ET05.pdf

If the crystal lattice provides electron carriers

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If the crystal lattice provides electron carriers, then each displaced electron leaves a positive hole at its former site. Such holes can appear to move or be diffused when a nearby electron falls into the first vacant hole and this electron in turn leaves its own place empty. Small proportions of impurities or elements added to an otherwise pure element in a semiconductor can influence the number of charge carriers and their lifetimes. Special combinations of elements (as from Groups III and V of the periodic table) can be selected to optimize selected response characteristics, such as the response of semiconductors to magnetic fields. Action of Magnetic Fields on Semiconductor Charge Carriers In semiconducting devices, when an external magnetic field acts on (1) the electrons, (2) the positive holes or (3) both types of charge carriers simultaneously, the external magnetic field creates transverse forces, in accordance with Eq. 15, on both the moving electrons and the oppositely moving positive holes. Because the signs (+ or –) of the electric charge e and the velocity v are both reversed, the magnetic forces tend to deflect both types of charge carriers in the same transverse direction. The net result is transverse deflection of the normal flow lines of the electric current within the semiconducting material. This magnetic disturbance of the current flow lines produces two detectable effects within hall effect detectors. The first effect is to develop potential differences at right angles to the current flow lines, through the phenomenon known as the hall effect . The second effect is to change the resistance along the direction of the current flow paths, through the phenomenon of the magnetoresistive effect. The hall effect is readily applied to detection of weak magnetic field intensities and directions, whereas the magnetoresistive effect is most evident with very strong magnetic fields. F evB e n = 152 Electromagnetic Testing P ART 3. Hall Effect Detectors 25
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Hall Element The hall effect results from the action of externally applied magnetic fields on charge carriers in metals or semiconductors. Figure 38 shows a sketch of a simple hall element, consisting of a thin layer of semiconducting material in the form of a rectangle. A typical hall device might consist of indium arsenide, indium antimony, germanium or other semiconducting materials selected for large hall effect and for minimum response to temperature variations. Typical dimensions of the rectangular layer might be 0.4 mm (0.015 in.) wide by 0.8 mm (0.03 in.) long by 0.05 mm (0.002 in.) thick. However, for such an element, the effective sensing area might be about 0.4 mm (0.015 in.) in diameter. Smaller hall effect detectors are feasible, with maximum dimensions of only 0.1 mm (0.005 in.). Precise placing of electrical connections on the hall element becomes more difficult as size is reduced.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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