- Does your sensor convert the stimulus directly into electric output?
The best energy resolution in modern radiation detectors can be achieved in semiconductor materials, where a comparatively large number of carriers for a given incident radiation event occurs. In these materials, the basic information carriers are electron–hole pairs created along the path taken by the charged particle through the detector. The charged particle can be either primary radiation or a secondary particle. The electron–hole pairs in some respects are analogous to the ion pairs produced in the gas-filled detectors. When an external electric field is applied to the semiconductive material, the created carriers form a measurable electric current. The detectors operating on this principle are called a solid-state or semiconductor diode detectors (Figure 1). The operating principle of these radiation detectors is the same as that of the semiconductor light detectors. It is based on the transition of electrons from one energy level to another when they gain or lose energy.
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- What steps are required to transform the signal into electric output?
To fabricate a solid-state detector, at least two contacts must be formed across a semiconductor material. For detection, the contacts are connected to the voltage source, which enables carrier movement. The use of a homogeneous Ge or Si, however, would be totally impractical. The reason for that is in an excessively high leakage current caused by the material’s relatively low resistivity (50 kΩ for silicon).When applied to the terminals of such a detector, the external voltage may cause a current which is three to five orders of magnitude greater than a minute radiation-induced electric current. Thus, the detectors are fabricated with the blocking junctions, which are reverse biased to dramatically reduce leakage current. In effect, the detector is a semiconductor diode which readily conducts (has low resistivity) when its anode (p side of a junction) is connected to a positive terminal of a voltage source and the cathode (an n side of the junction) to the negative. The diode conducts very little (it has very high resistivity) when the connection is reversed; thus, the name reverse biasing is implied. If the reverse bias is made very large (in excess of the manufacturer specified limit), the reverse leakage current abruptly increases (the breakdown effect), which often may lead to a catastrophic deterioration of detecting properties or to the device destruction.
- What types of physical effects and energies are involved in the sensing and in transformation?
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