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So it seems that a zener diode is "magically" able to change its resistance to allow more current to pass through it, such that the voltage drop across the diode remains the same. My question is simple: HOW?

I also have a question related to normal diodes (the humble non-zener type). I do know that a zener diode is still similar to "normal" ones but just have a much higher reverse break-down voltage. So here it goes: For resistors, their resistance is always the same thus depending on the circuit topology the voltage drop across them varies, considering that we have voltage source in the circuit, current shall vary and result in different voltage drops across the resistor. For diodes, the voltage drop across them is almost the same about 0.6v, what exactly do they vary within them that we always get no more than about 0.6v drop across them? It just looks like magic doesn't it?

I have asked one question about zener and one about normal diode, the questions are related and thus I put them into the same post.

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  • \$\begingroup\$ I believe it is actually magic and that there is, an as yet undiscovered nth dimension that once we find it, will explain these strange magical phenomena. \$\endgroup\$ – Andy aka Oct 1 '13 at 17:10
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    \$\begingroup\$ @Andyaka, We may not know which dimension it comes from, but we know the control mechanism involves magic smoke. \$\endgroup\$ – The Photon Oct 1 '13 at 17:47
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It just looks like magic doesn't it?

If you haven't taken a course in semiconductor device physics, I suppose it does look like magic.

PN junctions are not resistors. Whereas the voltage across a resistor is proportional to the current through

\$v_R =R \cdot i_R \$

the current through a diode is approximately:

\$i_D = I_S(e^{\frac{qv_D}{kT}}-1) \$

In words, the diode current goes up exponentially with the diode voltage.

enter image description here

The derivation of the diode equation isn't trivial but it isn't mysterious either; it's based on well understood device physics.

For zener and avalanche diodes, the physical explanation is a bit easier to grasp. From Wiki:

Under a high reverse-bias voltage, the p-n junction's depletion region expands, leading to a high strength electric field across the junction. A sufficiently strong electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor leading to a large number of free charge carriers. This sudden generation of carriers rapidly increases the reverse current and gives rise to the high slope resistance of the Zener diode.

The Zener effect is distinct from avalanche breakdown which involves minority carrier electrons in the transition region which are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bound electrons. Either the Zener or the avalanche effect may occur independently, or both may occur simultaneously. In general, diode junctions which break down below 5 V are caused by the Zener effect, while junctions which experience breakdown above 5 V are caused by the avalanche effect.

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The resistor functions as a "constant obstacle" to the current flow - the more "force" (voltage) you apply across the resistor, the more current flow you'll get.

The above description is not the most accurate one because it neglects the dependency of resistivity on temperature and frequency, electromigration and etc., but it is good enough for intuitive explanation.

Diodes, on the other hand, are not "constant obstacles". There is a region inside diodes which is called a depletion region (or, sometimes, space-charge region) which is the most interesting part of a diode - it is this region which makes diodes (and transistors) so different from resistors. Depletion region (and effects associated with it) is very sensitive to the externally applied voltage.

enter image description here

  • For regular PN diodes, when the forward voltage across the diode increases, depletion region shrinks in size and allows for the current to flow "more easily" (when depletion region shrinks the voltage across it decreases). You can think of it as "doubling" the effect of the voltage: it causes more current to flow, and causes depletion region to shrink which allows for even more current to flow.

  • When reverse voltage is applied to the diode, depletion region expands. It is not causing the current to become smaller (at zero voltage there is no current through diode, but there is some current when the diode is reverse biased), but the expansion increases the voltage across the depletion region thus "neutralizing" the externally applied reverse bias, which prevents from current flow to ramp up.

The regular PN diode is based on the two points above: it allows for high currents to flow under relatively constant voltage while it is forward biased, and prevents reverse currents while it is reverse biased.

Zener (and Avalanche) diodes are similar to the regular PN diode, but they exploit the reverse-bias as the operating mode. As we said, the current through reverse biased PN diode is negligible because depletion region "neutralizes" the externally applied voltage. However, everything has its limits: when the reverse-bias of the diode crosses some threshold, the depletion region can undergo Breakdown (usually, it is non-destructive effect). When Breakdown occurs, it is like the depletion region is not there at all, and the current ramps instantly to very high values, while the voltage across the diode remains essentially as it was a moment before the breakdown.

enter image description here

Many people will want to crucify me because what I wrote. Yes, this is very simplified and inaccurate description of PN (Zener, Avalanche) diode's principle of operation. However, I believe that this model allows for intuitive understanding of PN junction, without any preliminary knowledge in semiconductors.

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"...what exactly do they vary within them that we always get no more than about 0.6v drop across them?" Great question!

And here is an intuitive answer: What varies within them is the resistance... diodes are voltage-stable dynamic resistors which vary their resistance so that the voltage drop across them is almost constant...

enter image description here

"It just looks like magic doesn't it?" Yes, it does!

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Diodes conduct current after a certain threshold energy (the bias voltage) is surpassed and orienting (rather minimizing) the depletion region in forward bias such that current can flow.

For zener diodes they are manufactured such that the reverse bias will also properly bias the depletion region for current flow. It's reverse biased because current is flowing in the opposite direction.

I would also like to point out that resistors don't always have the same resistance. Passive components (resistors, capacitors, inductors) have parasitic elements. This means at certain frequencies you'll see capacitive or inductive behavior of these components. You can tell this component by the phase change of a time varying signal compared to what you expect of an ideal component.

In summary to the main question: diodes are conducting current when the bias voltage minimizes the depletion region such that current can flow in the desired direction.

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A simplistic description of a diode. A Diode has two type of materials mated together to form a junction. A base material such silicon for an example, silicon is a non conductor. As you may know from physics Atoms have different levels of Electrons. Silicons Outer band has 4 Electrons (::|::) so it is a balanced Atom sharing Electrons with other Silicon Atoms. To make Silicon a conductor chemical element are combined either adding electrons or taking away electrons. What is important about this is that the silicon with only 3 electrons can take in electrons, while silicon with 5 electrons can only give up electrons. This how a semiconductor works the 2 element are mated together. ( .:<|>::. ) If current is applied in different directions. The outcome is as follows. The element with the 5 electrons can fill the holes ->|-> (.::<|>:.) of the element with 3 electrons (Their-forth Current flow). Reversing the current the element with 3 electrons (.::<|>:.) <-|<- has none to give up and the one with 5 can not take any (NO Current flow.) the 0.6 to 0.7 volts is formed at the junction of the 2 materials because their is a diffusion of holes and electrons trying to equalize charges at the junction, the area around the junction becomes charged positively toward the element with 5 electrons. This charge has to be exceeded before electrons can flow. 0.6 Volts

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