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In a regular silicon junction diode the positive side is the anode an the negative the cathode.

But when someone uses a Zener diode that operates in reverse bias there is a problem with what constitutes an anode and cathode.

In a Zener diode operating in forward normally the anode is positive and the cathode is negative. But when the diode is operating in reverse bias, what constitutes an anode and a cathode?

Does anode and cathode describe the polarity of the terminal, meaning that now the old anode will be the new cathode, as the polarity has changed, or those terms mean the actual physical terminals of the diode, so now the anode will be negative?

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  • \$\begingroup\$ @PeterSmith: Please do not answer the question in comments, as this bypasses the normal review process for answers, as discussed in meta \$\endgroup\$ – Dave Tweed Apr 1 at 10:53
  • \$\begingroup\$ @DaveTweed - I really didn't think it was long enough for an answer. I can convert it, though :) \$\endgroup\$ – Peter Smith Apr 1 at 10:55
  • \$\begingroup\$ Especially when answers that are not 500 words,17 paragraphs and have at least3 references then get converted to comments... \$\endgroup\$ – Solar Mike Apr 1 at 11:07
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    \$\begingroup\$ Comment converted to a (more wordy) answer. \$\endgroup\$ – Peter Smith Apr 1 at 11:18
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A silicon diode (of any type) is basically just two doped regions of silicon (with the exception of PIN diodes which have an intrinsic region).

Silicon diode

Source: Wikibooks

The Anode and cathode are defined by the dopants (group III for P and group V for N). All diodes exhibit avalanche behaviour in reverse; a zener is simply defined for where reverse conduction occurs.

As a note, most zeners below about 6V operate in true zener effect; above this they are generally avalanche effect devices.

As noted, the anode and cathode remain that way even in reverse bias.

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