X ray detectors and xrf detection channels 219

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X-Ray Detectors and XRF Detection Channels 219 Valence band Conduction band Electrons Energy gap Holes Fig. 4.12. Band structure of a semiconductor material band gap. Once promoted in the conduction band, an electron can move un- der the influence of an externally applied electric field and can be collected at an electrode. The vacancy or hole left in the valence band by the pro- moted electron can also move by the applied field in the opposite direction of the electron. The velocities of electrons and holes are different according to their different mobilities in the crystal (e.g. 1450 cm 2 V 1 s 1 for electrons and 505 cm 2 V 1 s 1 for holes in silicon at room temperature [22]). When an X-ray photon interacts in the crystal, the primary electron cre- ated by the ionization process excites bound electrons to the conduction band. These secondary electrons, if sufficiently energetic, can further create addi- tional electrons by a cascading process which finally leads to a large number of electron–hole pairs that can be collected at the electrodes of the device. Impurity-free Si and Ge materials are called intrinsic semiconductors. When dopants are added to an intrinsic semiconductor, the conduction prop- erties of the material can be modified. Adding pentavalent dopants like P or As into a tetravalent Si (or Ge) semiconductor will increase the conductivity of the material because only four electrons of the pentavalent atom are involved in binding it with the Si (or Ge) atoms and therefore the fifth electron can easily be promoted to the conduction band with just a small amount of energy. Dopants of this kind are called donors because they introduce free electrons, and the doped semiconductor is referred to as n-type material. On the con- trary, trivalent dopands like B, called acceptors , provide free holes, increasing the conductivity of the material. Semiconductors doped with acceptors are called p-type materials. The basic principle of a simple Si diode detector based on a junction between two p and n semiconductor materials is shown in Fig. 4.13. When the pn diode is reverse biased, the bulk material (n-type in our example) is depleted from the free charge and the carriers produced by the interaction of photon in the bulk can be suitably collected at the electrodes by means
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220 A. Longoni and C. Fiorini p + n+ n - bulk - V bias - + + - - - + Ionizing radiation e Fig. 4.13. Working principle of a pn diode detector. The same voltage applied to the device provides the depletion of the semiconductor bulk and the drift field ε responsible for the collection of the charge carriers, created by the ionizing radiation, by the electrodes of the applied field. A continuous flow of charge thermally generated within the bulk material (dark current) is also collected at the detector electrodes, contributing to a statistical spread in the measurement of the signal charge.
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  • Spring '14
  • MichaelDudley

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