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I'm working on the irradiation of a pool of 1,000 avalanche photodiodes. To make them radiation hard we irradiated them with 30 Gray before we mount them into a detector. The distance from the cobalt source to the devices is about 0.5 m for each diode. Our aim to do that is that the devices shall not change that much in the experiment later due to irradiation from the experiment, that's why we did that on our own before.

Now I can easily see that the V-I curves have been changed after the irradiation. To get an overview of all photodiodes I plotted the two parameters slope of the q-point and bias voltage - before and after irradiation. The q-point/amplification is the same for all devices but the bias voltage is individually (to obtain the same q-point) and also the slope at the q-point.

Pool

Question: After irradiation, to my amazement, the spread of the parameters has been reduced. Is there any explanation for that? Or just coincidence (can't really believe that)?

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For the original diodes, their characteristics are the result of small fluctuations in recombination centers as made. Radiation introduces additional centers in proportion to the dose, and if the introduced centers are greatly in excess of the as-made defects, they will dominate. Since all devices got the same dose, their characteristics will be fairly similar for a high enough dose.

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  • $\begingroup$ Cluster: Thanks a lot! Taking up this assumption shouldn't they change the same in one way, e.g., the bias voltage of all diodes should increase or something similar? $\endgroup$
    – Ben
    Commented Jan 3, 2018 at 18:30

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