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Why do the free electrons in the N-type want to diffuse?

  1. Mar 5, 2015 #1
    I'm trying to understand how a diode works and for this I've used(among other resources) the book written by Albert Malvino, Electronic Principles.

    Everywhere I read about this topic, it says that when the N-type and P-type semiconductors are joined together, the free electrons from the N-type diffuse to the P-type.

    I don't understand why is the diffusion happening.

    The book Electronic Principles contains an attempt to explain this and states that the electrons diffuse because they have the same charge and they repel each other, but in my understanding the P-type and the N-type semiconductors have a neutral charge, because the number of positive charges (protons in the nuclei) is equal to the number of negative charges (free electrons and covalent bonds electrons), so the electrons repeling each other can't actually be the cause of the diffusion.
     
  2. jcsd
  3. Mar 5, 2015 #2
    When making a P-type, impurities called electron acceptors are added to the material (look up 'doping'). These ions capture free electrons from the surrounding material, leaving behind 'holes' - spaces that may be filled with electrons. In an N-type, electron donors are added which provide free electrons to the surrounding material. So the P-type has a high concentration of holes but not many free electrons, and the N-type has a high concentration of free electrons but not many holes. Hence when the two are put in contact we have a concentration gradient and diffusion occurs.
     
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