Electronic – Zener diode can vary current flow to maintain voltage drop, how does this magic effect work

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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.

Best Answer

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.

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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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