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Diode reverse-bias resistivity?

Discussion in 'Electronic Basics' started by eromlignod, Jan 17, 2008.

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  1. eromlignod

    eromlignod Guest

    Hi guys:

    I'm an ME, so I have a basic knowledge of semiconductors, but not as
    in-depth as you EE's.

    When a diode is connected in reverse (the direction where current
    doesn't flow), does it behave like an insulator or like a
    semiconductor? In other words, does it have an extremely high
    resistivity, like a dielectric, or does it just have a high
    resistivity and pass some very small amount of current, like a
    semiconductor?

    I know about avalanche breakdown; that's not what I'm talking about.
    I'm talking about at voltages below breakdown.

    Thanks for any replies.

    Don
     
  2. Greg Neill

    Greg Neill Guest

    Check the datasheet specs for the terms Leakage Current
    or Peak Reverse Current. Typically on the order of a
    few microamps for small silicon diodes in the region
    before breakdown. This can be modeled as a high
    resistance of tens of megohms.

    A sample data sheet:
    http://www.diodes.com/datasheets/ds28002.pdf

    Beware of another property, junction capacitance, which
    can be on the order of several picofarads. This can
    permit AC components of a signal to leak through even
    when reverse biased.
     
  3. They pass a current called the saturation current that can
    be very small, depending on the materials and doping of the
    junction. But for some, especially high current or low
    voltage devices this reverse leakage can be in the milliamps
    or even hundreds of milliamps. Typical leakage for 1 amp
    silicon PN junctions is in microamps.
    For each particular device, you find this information on the
    data sheet, which sometimes just lists a maximum value.

    Here are a couple examples with graphs of reverse leakage
    versus voltage and temperature. Not that below breakdown
    the current is less than proportional to the reverse voltage
    (not resistive that follows Ohm's law, but closer to a
    constant current).

    figure 5 on this one:
    http://www.vishay.com/docs/88503/1n4001.pdf

    figure 2 in this one:
    http://www.onsemi.com/pub/Collateral/MUR8100E-D.PDF

    much higher leakage for Schottky low voltage type, figure 2:
    http://www.nxp.com/acrobat_download/datasheets/PMEG2010EH_EJ_ET_4.pdf
     
  4. eromlignod

    eromlignod Guest

    Great Info. Thanks for the replies.

    Should I assume that a transistor in its "off" state would exhibit
    similar behaviour?

    Don
     
  5. For sufficiently vague concepts of "similar", yes. There is
    definitely a reverse biased diode (collector to base
    junction) leakage current component that dominates the
    character of the leakage if the transistor has a reverse
    biased base emitter junction. If the transistor has a small
    positive base emitter bias it might still be considered to
    be off, but an additional leakage current component (gain
    times base current) is added.

    The main similarity is that the current is less than
    proportional to the voltage over a considerable voltage
    range, till breakdown is approached. Then it takes off and
    becomes more than proportional to voltage.
     
  6. John Larkin

    John Larkin Guest

    Typically a nearly constant current, with a much less than resistive
    slope, as JP states. Schottkies are leakier and more "resistive" than
    silicon PN junctions. The reverse leakage current typically doubles
    with every 10 degree C temp increase.

    It can vary wildly between "identical" devices, especially power
    diodes.

    Some glass diodes are photosensitive, too!

    John
     
  7. Joerg

    Joerg Guest

    When I was a kid that discovery made me jump for joy. No more plunking
    down more than a buck for an LDR. Just take a diode or transistor from
    the TV scrap heap, scrape the black paint off the glass body, and bingo!
     
  8. whit3rd

    whit3rd Guest

    Oddly, all (four, I think) answers (in other posts) are correct.

    (1) If you don't have sub-picoamp current meters, a good
    quality instrument diode is an insulator.

    (2) If you DO have sub-picoamp current meters, a good
    quality instrument diode leaks nearly constant 'satuation current'.
    The diode equation scales all diodes into a simple function,
    and the saturation current is the scale factor, I_s

    (3) if your diode has significant surface area, and has
    aged and/or gotten 'dirty' there can be large leakage
    currents on the exterior crust of the semiconductor, not
    really covered by the diode equation at all, and the manufacturer
    has some data on how bad this can be for his design and
    with his chosen materials.

    (4) It varies from device to device (i.e. a rectifier is 'good enough'
    if the leakage is a milliamp, but few are that bad, and NONE
    are that bad straight from the factory).
     
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