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Author Topic: How does radiation exposure affect free carrier electrons in a material?  (Read 2178 times)

Offline Unknown_Guy

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This question I am posing is very specific although not fully thought out as of yet. I would at first like to tell all of you reading this now I take science more as an enjoyment and hobby, furthermore I am by no means a professional. So I was wondering what are the effects of different types of radiation on free carrier electrons in elements like silicon.
« Last Edit: 25/05/2013 23:01:01 by chris »


Offline evan_au

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Low-frequency radiation like long-wave infra-red and radio waves pass straight through silicon, just like visible light does through glass.

Silicon has a bandgap of 1.1 electron-Volts (eV). At room temperature, nearly all the electrons are orbiting a single atom, and so do not roam freely through the material, so they can't conduct electricity. A very small fraction gain enough energy from thermal vibrations to move into the conduction band, and these electrons can conduct electricity. This makes Silicon a semiconductor.

When a photon with energy 1.1eV or higher hits silicon, it can kick an electron into the conduction zone, where it will respond to external electric fields, and conduct electricity. This effect allows building silicon photodetectors, such as used in light sensors. Large banks of these silicon detectors are used in the Large Hadron Collider, as high-energy radiation like gamma rays can kick electrons out of its parent atom, as can charged particles like protons and mesons.

Pure Silicon has a fairly high resistance, so these electrons don't travel easily through the material. So the silicon is usually "doped" with elements from adjacent columns on the periodic table to introduce more charge carriers and reduce the resistance.
To trap generated electrons more effectively, part of the silicon is doped with elements like Phosphorus to give it extra electrons, and part is doped with elements like Boron to give it conductive "holes". The junction between two regions develops a voltage, and this voltage can deliver more current when light shines on it; this effect is used to generate electricity in solar cells. This diode structure is used in light detectors for DVD players and optical fiber communications.

With some semiconductors, the diode structure can also be used "in reverse": put in electricity and it will produce light with the energy of the bandgap - the Light Emitting Diode.

Offline graham.d

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Just to add to evan_au's reply... In general, any radiation "particle" (not just a photon) will generate electron-hole pairs as it gives up its energy within the crystal lattice of Silicon. This is a problem for memory devices, especially for aerospace use, where cosmic rays can generate enough electron-hole pairs in the depletion region (between a p-doped and n-doped regions) to neutralise any charge that may be stored on a capacitor for which one of the regions is a part. A good deal of error correction circuits need to be deployed. Generally this does not damage the device but just produces a "soft" error. This was a particular problem with Dynamic Memories (DRAMs) in the 1980s which was traced to material within the device package which was slightly radioactive and emitted alpha particles. These caused soft errors, particularly as the radioactive material was part of the filler in the package seal which meant it penetrated the device's memory cells at just the right angle to maximise the number of electron hole pairs within the depletion region within the memory cells. The alpha particle energy was such that in typically penetrated about 20 microns. It was solved by applying a polyimide coating to the die although now this type of package is not used so the polyimide is unnecessary.

In high levels of radiation semiconductor devices can be permantly damaged and devices for aerospace use have to be designed with this in mind. Typically the transistor characteristics change and eventually will prevent a device from working. Devices have to be designed to tolerate this change which can be done by use of specific technology which is less susceptible to damage and also designing with wide tolerances. Earth-bound aerospace (unless military) is usually OK with fairly conventional processes but Space use (or military where it has to work despite nuclear attack) has to tolerate a very large radiation - often 1 Mrad.

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