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I think potential difference across disconnected diode should be zero but how? enter image description here

the last graph tells that there is a potential difference between the two ends of a diode. then how can be it zero?

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    $\begingroup$ why would you expect a current? $\endgroup$ – jim Mar 31 '16 at 11:41
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If you take a diode that isn't connected to anything we get the usual depletion layer at the PN junction:

Disconnected diode

And we get a potential difference generated across the junction. Suppose we now connect the two side of the diode with an external wire, then a second depletion zone develops at the connections with the wire:

Connected diode

And this depletion zone has a potential equal and opposite to the depletion zone at the junction.

So when we first connect the wire there is a transient current while the second depletion zone is building up, but the current stops as soon as the potential across the second zone balances the one across the junction. If we now disconnect the external wire electrons flow through the PN junction and the second depletion zone disappears.

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  • $\begingroup$ what do the colours mean? I get the feeling that the outer two bands should be reversed with respect to the inner two. $\endgroup$ – Gremlin Mar 31 '16 at 12:18
  • $\begingroup$ @Eoin: pink is an excess of holes and blue is an excess of electrons. The second zone is formed by the same mechanism as the first. Electrons diffuse (through the wire) from the N to the P side until balanced by the potential across the depletion zone. $\endgroup$ – John Rennie Mar 31 '16 at 12:21
  • $\begingroup$ I beg to disagree, at least as far as the original question goes. The diagram shows a voltage across the diode, but that's only because no current is allowed to flow. Your shorting argument is perfectly true, but doesn't address the OP's question. $\endgroup$ – WhatRoughBeast Mar 31 '16 at 15:26
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Current only flows between two points if there is a potential difference between the points. No current means no pd. The figure you've shown is how the charge distributes at the PN junction, this creates a potential barrier that charge must overcome to cross the barrier, it is not a voltage that can be used to cause current flow. For example, if you were to connect the leads of the diode together then current wouldn't be expected to flow from the conservation of energy.

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  • $\begingroup$ So, are you saying there's no potential difference across a battery that's not connected to a circuit or an electrical outlet with nothing connected to it? $\endgroup$ – M. Enns Mar 31 '16 at 11:54
  • $\begingroup$ it isn't a battery, it is a barrier $\endgroup$ – jim Mar 31 '16 at 11:56
  • $\begingroup$ @jim, this is a limited view of what a potential difference is. In free space, there can be a difference in the potential between two points, which gives rise to an electric field between them. $\endgroup$ – Gremlin Mar 31 '16 at 12:00
  • $\begingroup$ True, but am restricting the discussion to batteries connected to components where mobile charge carriers can move. $\endgroup$ – jim Mar 31 '16 at 12:02
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I'm assuming now that what confuses you is that this is not an active element, but still from the picture you would assume that connecting the ends of the diode would create an electrical current because of the potential difference that you see.

To me the confusion always was, how this diode is able to "produce energy" or "current" out of nowhere.

The clue is: If you connect the two ends of the diode, the same thing that happens at the junction of the p-part and the n-part will just happen again: There will be a current from the p-part to the n-part because of the potential-difference, but at the same time there will be a diffusion current because of the different densities of electrons and holes in the p and the n part. After short Time, equilibrium is reached, and thus there will be no more current, but still the same potential difference.

It would be another case, if you connect both ends of the diode to a conductor material. In that case at the two junctions there will happen the same thing that happens at the p-n-junction. This will bring the potential in the conductor-end-parts to the same level again, and there will be no potential difference.

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The charge separation shown in the diagram is true, as far as it goes. Keep in mind that the diagram shows an isolated diode, so the resistance between the ends of the diodes is infinite and no current flows.

The driving mechanism behind the separation of charge is thermal motion of the electrons in the diode. If you were to place a large enough resistor between the ends of the diode, you could in principle get a measurable current at something like the diode voltage. Perpetual motion? Nope. The process of separating the charges will cause the material to become cooler, and if unchecked this process will continue until thermal motion ceases. in other words, you have to heat the diode to get it to work as a generator.

And there's another way to goose the process: shine a light on the diode. Electrons produced by the absorption of photons can, indeed, produce useful amounts of energy - an illuminated diode is neither more nor less than a photovoltaic solar cell.

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