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Questions about Silicon Diode Detectors

  1. Mar 3, 2010 #1
    I have some questions about how silicon diode works, specifically as a ionizing radiation measurement instrument. I will write what my understanding is so far from different textbooks/websites (please comment whether I have it correct) and also write question where I get confused. Thank you.


    - For radiation measurement, can be used in non-biased and reverse-biased mode.

    CONSTRUCTION:

    - When p-type silicon and n-type silicon come into contact

    The majority carrier (hole) of p-side diffuse to n-side, leaving negatively-charged acceptor ion.

    The majority carrier (e-) of n-side diffuse to n-side, leaving positively-charged donor ion (the minority carrier).

    This forms the space charge region or depletion layer, with built-in potential (positively charged on the n-side and negatively charged on the p-side).

    This built-in potential prevents any further movement of majority carriers.

    The space charge region is a zone with a net charge provided by the fixed ions (donors or acceptors) that have been left uncovered by majority carrier diffusion

    There are two types of p-n junction diode detectors, p-type diode (made with p-type silicon substrate doped with small amount of n-type silicon) and n-type diode (made with n-type silicon substrate doped with small amount of p-type silicon)


    - For radiation measurement, can be used in non-biased and reverse-biased mode.

    FOR OPERATION IN NON-BIAS MODE

    - Due to the built-in potential, electrons from the electron-hole pair (created in the depletion zone by ionizing radiation) moves to the n-side, where the connection to the electrometer allows measurement of charge

    FOR OPERATION IN REVERSE-BIAS MODE
    - Because the p-type material is now connected to the negative terminal of the power supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the depletion zone to increase. Similarly, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore the depletion region widens, and does so increasingly with increasing reverse-bias voltage. This increases the voltage barrier causing a high resistance to the flow of charge carriers thus allowing minimal electric current to cross the p–n junction.

    When ionizing radiation creates electron-hole pairs in the depletion zone, the electrons are swept to the n-side by the positive external potential on the n-side and the charges are collected by the electrometer.


    QUESTIONS

    - in reverse-bias mode, “there is no current, except some leakage”. But isn’t this also true for non-bias mode (once equilibrium is established, there’s no more migration, isn’t it?)

    - Between p-type diode detector (with p-type silicon substrate doped with small amount of n-type silicon) and n-type diode detector (with n-type silicon substrate doped with small amount of p-type silicon), why is p-type detector better for collecting electrons at the n-side. Wouldn’t electrons (liberated by ionizing radiation migrate to the n-side in p-type or n-type just the same? I believe most radiation measurement diodes are p-type.

    - I read that p-type diode suffers less damage over time than n-type, why is this true?

    - why does operating in reverse-bias mode prolong the life of diode (as compared to non-biased mode)?

    - what is the significance of the increased width of the depletion zone in reverse-bias mode? Does it give you better detection efficiency? Better signal to noise ratio?
     
  2. jcsd
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