Detection and Measurement of Nuclear Radiation (1962) / Chapter Skim
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Semiconductor Radiation Detectors
Semiconductor Radiation Detectors
Pages 58-74
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From page 58... ...
6. Energy Spectra In several laboratories, gridded ionization chambers are used routinely for analyzing energy spectra of charged particles, especially alpha particles from radioactive samples.
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From page 59... ...
Except at very low temperatures, a highly purified semiconductor exhibits intrinsic conductivity, as distinct from impurity conductivity of specimens which contain foreign atoms at some of the lattice sites. The electronic band scheme which explains this behavior can be discussed with reference to Fig.
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From page 60... ...
is called the Fermi energy or the Fermi level. Without resorting to a mathematical treatment, it is sufficient to state qualitatively that in a p-type semiconductor, the Fermi level lies in the energy gap, near the valence band; on the other hand, because n-type material contributes a large number of electrons into the conduction band, the Fermi level is displaced to an energy near the conduction band.
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From page 61... ...
Recent data on the response of surface barrier and diffused junction diodes to fission fragments does give some evidence for a defect. It is likely that this isolated case of nonproportionality of pulse height with energy involves a failure to collect all the current carriers formed in high density by the intensely ionizing fragments.
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From page 62... ...
Thus, the p-type region acquires a negative charge and the n-type region becomes positively charged, until the two regions are aligned about the constant Fermi level energy of the system. The result of this equilibrium is that a potential barrier is established which opposes any further flow of electrons or holes across the junction.
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From page 63... ...
This capacitance arises because the space-charge region resembles two charge sheets of finite thickness, separated by a thin, high-resistivity layer, the barrier itself. A modification of the p-n junction detector, which promises to yield sensitive regions deep enough even for beta particles 63
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28. Nomograph which relates the applied reverse bias voltage, barrier depth, dynamic capacitance, and impurity concentration for a Schottky-type barrier in silicon.
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. If reverse bias is applied to this junction, the electric field in the region around the point c will exert a force which will move the positively charged Li+ ions from the Li-rich side of the junction to the Li-deficient side.
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The positive space charge within the barrier arises because the donor sites are nearly completely ionized, and there are few if any electrons to compensate. Further, the space charge is enhanced near the surface by the presence of minority carriers (holes)
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From page 67... ...
Many of the general remarks on the operation of gas ionization chambers apply equally to the semiconductor type as well. It will be recalled that, in gas ionization chambers, the low mobility of heavy positive ions gives rise to several problems associated with the collection of charge; on the other hand, the semiconductor detector possesses the great advantage that both the hole and the electron are highly mobile current carriers.
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2. Fabrication Techniques Semiconductor radiation detectors resemble conventional photovoltaic cells, and the usual semiconductor fabrication 68
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31. Sketches showing semiconductor radiation detector construction (not to scale)
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Lower leakage currents and higher Some manufacturers are: Harshaw Chemical Company, 1945 East 97th Street, Cleveland 6, Ohio. Hughes Aircraft Company, P
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From page 71... ...
Also, the leakage noise increases with the reverse bias applied, and the noise may limit the usable bias to an unacceptably low value in cases where a deep depletion layer, and hence a high reverse bias, is required. As was mentioned earlier, some form of edge protection is required; various materials have been suggested for this purpose, e.g., Apiezon W, epoxy resins, oxide films, and silica or glass films.
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33. The bias supply must be well filtered, and it is usually helpful to monitor the leakage current and the rms noise as the bias voltage is varied.
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The clipping time of the main amplifier may be chosen for optimum signal to noise ratio, since the collection time is extremely rapid compared to gas ionization chambers. Recommended 54 clipping times are 0.5 to 2 ^sec.
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From page 74... ...
Figure 34 shows the counting chamber designed by Chetham-Strode, et al., for precision alpha spectrometry with silicon surface-barrier detectors. The relatively large internal dimensions were chosen to remove scattering surfaces from the source and detector.
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