NRDC is focusing on the solid-state detectors/sensors. Solid-state detector also called Semiconductor Radiation Detector in which a semiconductor material such as a silicon or germanium crystal constitutes the detecting medium. One such device consists of a p-n junction across which a pulse of current develops when a particle of ionizing radiation traverses it. In a different device, the absorption of ionizing radiation generates pairs of charge carriers (electrons and electron-deficient sites called holes) in a block of semiconducting material; the migration of these carriers under the influence of a voltage (electric field) maintained between the opposite faces of the block constitutes a pulse of current. The pulses created in this way are amplified, recorded, and analyzed to determine the energy, number, or identity of the incident-charged particles. The calibrated intensity of the current gives the amount of radiation dose. In order to integrate p-n junctions to microelectronics we need to fabricate Field Effect Transistors (MOSFET). This type MOSFETS were named as RadFET (Tyndall, Ireland). The RadFET measures all ionizing radiation and is a total dose integration system. The RadFET has an advantage over other types of dosimeter in that it is small, lightweight and has low power consumption. Following a number of years of development, the Tyndall now has a range of commercially available dosimeters. These have different radiation sensitivities and these products are already in use in satellites in space including the nuclear industry, clinical applications and personnel dosimetry. The sensitivity requirements of clinical and personnel dosimetry are much more stringent than that of space or of the nuclear industry. The main task is to increase the sensitivity of RadFET types detectors/sensors.
When an MOS transistor is exposed to high-energy ionizing irradiation, electron-hole pairs are created in the oxide. Electron-hole pair generation in the oxide leads to almost all total dose effects. The generated carriers induce the buildup of charge, which can lead to device degradation as in figure.

 

The plot is an MOS band diagram for a p-substrate capacitor with a positive applied gate bias. Immediately after electron-hole pairs are created, most of the electrons will rapidly drift (within picoseconds) toward the gate and holes will drift toward the Si/SiO2 interface. However, even before the electrons leave the oxide, some of the electrons will recombine with holes. The fraction of electron-hole pairs that escape recombination is called the electron-hole yield or charge yield. The number of electron-hole pairs gives the intensity of current, which is calibrated to radiation dose.


 


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