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This term is used frequently to describe the mechanism of current flow in a forward biased PN junction. It is called "recombination current", where recombination is the process of conduction electrons filling holes in the valence band, thus eliminating an electron-hole pair.

The issue with this terminology, is that elimination of an electron-hole pair seems to stop current, not cause or allow it. So to call a current "recombination current" makes it seem as though the current is made up of disappearing electrons and holes, a seeming contradiction. You could as well say that at rush hour on the highway, the traffic is made up of "parking traffic".

So with this in mind, how is the phrase "recombination current" supposed to be interpreted? In effect, what is recombination current through a PN junction?

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  • $\begingroup$ Before recombining, the electron and holes are flowing right? That gives right to a current, then they recombine and emit some energy. This process continues leading to the current. $\endgroup$ – Kalpak Gupta Feb 24 '17 at 15:12
  • $\begingroup$ I understand what you are getting at, but disagree with your reasoning. The recombination current, it seems, is due to charge carriers that will recombine eventually. They must come from somewhere, though, and some must not recombine, or else current would never get to the other side of the junction. $\endgroup$ – Sam Gallagher Feb 24 '17 at 15:37
  • $\begingroup$ That's what I am saying. They come from the battery, and will recombine, giving energy. Before recombining, they will give rise to current due to their movement. $\endgroup$ – Kalpak Gupta Feb 25 '17 at 3:19
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The current in a forward biased pn-junction can be called recombination current because the current is finally due to recombination of electrons and holes inside the junction, either in the depletion zone or in the neutral n- or p- regions. It is important to realize that this forward current is due to recombination of electrons and holes coming from different sides (n-doped and the p-doped) of the junction. Thus each electron-hole recombination event corresponds to the transport (and thus current) of one elementary charge across the pn-junction.

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  • $\begingroup$ Thank you for your helpful answer. My only question is how charge carriers "escape" from the PN junction, if they always recombine? (Do they always recombine?) $\endgroup$ – Sam Gallagher Feb 24 '17 at 15:40
  • $\begingroup$ They always recombine. Electrons come in one side, holes the other. They meet in the middle and die. Electrons (or holes) do not flow through the diode. A hole moving to the right looks just like an electron moving to the left from a current point of view. $\endgroup$ – Jon Custer Feb 24 '17 at 16:01
  • $\begingroup$ @Sam Gallagher - The p-region has a positive bias with respect to the n-region. An electron enters the n-region from the contact and moves towards p-region. A hole enters from the contact the p-region and moves towards the n-region. The electron and the hole recombine in the depletion zone or in the adjacent n- or p-region. Thus, effectively, a positive elementary charge has moved from the positively biased contact to the recombination site and from there to the negatively biased contact. All these recombining carriers are resupplied from the contacts causing the junction current. $\endgroup$ – freecharly Feb 24 '17 at 16:11
  • $\begingroup$ Thank you Jon and Freecharly for the replies. I was forgetting about how holes moving towards the depletion region is the same as electrons moving away from the depletion region, hence conduction! $\endgroup$ – Sam Gallagher Feb 24 '17 at 18:30
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Use Current = V/R, where the holes are moving. We don't think about electron movement for voltages, that's for physicists. Think about the positively directed current and the theory works. It's just confusing, because of all the charges and lack of good notation.

The recombination current is just the current across the junction. Outside the junction, there's no current until the depletion layer extends across the n-terminal. You just get the resistive p-block insulating flow. To get the total recombination current, you've got to add both the electron and the hole flow; but consider only one first. the generation is from heat, or light, and happens with a frequency.

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The holes disappearing from the negative terminal cause the electrons from the battery to be used up. Which causes more electrons to be pushed from the negative terminal of the battery. similarly the electrons disappearing from the positive terminal cause electron in the p junction to be used up and delivered to the positive terminal of the battery.so if you consider a diode to be a black box, the electrons disappearing from the left are reappearing on the right (from the right to the left) and there is a net "FLOW" of charges hence the current.

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