Thanks for the info Jim,
    I did see this equation, in the "Art of electronics" book, but in a
    slightly different form. I can see now why it would be the reverse
    leakage current due to the Base-Collector junction being reverse
    biased in the Forward Active region and a base-emitter voltage
    increase corresponding to a collector current increase (60mV to a 10
    fold increase in Collector current).
    Also I've found something that states that :
    Is = Js.A
    where Js is the Transport current density and
    A is the emitter area.
    Can you recommend anything that goes into this a little deeper?


  4. Mantra

    Mantra Guest

    For far more detail, especially the "next model up" (Gummel-Poon)
    which is a superset of Ebers-Moll:
  5. Jim Thompson

    Jim Thompson Guest

  6. John Larkin

    John Larkin Guest

    Seems to me that actual device leakage currents are somewhat larger
    (as in, say, 1e6 times larger) than is suggested by a value of Is that
    explains the forward curves.

  7. Jim Thompson

    Jim Thompson Guest

    That's why I put "leakage" in quotes... it's called "leakage" in the
    literature, but real "leakage" is due to crystal misalignments,
    extraneous dopants and just plain dirt.

    ...Jim Thompson
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