P-n junction

  1. Hello.

    I have a question regarding the p-n junction. When I look at figure B here:


    it looks completely wrong to me. Take the E-field. It is going to be positive outside the depletion region, which in turn will make the voltage tend towards zero for x→±∞. Right ??
  2. jcsd
  3. mfb

    Staff: Mentor

    The figure looks good.
    (explain more what you don't understand to get more specific answers)
  4. Well the depletion region is a dipole, isn't it? So the e-field will look like this


    So if the field is negative "inside" the dipole, it is positive outside (but of small magnitude, of course).
  5. mfb

    Staff: Mentor

    It is not, you have two "infinite" sheets, and infinite sheets have a constant field around them - two sheets with opposite charge of the same magnitude lead to fields that cancel everywhere outside.
  6. Well, clearly the depletion region is not infinite in extent. Also, the field is not constant in the figure that you said looks good.
  7. mfb

    Staff: Mentor

    That is a very good approximation for most transistors.
    The field is zero outside the regions with net charge, so the potential is constant. The field is not constant in the region with net charge, as there is net charge on the sides.
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