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P-N junction - Need explanation

  1. Sep 6, 2009 #1
    "...Recall that the n-side is studded throughout with positively charged donor ions, fixed firmly in their lattice sites. Normally, the excess positive charge of each of these ions is compensated electrically by one of the conduction band electrons. When an n-side electron diffuses across the junction plane, however, the diffusion uncovers one of these donor ions, thus introducing a fixed position charge near the junction plane on the n-side. When the diffusing electron arrives on the p-side, it quickly combines with an acceptor ion (which lacks one electron), thus introducing a fixed negative charge near the junction plane on the p-side. In this way, diffusion of electrons results in the build up of space charge on each side of the junction plane. HOLES DIFFUSING THROUGH THE JUNCTION PLANE FROM P TO N-SIDE HAVE EXACTLY THE SAME EFFECT. (TAKE THE TIME NOW TO CONVINCE YOURSELF OF THAT)......"
    There's the problem for me, I'm unable to grasp what will happen when the holes enter the n-side.
    Can anyone please fill that in for me?
    Can you also explain... "...introducing a fixed position charge near the junction plane on the n-side...." why near the junction plane? Why not in the centre or the side away from the plane?

    Thanking in advance...
  2. jcsd
  3. Sep 6, 2009 #2
    The donor atom is effectively dissociating into a positively charge ion and an electron which are loosely bound by electrostatic attraction. The electron may diffuse through the lattice, but the ion is fixed in place by chemical bonds to the surrounding atoms. If the electron diffuses into a region rich in acceptor atoms, it drops into the valence band, effectively filling the hole associated with an acceptor atom.

    Likewise an acceptor atom dissociates into a negative ion (by accepting an electron) and a hole. If the hole diffuses into a region rich in donor atoms, it combines with an electron and ceases to exist.

    Since donor atoms are found in n-type material and acceptor atoms are found in p-type material, the only place where donor and acceptor atoms are nearby is in the vicinity of junction.

    This gives you a region near the junction where charge carriers are depleted, although the size of this region can be altered by a bias potential.
    Last edited: Sep 6, 2009
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