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Consider this datasheet for a family of Zener diodes. All of the parts 3V and under have their leakage current rated as some current at 1V. I've seen this is multiple family datasheets, but it really doesn't make sense. There is no relationship between the test voltage and Zener voltage. I could see a spec such Zener voltage at 1uA having utility. At least the test condition would remain consistent and have the same meaning for each part in the family. Why are Zeners spec'd the way they are?

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  • \$\begingroup\$ Not sure what you mean. Page 3 of your PDF shows an increase in test voltage and test uAmps with increasing zener clamp voltage. \$\endgroup\$
    – user105652
    Commented Jun 25, 2016 at 1:02

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Leakage current in processes such as these are not primarily controlled for. As long as the devices met some typical value at test then the product is released. The reason for this is that leakage (in it's various forms , Band to band, tunnelling, Hopping) is driven to a first order by impurities and defects which are by definition random.

Principally these devices are not usually operated in any really different mode other than as Zeners, so this is measurement is more of a reality check. And while I can see your argument, if the leakage at 1V is X then it should behave predictably at higher voltages. Of course this is swamped by the zener current once it hits its knee.

Your alternative suggestion, while also understandable would be very hard to do in a reproducible way. Here is a snip from that data sheet.

enter image description here

You can see from the curve in the 3rd quadrant that a fixed voltage (a vertical line) with a variable curve (variable from sample to sample) will show a little variation in the Leakage current. Due to the slope of the curve the sensitivity is low.

Now, flip it around, pick a fixed current as you suggest (a horizontal line) and imagine multiple curves occupying the 3rd quadrant from device to device. Due to the slope of the curve, a slight process variation would manifest itself in a wildly varying voltage. So much so that it would be useless as a measure.

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  • \$\begingroup\$ Are you sure you have your quadrants right? Forward operation is Q1 and reverse-breakdown operation is Q3. \$\endgroup\$ Commented Jun 25, 2016 at 1:35
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When using a zener diode to protect a sensitive input from overvoltage, you want to know what effects it will have on the signal, which is typically at a low voltage, well below the zener "knee".

Therefore, you'll want to know its low-frequency and DC loading effect (leakage) and its high-frequency loading effect (capacitance).

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  • \$\begingroup\$ Definitely. These parameters are particularly important in TVS-type devices. \$\endgroup\$ Commented Jun 25, 2016 at 1:45
  • \$\begingroup\$ @user2943160 I've never seen a TVS datasheet that doesn't spec reverse leakage at the reverse working voltage. That makes sense, but I'd call it an apples-potatoes comparison. \$\endgroup\$
    – Matt Young
    Commented Jun 25, 2016 at 15:21
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Old question, but the real answer is a bit different.

  1. The leakage current has nothing to do with random impurities or defects, but is described by the regular diode equation. You can look this up in basic electronics texts.

  2. The leakage current at 1V for a zener of 3V is very important. Consider this basic schematic, shamelessly ripped *) from another stackoverflow posting:

basic zener application

Suppose VS may vary between 4 and 20 V, but there is a requirement that the voltage over RL should be kept at about 3 V. Then you may need a zener for 3 V. Now, your RL may be quite large (think of a FET gate), so you may be tempted to use a large value for RS, so your battery does not drain too fast.

But the characteristic of a zener does not have a 90-degree sharp 'knee' at 3 V. At a lower voltage, say 1 V, the zener will already draw some current, what you may call 'leakage' current, which is just jargon, but in any way, the zener will draw a non-zero current at voltages below the zener voltage. If you choose RS too large, the voltage over RL may never reach 3 V but may get stuck at a much lower voltage like 1 V.

You cannot make RS too small either, because you have to consider the maximum current that the zener can handle. But that is another issue.

Do you see how electronics design is a craft of balancing lots of rules?

*) I just copied from the google search page, but to honor the hard work of the original author, I found the original posting from 2017 by Bowpark

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  • \$\begingroup\$ Roland - Hi, Regarding: "Consider this basic schematic, shamelessly ripped from another stackoverflow posting" - Thanks for mentioning the source. To comply with the site rule on referencing (and to comply specifically with the CC BY-SA license for copying material from Stack Overflow) please add the original author's name & link to the original post from which that was copied, next to the copied image. Thanks. \$\endgroup\$
    – SamGibson
    Commented Dec 21, 2023 at 1:43
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    \$\begingroup\$ @SamGibson Thanks, I included the link and author \$\endgroup\$
    – Roland
    Commented Dec 21, 2023 at 16:25
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Another circuit that is sensitive to leakage currents at voltages clearly below the Zener voltage is a Zener-referenced crowbar circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

The idea of this circuit is that an overvoltage on V1 triggers SCR1 via D1, shorting the supply and blowing F1, so RL (the load resistor) is protected. You need to make sure that there are no false triggerings. The SCR triggers at a gate voltage of around 0.7V, so in this circuit, a leakage current of 70µA would trigger the SCR even if the supply voltage does not exceed 5.4V. Knowing an upper bound of the leakage current at some voltage clearly below the Zener knee helps dimensioning R1.

If you need a precision crowbar circuit, please use a circuit based on a voltage reference like the TL431 instead of this circuit, though.

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