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Does current flowing through a zener diode, when forward biased, increase due to heavy doping?

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This depends entirely on the particular diode.

All diodes have a design tolerance for not only reverse bias, which is specialised with zener diodes as they are able to conduct once the breakdown voltage is reached, but also forward bias. If the forward current exceeds the design tolerance a diode can be destroyed just like any other component. In the same way, if the reverse bias voltage and/or current design limitations are exceeded then zener diodes will be destroyed just like regular diodes.

In general, a forward biased zener diode will perform the same as a regular diode. It will have a voltage drop across the PN junction of about 0.7 V. The data sheet for the particular diode, whether it is a zener diode or 'ordinary' diode, will provide specific details on how much current it is able to conduct (in either direction).

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  • $\begingroup$ If the diode didn't have a fixed design tolerance, would the forward current be greater than an ordinary pn junction diode? $\endgroup$ – Prentice Apr 18 at 11:47
  • $\begingroup$ What is an ordinary PN junction diode? A 1N400x series diode typically can handle up to 1.0 A average forward current and a peak surge current of 30-50 A depending on the manufacturer while a 1N4148 is only rated to handle an average forward current of 200 mA and a peak surge current of 4 A. The 1N400x series are typical rectifier diodes while the 1N4148 is a typical signal diode, which explains the difference in values - they have different purposes. A zener diode has a different purpose again, and the forward current characteristics are often not supplied as this is not their normal use, $\endgroup$ – Mick Apr 23 at 7:10
  • $\begingroup$ but I would expect that, for an identical package, an 'ordinary' diode would conduct more forward current than a zener. $\endgroup$ – Mick Apr 23 at 7:11

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