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Traditionally, a diode is a 2-terminal device that limits current to flow in one direction, i.e. a rectifier. But a "tunnel diode", according to wikipedia, is not rectifying:

In the tunnel diode, the dopant concentrations in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction.

Then why call this device a diode if it's not rectifying? Isn't it "just" another device that exhibits negative differential resistance?

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  • $\begingroup$ After reading the various comments, and re-reading the question, I no longer feel like I understand the question. Can you clarify what your issue is? $\endgroup$ – garyp Nov 22 '14 at 18:08
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It's called a diode because the device has two terminals. Devices that have three terminals are called triodes, and those with five pentodes. Words of that type have fallen by the wayside except in the realm of vacuum tube electronics ... except for the word diode, which has hung on.

Note that the Wikipedia article that you cite refers specifically to the tunnel diode, which is a type of semiconductor diode. If you go to the Wikipedia page for diode you will find this:

In electronics, a diode is a two-terminal electronic component ...

The vacuum tube diode was, of course, around long before the semiconductor diode. The vacuum tube diode has asymmetric conductance. Both types of diode play similar roles in electronic circuits. When the semiconductor device was invented, the natural name for is was "semiconductor diode". Since we don't see many vacuum tube diodes anymore, we shorten the name to diode, and there is little chance for confusion.

Update following question edit

It seems that the first semiconductor diode was named in analogy to the vacuum tube diode. Subsequently, several related devices, all employing semiconductor junctions, and all having asymmetric conductance, were developed. I suppose the word "diode" became the name of the category of such devices. (I think negative differential resistance has nothing to do with the name.)

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  • $\begingroup$ So what you mean is that "diode" is not a necessarily a rectifier? $\endgroup$ – Sparkler Nov 22 '14 at 15:42
  • $\begingroup$ Well ... you asked why it's called a diode. Now every device that we call a "diode" that I can think of is a rectifier, or closely related (Zener diode, for example). A resistor has two terminals, but we don't call it a diode. If I re-interpret your OP, I'd say that a tunnel diode is very closely related to a simple rectifying diode, so it borrows the name. Physically, inside the can, there is very little difference between a simple diode and a tunnel diode. $\endgroup$ – garyp Nov 22 '14 at 15:52
  • $\begingroup$ in your answer+comment you suggest that a diode is a 2-terminal rectifier. but a tunnel diode is not rectifying, and physically, tunnel diodes can be very different "inside the can", for example see the resonant tunneling diode. back to square 1... $\endgroup$ – Sparkler Nov 22 '14 at 15:57
  • $\begingroup$ @garyp your comment in fact (seemingly) contradicts the body of your answer. Could you make a more even mixture of these statements inside the answer? $\endgroup$ – Ruslan Nov 22 '14 at 16:28
  • $\begingroup$ @Sparkler presence of heterojunctions shouldn't confuse you. A typical resonant tunneling diode, as a usual homo-pn-diode, contains a single pn-junction. $\endgroup$ – Ruslan Nov 22 '14 at 16:31
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The negative differential resistance creates a special phenomena: under a certain bias and certain incident electron energies, the transmission function through the double barrier is nearly zero. In other words, for a finite voltage domain, the current is nearly zero, i.e. a rectifying behaviour. This can be referred to as "generalized diode", since the rectifying behaviour is not passing through the origin of the IV curve.

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