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I am confused about a couple of points:

When the electrons diffuse from N side to P side, and holes diffuse from P side to N side, negative charged region is formed at the junction in P type and positive charged region is formed near the junction in the N type.

The question is:

  1. Why is the positive charge not distributed in the entire N type region (as there are free electrons for conduction in the N type region)?
  2. Why are the charges fixed at junction despite the presence of holes and free electrons?
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  • \$\begingroup\$ A few pointers for you: You only need to use one question mark at the end of a question. Observing English capitalization and grammar rules will go a long way to getting your questions the attention you would like. Almost all of your previous questions have been edited by other volunteers to help you out. \$\endgroup\$ – JYelton Feb 7 '14 at 16:35
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Why is the positive charge not distributed in the entire N type region (as there are free electrons for conduction in the N type region)?

It actually is. The diagrams are shown that way because we're primarily interested in the electric field produced by that charge, which is strongest near the junction, since that's where it's nearest the corresponding negative charge.

Why are the charges fixed at junction despite the presence of holes and free electrons?

Once the diffusion has stabilized, there are no free carriers in the central region, which becomes insulating. This insulating barrier becomes the dielectric between the two conductive regions, effectively forming a capacitor.

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  • \$\begingroup\$ That means that if we connect the outer end of the P type semiconductor with the outer end of the N type semiconductor , curent should flow ? \$\endgroup\$ – user28804 Feb 7 '14 at 17:01
  • \$\begingroup\$ You'd like to think so, but no. Additional junctions are formed between the semiconductor and whatever wire you might use to create such a connection, and it will always turn out that the fields at these junctions cancel the original P-N field. \$\endgroup\$ – Dave Tweed Feb 7 '14 at 17:04
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Why is the positive charge not distributed in the entire N type region (as there are free electrons for conduction in the N type region)?

It is, but its normally cancelled out by the presence of an equal number of electrons. In a semiconductor there are 4 main sources of charge. Electrons and holes are mobile charge carriers, and ionized donors and acceptors which are fixed charges. In a neutral chunk of semiconductor the sum of all of these charges cancels out to zero. When you create a p-n junction diffusion of the electrons and holes cause a decrease in the number of mobile charges in the region near the junction. The result is a net charge now that the ionized dopants no longer have electrons/holes to cancel out their charge.

Why are the charges fixed at junction despite the presence of holes and free electrons?

The charges are fixed at the junctions because they are fixed everywhere, and that includes near the junction. Throughout the entire semiconductor there are ionized donors and acceptors that have been integrated into the crystal lattice replacing silicon atoms. This is where the free electrons and holes come from.

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