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I'm trying to learn how Zener diodes work and I think I get the overall concept (regulating voltage to the zener voltage level), but there's one thing I don't understand. I'm looking at this diode and see that it's zener voltage is 3.3V, but it's max forward voltage is 0.9V.

Why is the forward voltage so much lower than the zener voltage? Is it because the diode is generally never placed in a forward biased direction (i.e. current should only generally travel in a reverse biased direction)?

Update
I was originally under the assumption that the max forward voltage was the maximum voltage the diode could handle in a forward biased direction. Thanks to @SomeHardwareGuy for setting me straight. The max forward voltage is the voltage required to have current flow through the diode in a forward direction.

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It's just the basic operating principle of a diode. An ideal diode would allow current to flow in one direction but block current flow in the other direction. This is based on how it's made, with a p-type region, an n-type region, and a depletion zone in between. Like the bottom diode in this picture:

enter image description here

When you apply a some voltage, in your case 0.9V then the p-type holes and the n-type electrons move into the depletion region because they are repelled by their respective battery terminal. With enough voltage (0.9V in your case) the free electrons in the depletion region get moving and current begins to flow like this:

enter image description here

Now in the ideal case if you were to reverse that battery the opposite will happen and you'll get no current flowing:

enter image description here

In the real world though you can only apply so much reverse voltage or push before you hit the breakdown voltage and current begins to flow freely in the reverse direction. Zener diodes take advantage of this fact and are constructed to breakdown at lower voltages such as your 3.3V.

Sources:
You can read more about how zeners are made here Or see the article I got all the pictures from here

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  • \$\begingroup\$ Thanks, so the 0.9V in the datasheet isn't referring to the max voltage that can pass in a forward direction through the diode, but rather the breakdown voltage in the forward direction? Whereas 3.3V is the breakdown voltage in the reverse direction? I was originally thinking 0.9V was the max forward voltage the diode could handle without being damaged. \$\endgroup\$ – Matt Ruwe Nov 19 '12 at 21:00
  • \$\begingroup\$ You're on the right track but we usually don't call it break down voltage. It's just the voltage required to get current moving in the forward direction. If you look at figure 4 in the datasheet you'll see chart of forward voltage vs forward current. The spec you don't want to exceed is the max forward current stated here to be 250mA. \$\endgroup\$ – Some Hardware Guy Nov 19 '12 at 21:07
  • \$\begingroup\$ Ok, that's very helpful. I guess it's not really the "max" forward voltage. After looking at the datasheet this seems more clear, but is it just me or is that a sort of confusing way to think about it? \$\endgroup\$ – Matt Ruwe Nov 19 '12 at 22:30
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Zener diodes (most are actually breakdown diodes and don't use the Zener effect) are always reverse-biased.

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http://hyperphysics.phy-astr.gsu.edu/hbase/solids/zener.html Forward drop varies with ESR and current but standard drops for a given chemistry and doping level exist for Germanium, Silicon, hot-carrier, LED's and Schottky diodes.

The reverse breakdown <5.6V is mainly the "zener effect" which has an NTC and > 5.6V is usually the "avalanche effect" with a PTC.

Side comment

There are more precise low-voltage references using bandgap and floating gate analog cell that is essentially unaffected by external influences such as variation in temperature, input voltage, and time. ( but affected by airport X-RAY machines)

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