In forward bias the electrons from n region move to p region after crossing the depletion region and holes from p region move to n region. But shouldn’t they recombine and hence the width of the depletion region width should remain constant? Why is it decreasing? And I read that the electrons that migrated to the p side now move to the positive terminal of the battery and vice versa for the holes on the n side. But how can they do so without recombining with majority carriers en route( since both the n and p sides outside the depletion region will have majority carriers)??
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$\begingroup$ Holes cannot travel to the battery through conducting wires . Only the electrons travel. $\endgroup$– Nehal SameeCommented Feb 25, 2018 at 12:55
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$\begingroup$ Ok. I see . And what about the electrons? Also why does the depletion region width decrease? $\endgroup$– Semanti ChakladarCommented Feb 25, 2018 at 12:57
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$\begingroup$ @SemantiChakladar...In a p-n diode, as soon as the barrier potential is crossed by the applied voltage , the electrons of potential barrier near the n region move to the the +ve potential of battery and the holes of potential barrier near p region move to the -ve potential if battery...So width decreases... $\endgroup$– Nehal SameeCommented Feb 25, 2018 at 17:27
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$\begingroup$ A potential barrier is the region with electrostatic charges where the barrier potential exists...The end of it near p part has holes and that near n part has electrons... $\endgroup$– Nehal SameeCommented Feb 25, 2018 at 17:31
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2 Answers
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Thats because there is no such "real" hole in a pn junction. It is actually the electron which is getting enough energy to cross the depletion region and thus reducing its width. Therefore the potential barrier also decreases.
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When you apply a forward bias voltage in a pn-junction, the width of the depletion region decreases because the applied voltage decreases the voltage drop corresponding to the built-in voltage of the pn-junction. In forward bias electrons will be injected from the n-region into the p-region and holes will be injected into the n-region, where they will flow as minority carrier diffusion currents. In thick p- and n-regions, they will recombine with the majority carriers there and the currents will continue gradually as majority carrier currents to the contacts. Majority holes reaching the contact recombine with electrons of the contact so that the eeltrical current continues as electron current in the contact. In short regions, the minority diffusion currents flow directly to the contacts. Holes recombine there again with the electrons of the contact. Depending on the semiconductor and recombination centers, recombination of electrons and holes can also occur already to a large part in the depletion region. This happens in typical Si pn-junctions.
answered Feb 25, 2018 at 14:20