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Electrochemistry in diodes?!

Discussion in 'Electronic Design' started by Paul, Jan 12, 2009.

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  1. Paul

    Paul Guest


    I'm doing some diode research, and would like to know of any possible
    DC voltages produced in the diode due to electrochemical reactions.
    It's a palladium-silicide diode, part number SMS7630, but also has
    gold. Over time the atoms could migrate. The SMS7630 is a microwave
    diode used for small signals. It is not terribly uncommon for the DC
    voltages produced by small AC signal rectification from such ZBD's
    (zero bias diodes) to be as low as one microvolt DC. The SMS7630 zero
    bias resistance is ~ 5400 ohms. Even the slightest *possible*
    electrochemical reaction is important-- e.g., diode square law

    It seems possible, for example, for silicon atoms migrating some
    distance into the palladium side to bond with palladium atoms that
    previously had no such bond-- diffusion.

    I would like to build an experiment to measure this DC voltage
    produced by electrochemical reactions. Does anyone have any idea what
    DC voltages I could expect? Nano volts, pico volts, etc. The DC
    voltage produced by a lead acid battery is temperature dependent,
    where the electrochemical DC voltage decreases with an increase in
    temperature; -0.022V/°C-- ref:

    Therefore, how about measuring the change in DC voltage produced by
    the SMS7630 at say 60F versus 90F? I have a two layer metal shield
    system (small and medium shields) and an electrometer that produces a
    few femto amps bias current. It could detect down to 0.5uV DC, but if
    there's no appreciable chance of these diodes producing up to 0.5uV DC
    from electrochemical reactions then it could save a lot of research

    Please see page 3 of this pdf for details on this diodes structure:

  2. dlzc

    dlzc Guest

    Dear Paul:

    Non-detectable directly. Your only hope is measuring conduction
    carrier density over time.
    This does not happen over decades, unless they experience high
    temperatures for extended periods.

    I'd plan on looking for an secular increase in applied voltage to
    drive a given current, or such a decrease in current resulting from an
    applied voltage.

    David A. Smith
  3. Paul

    Paul Guest

    Thanks for reply David. If I understand you correctly, then that's my
    thought as well; i.e., that the DC voltage produced by this diodes
    electrochemical reactions would be undetectable. The junction
    diffusion occurring on the order of decades sounds significant. To all
    intents and purposes, I'm guessing that would amount to *zero* DC
    volts. Hmmm, maybe it's a few femto volts, lol. I had no idea
    commercial diodes diffused at such a slow rate.

    Even if it diffused at a faster rate, I'd imagine the electrochemical
    potentials produced from palladium and silicon would be small, perhaps
    less than one microvolt.

    Correct me if I'm wrong, you're suggesting the electrochemical DC
    voltage is too small to be directly measured, so it's better to
    measure the long term change in resistance due to diffusion? That's an
    interested method that should work. It's a bit time consuming though,
    waiting for the resistance to change, but at least doable. Although it
    would be interesting to know what the electrochemical DC voltage would
    be in such a diode.

    Many Thanks,
  4. dlzc

    dlzc Guest

    Dear Paul:

    You will have some sort of "thermoelectric junction" voltage, as you
    would with a "thermopile", but there are some constraints on the
    conductors you would use between the two junctions.
    You can google:
    dopant diffusion semiconductor
    .... to get a better feel for this. Until you get up near something
    like 200 degC, diffusion is near zero for all intents and purposes.

    You can try using two devices at different temperatures, with
    identical wiring between them. See if you measure a potential
    difference. But diffusion will be a *very* small signal.
    Let's see what the other folks in the groups you posted to think...

    David A. Smith
  5. Marvin

    Marvin Guest

    There are ion-sensitive diodes and transistors that respond
    to the Nernstian potential of ions, made selective by
    certain coatings. to read up on this, google with the words
    "ion sensitive diode" without the quotation marks.
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