Nükleer Radyasyona Duyarlı Alan Etkili Transistör Tabanlı Radyasyon Sensörlerinin Uzay Uygulamaları için Geliştirilmesi ve Karakterizasyonu
Özet
Transistors are one of the most important application areas of nanotechnology. It has been possible to produce smaller, more efficient, and faster transistors with the development of production techniques. In addition to their use as electronic circuit components for switching and power regulation purposes, transistors can also be used for other purposes. There are many studies in which Metal Oxide Semiconductor Field Effect Transistors (MOSFET) -namely one of the most common transistor types- are used as radiation sensors. However, studies on space radiation applications using sensors that have oxide layer thicknesses below 100 nm have been found to be insufficient in the literature. The main objective of this study is developing and characterizing a radiation detector using radiation sensors called Nuclear Radiation Sensitive Field Effect Transistor (NürFET) with 40 nm, 60 nm, and 100 nm gate oxide thicknesses for space applications. The material type of gate oxide layer is silicon dioxide (SiO2). A radiation detector, which can also be utilized in satellites, was designed and manufactured to measure the performances of sensors. To determine the performances of the sensors, the radiation tests were performed and all measurements were collected in real-time. Additionally, there are two (2) sensors on the detector which were used in space missions before to compare and analyze the performances of Nuclear Radiation Sensitive Field Effect Transistor type radiation sensors. One of these sensors is Radiation Sensitive Field Effect Transistor (RADFET) and the other one is Floating Gate Dosimeter (FGDOS). The performances of the detector and sensors were tested with Cobalt-60 (Co-60) Gamma radiation source. The radiation detector was exposed to radiation in two steps. The first irradiation step lasted in 256 seconds and all radiation sensors (NürFET, RADFET, and FGDOS) were operated successfully. In the first irradiation, after a while, FGDOS did not work because of the high radiation dose and it congested the detector electrically. In the second part of the test, FGDOS was removed from the circuitry with the help of software. The second irradiation step lasted for 416 seconds. Nuclear Radiation Sensitive Field Effect Transistor and RADFET sensors were operated successfully. It has been shown that the sensitivity of radiation sensors increases with the increasing oxide layer thicknesses. It is recommended by the manufacturer of the Nuclear Radiation Sensitive Field Effect Transistor type radiation sensor that the shifts in threshold voltages of Nuclear Radiation Sensitive Field Effect Transistor type radiation sensors should be read with 10 μA constant current source. However, it is not easy to supply "10 μA" constant current precisely because of the harsh conditions of the space environment. Therefore, a second constant current source of 100 μA is used which is relatively easier to use on the detector circuit. In the first radiation test, the threshold voltage shift for a 40 nm Nuclear Radiation Sensitive Field Effect Transistor type radiation sensor was measured 3.698 mV with 10 μA and 3.884 mV with 100 μA. In the second radiation test, the threshold voltage shifts for a 40 nm Nuclear Radiation Sensitive Field Effect Transistor type radiation sensor was measured 5.668 mV with 10 μA and 6.081 mV with 100 μA. In the first radiation test, the threshold voltage shift for a 60 nm Nuclear Radiation Sensitive Field Effect Transistor type radiation sensor was measured 7.134 mV with 10 μA and 8.091 mV with 100 μA. In the second radiation test, the threshold voltage shifts for a 60 nm Nuclear Radiation Sensitive Field Effect Transistor type radiation sensor was measured 10.103 mV with 10 μA and 10.330 mV with 100 μA. In the first radiation test, the threshold voltage shift for a 100 nm Nuclear Radiation Sensitive Field Effect Transistor type radiation sensor was measured 8.826 mV with 10 μA and 9.342 mV with 100 μA. In the second radiation test, the threshold voltage shifts for a 100 nm Nuclear Radiation Sensitive Field Effect Transistor type radiation sensor was measured 13.603 mV with 10 μA and 14.066 mV with 100 μA. The radiation detector was designed to measure the Total Ionizing Dose (TID) effect, one of the effects of Space radiation. The radiation dose level to be exposed in orbit is related to the type of material used in the structural model (outer surface/panels) of the satellite. For this purpose, a conceptual satellite model was created and the radiation dose to be exposed in orbit for four different material types (satellite outer surface/panel) was calculated with the analysis program. Polymethyl methacrylate (PMMA), glass (SiO2), aluminum (Al), and lead (Pb) materials were used in the satellite structural model. The thicknesses of the materials were chosen as 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm. On these materials, silicon (Si) point detectors were identified on the inner and outer surface of the satellite and corresponding dose values were obtained for each point. The exposed dose levels for the silicon point detector which was assigned at the center of the satellite were calculated on orbit - for polymethyl methacrylate material - as 291 krad, 90.56 krad, 31.69 krad, 17.03 krad, 10.57 krad and 7.171 krad for shielding thicknesses of satellite 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm, respectively. The exposed dose levels for the silicon point detector which was assigned at the center of the satellite were calculated on orbit - for glass material - as 74.53 krad, 26.25 krad, 8.504 krad, 4.320 krad, 2.945 krad and 2.428 krad for shielding thicknesses 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm, respectively. The exposed dose levels for the silicon point detector which was assigned at the center of the satellite were calculated on orbit - for aluminum material - as 74.39 krad, 26.10 krad, 8.356 krad, 4.281 krad, 2.965 krad, and 2.472 krad for shielding thicknesses 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm and 5 mm, respectively. The exposed dose levels for the silicon point detector which was assigned at the center of the satellite were calculated on orbit - for lead material - as 5.822 krad, 3.270 krad, 2.428 krad, 2.067 krad, 1.839 krad and 1.641 krad for shielding thicknesses 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, and 5 mm, respectively. The radiation analysis result shows that the exposed dose for the point which is inside the satellite decreases with the increase of thicknesses and material density. According to the radiation test and analysis results obtained within the scope of this thesis study, a radiation detector using the Nuclear Radiation Sensitive Field-Effect Transistor based radiation sensors that can measure space radiation dose has been successfully designed and manufactured for space applications. When all findings obtained are evaluated, it can be concluded that the designed and produced radiation detector, utilizing Nuclear Radiation Sensitive Field-Effect Transistor based radiation sensors, could have an important potential for space applications.
