Can someone explain me what a zener diode does?

  • Thread starter Fys
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  • #1
Fys
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Can someone explain me what a zener diode does? I don't know what the turn on and breakdown voltage mean, and what forward and reverse biased means.
And what the difference between a zener and a normal diode is?


Thx very much
 

Answers and Replies

  • #2
LURCH
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I recently built a circuit in which we ued Zeners as a sort of presuure-regulating valve. A Zener can work like a regular diode. If a voltage is applied in one direction (any voltage above about .6V), it will allow current to flow. That is the "forward-biased" direction for both the Zener and the regular diode. But, with the regular diode, voltage applied in the opposite direction will not cause the diode to conduct, no matter how much voltage is applied. Of course, you could apply enough voltage to make the charge arch across the diode, but the diode would be destroyed, and the voltage required would be ridiculously large.

With the Zener, you can apply a certain voltage in the reverse direction (reverse biasing), and the Zener will fire and allow a controlled current to pass through. The required voltage is dtermined by what kind of Zener you use. In our circtuit, the Zener fires whenever the output exceeds 5V. That is its firing votage, or turn-on voltage. This ensures that our output device, which is an OpAmp capable of sending 15V + or-, will never put more than 5V into the next input, a 5V Nand Gate.

For this reason, Zeners are almost exclusively used in reverse-biased applications, because in forward-bias, they behave exactly like a regular diode, but cost a lot more. Whereas, in reverse-bias, they do a job that a regular diode can't.
 
  • #3
stewartcs
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Just to add to what Lurch said, the breakdown voltage is the minimum reverse voltage needed to make the Zener diode conduct in reverse.
 
  • #5
zeners are good for limiting the voltage on a node. They are used for TVS/ESD protection to shunt any extra voltage above its breakdown voltage, thereby protecting the node its on from going too high in voltage. They are also used in different power regulators, because they can force a constant voltage output of your regulator (if the regulator gives too much voltage out, the zener will just break down and drop only its breakdown voltage across itself, the rest is eliminated through current passing through the zener).

They have a "knee" in the reverse bias region of a Vd Vs. Id curve, and once you get to this knee, the diode begins to conduct.
 
  • #6
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I am getting confused with the effect of Zener diodes on the width of the depletion region. Does it inc or dec? I know that its of the order 10^-6..but in general does depletion rgn inc/dec. Also, the Juction Field..what happens to it?

Thanks a lot
Aparna
 
  • #7
Defennder
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I'm a little confused about something I read about diodes in Wikipedia:

[PLAIN said:
http://en.wikipedia.org/wiki/Diode]At[/PLAIN] [Broken] very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current that usually damages the device permanently
This seems to me to suggest that reverse-bias avalanche breakdown is destructive, but the article on p-n junction suggests that the reverse-bias avalanche breakdown is non-destructive:

[PLAIN said:
http://en.wikipedia.org/wiki/P-n_junction]The[/PLAIN] [Broken] strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p-n junction depletion zone breaks-down and current begins to flow, usually by either the Zener or avalanche breakdown processes. Both of these breakdown processes are non-destructive and are reversible, so long as the amount of current flowing does not reach levels that cause the semiconductor material to overheat and cause thermal damage.
The last sentence suggests that the device is damaged because of thermal damage due to current heating. I'm assuming that "critical level" here refers to the peak inverse voltage.

To make things worse, the article on Peak Inverse Voltage (with a factual accuracy dispute tag) says that diodes may be destroyed if the applied voltage exceeds PIV:
[PLAIN said:
http://en.wikipedia.org/wiki/Peak_Inverse_Voltage]As[/PLAIN] [Broken] a general term applied to semiconductor diodes, peak reverse voltage or peak inverse voltage is the maximum voltage that a diode can withstand in the reverse direction without breaking down or avalanching. If this voltage is exceeded the diode may be destroyed.
So when exactly does the diode break down? When the applied voltage is greater than the peak inverse voltage? And isn't the peak inverse voltage the point where avalanche breakdown begins to occur and the diode begins to conduct (without being destroyed) in reverse bias?
 
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