MOSFET dosimeter is a small portable device for monitoring and direct reading of radiation dose rate. Since it is based on the MOSFET transistor, the metal-oxide-semiconductor field-effect transistor (MOSFET), the principle of operation is similar as for semiconductor detectors. MOSFET dosimeters are now used as clinical dosimeters for radiotherapy radiation beams. Their main advantage is their physical size, which is less than 4 mm2. In radiation therapy dosimetry, MOSFET dosimeters often replace TLD dosimeters, since they offer immediate read out.
Principle of Operation of MOSFET Detectors
The operation of MOSFET detectors is summarized in the following points:
- Ionizing radiation enters the sensitive volume of the detector and interacts with the semiconductor material.
- Particle passing through the detector ionizes the atoms of semiconductor, producing the electron-hole pairs. Electron-hole pairs are generated within the silicon dioxide by the incident radiation. Electrons, whose mobility in SiO2 at room temperature is about 4 orders of magnitude greater than holes, quickly move out of the gate electrode while holes move in a stochastic fashion towards the Si/SiO2 interface where they become trapped in long term sites, causing a negative threshold voltage shift (∆VTH), which can persist for years.
- The difference in voltage shift before and after exposure can be measured, and is proportional to dose.