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Is the P-N junction the only means to acheive a depletion region?

  1. Sep 27, 2013 #1

    I have a quick question. When constructing a semiconductor diode, the P-N junction is used to create a depletion region (depleted of mobile majority carriers) which is the active volume of the diode/detector. The region enables an electric current generated from the incomming radiation to be measured, due to the fact that the majority carriers are removed (they contribute to a current of order 0.1A, and radiation induced current is like 0.00001A).

    My question is, is the P-N junction the only means to acheive a depletion region?

    E.g if I took a (100%) pure crystal and applied a bias voltage to sweep away the electrons that were thermally excited, I would remove the majority carriers just like in the P-N junction. Does this statement fail to hold only due to the fact that there is no 100% pure crystal? And that any small amount of impurities drastically alters the crystals properties? Or is there another reason?

    Thank you for your time!
  2. jcsd
  3. Sep 27, 2013 #2
    No, you can also get it by applying an electric field across an insulating layer, e.g. in a FET.


    Even in a pure crystal you always get thermal production of electron-hole pairs. So you either have to cool the semiconductor to a temperature where k_B T << band gap, or use a material with large band gap at room temperature. Such materials are usually called insulators and not semiconductors.

    For this reason Germanium detectors for x- and gamma rays are usually operated at liquid nitrogen temperature.

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