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

  1. Hi,

    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. 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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