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I test my Zener diode IV characteristic (reverse bias). IV curve

It is a 1N4728 3.3V Zener diode, but its exponential IV curve seems like a normal diode with lower I_sat(?) From its datasheet

Willis Lin
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2 Answers2

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No, this Zener diode is not broken, it behaves exactly as expected. Low-voltage Zeners have very soft "knees"; that is, they transition slowly to the conducting state as you increase the voltage, which means that they pass a significant amount of current below their rated Zener voltage. Zeners with higher breakdown voltage have sharper knees.

Zener characteristics vs. breakdown voltage

Image source

Physically, this is caused by two competing breakdown mechanisms in the Zener diode: The Zener effect, which is based on tunneling, and the Avalanche breakdown, which is caused by impact ionization. Tunneling is a stochastic process, which means that the Zener effect has a slow onset (the higher the voltage, the more likely it becomes for an electron to tunnel), leading to a soft knee. Avalanche breakdown on the other hand has a sudden onset with a clearly defined voltage threshold, causing a sharp knee, because the semiconductor material goes into breakdown immediately when a critical electric field strength is reached - similar to a lightning bolt ionizing air.

Jonathan S.
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    @WillisLin Some advice: if you need a sharp knee at a lower voltage, you may want to look into the TLV431. It's functionally a programmable zener diode with a very sharp knee regardless of the voltage it's set to. – Hearth Mar 31 '23 at 00:19
  • I know. I have some of them. I use them with power NPN to make precise linear voltage regulator. The reason I do the zener test is because of curiosity. – Willis Lin Mar 31 '23 at 02:49
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Zener diodes are not that great. They do conduct significant (and sometimes unusable) currents below their so-called threshold voltage and, if you compare your graph against the table that you posted in the question, they match: -

enter image description here

Andy aka
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