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I was studying about the semiconductor physics.I learned about the concepts of holes and all.If the electrons present in n type is in conduction band how can they fall into holes in valence band of p type which is of much lower energy state. If it is by losing energy while crossing the depletion region then how in the first place ,on joining p and n type materials, the electrons on n side combine with the holes on the other side forming the depletion layer itself. The question may be utter foolish but correct me if i am wrong.

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  • $\begingroup$ Actually, You don't join the p and n materials, you just dope the impurities on either side of an intrinsic semiconductor $\endgroup$
    – Elendil
    May 7, 2020 at 6:52

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I'm hoping the diagram at the bottom may help a little, it's one I made when I was writing up to show the basics of what happens in the p-n junction (without any bias). I would highly recommend also the website pveducation.org, which takes you through step-by-step what happens on the formation of a p-n junction and how the depletion region is formed.

The formation of the depletion region occurs chiefly through diffusion of carriers - i.e. the electrons, which are the majority carrier in n-type materials diffuse towards the p-type side, and vice versa for holes. As a result the 'ion' cores of the opposing charge are left behind, which causes the build up of an electric field between the cores (positive in n-type, negative in p-type).

P-n junction corrected

The recombination of the electrons with the holes can happen through many pathways - for example through defects in the material. It doesn't happen straight away, and the electron lifetime is an important property in a semiconductor!

I hope I have helped, if there is anything I've missed do let me know. (Edit as I realised I had my band labels the wrong way around)

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  • $\begingroup$ Thank you for your answer.Is the electron from n side on crossing the depletion region falls into the hole and move through the hole to leave p type material? $\endgroup$
    – Hari
    May 7, 2020 at 10:21
  • $\begingroup$ Once the electron has crossed the depletion region it does not necessarily recombine straight away - and once it has both carriers are effectively "out of the game" for the time being. $\endgroup$
    – Sam Pering
    May 7, 2020 at 10:29
  • $\begingroup$ They can't really leave the material so to speak - the only way it will is either by entering an external circuit (which it would have to do from the conduction band as a free carrier) - at which point the electron comes back the other side and the cycle starts again. $\endgroup$
    – Sam Pering
    May 7, 2020 at 10:31
  • $\begingroup$ Please correct me where I'm wrong.The electron from n type after crossing the depletion region at some part of the time has to recombine with hole to subsequently move out of the p type material or the electron from n type can just move through the conduction band of p type out of material without recombining in a sense that it is a part of a circuit. $\endgroup$
    – Hari
    May 7, 2020 at 10:36
  • $\begingroup$ Upon recombination with the hole the electron is back in the valence band, in the 'bulk', so will undergo some movement, but not anything particularly important. The only way it can leave the material is if it doesn't recombine - for example, once an electron is excited it has to leave the junction through the external circuit before it recombines to do any work. $\endgroup$
    – Sam Pering
    May 7, 2020 at 10:41

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