Radiation Sensitive Field Effect Transistors Research Articles

Sensitivity and fading of irradiated RADFETs with different gate voltages

The radiation-sensitive field-effect transistors (RADFETs) with an oxide thickness of 400 nm are irradiated with gate voltages of 2, 4 and 6 V, and without gate voltage. A detailed analysis of the mechanisms responsible for the creation of traps during irradiation is performed. The creation of the traps in the oxide, near and at the silicon/silicon-dioxide (Si/SiO2) interface during irradiation is modelled very well. This modelling can also be used for other MOS transistors containing SiO2. The behaviour of radiation traps during postirradiation annealing is analysed, and the corresponding functions for their modelling are obtained. The switching traps (STs) do not have significant influence on threshold voltage shift, and two radiation-induced trap types fit the fixed traps (FTs) very well. The fading does not depend on the positive gate voltage applied during irradiation, but it is twice lower in case there is no gate voltage. A new dosimetric parameter, called the Golden Ratio (GR), is proposed, which represents the ratio between the threshold voltage shift after irradiation and fading after spontaneous annealing. This parameter can be useful for comparing MOS dosimeters.

A design concept for radiation hardened RADFET readout system for space applications

Instruments for measuring the absorbed dose and dose rate under radiation exposure, known as radiation dosimeters, are indispensable in space missions. They are composed of radiation sensors that generate current or voltage response when exposed to ionizing radiation, and processing electronics for computing the absorbed dose and dose rate. Among a wide range of existing radiation sensors, the Radiation Sensitive Field Effect Transistors (RADFETs) have unique advantages for absorbed dose measurement, and a proven record of successful exploitation in space missions. It has been shown that the RADFETs may be also used for the dose rate monitoring. In that regard, we propose a unique design concept that supports the simultaneous operation of a single RADFET as absorbed dose and dose rate monitor. This enables to reduce the cost of implementation, since the need for other types of radiation sensors can be minimized or eliminated. For processing the RADFET's response we propose a readout system composed of analog signal conditioner (ASC) and a self-adaptive multiprocessing system-on-chip (MPSoC). The soft error rate of MPSoC is monitored in real time with embedded sensors, allowing the autonomous switching between three operating modes (high-performance, de-stress and fault-tolerant), according to the application requirements and radiation conditions.

Radiation and Spontaneous Annealing of Radiation-sensitive Field-effect Transistors with Gate Oxide Thicknesses of 400 and 1000 nm

This work was supported in part by the European Union's Horizon 2020 research and innovation programme (Grant No. 857558) and the Ministry of Education, Science and Technology Development of the Republic of Serbia (Project No. 43011).

Utilization of RADFET sensors for x-ray undulators: experiential considerations on magnetic field profile before commissioning

Concerning accelerator-based light sources, electrons are accelerated in conformity with specific time structures (i.e. continuous wave, ‘cw’, or bunched), which arise out of accelerating frequencies of employed radiofrequency cavities. Thus, the concept of transverse emittance of electron bunches comes into prominence, simply defining the area of an ellipse in phase space where the electrons are occupied. In nonideal cases (i.e. in real life), when some fraction of these electrons sneak out of a bunch, the concept of ‘energy spread’ automatically arises. Since the energy spread resulting from off-axis relativistic electrons in an insertion device induces considerable radiation damage on magnetic structures of x-ray undulators, radiation-sensitive field effect transistors (RADFETs) are utilized for quantification of radiation intensity. Hence, it is crucial to bring out their influence on magnetic field profiles before commisioning in tunnels. In this respect, two different types of RADFET sensors were investigated over the magnetic poles of an undulator with 40 mm period length (shortened as an U40 undulator). The effects of the sensors on magnetic field were measured using transverse and longitudinal scans through 6.5 m-long magnetic measurement benches (MMBs). The results of two RADFET standard systems are presented, wherein one contains the magnetic material nickel. Finally, novel magnetic measurement techniques for x-ray undulators via unique MMBs are thoroughly explained as well.

Use of High-Field Electron Injection into Dielectrics to Enhance Functional Capabilities of Radiation MOS Sensors

The paper suggests a design of radiation sensors based on metal-oxide-semiconductor (MOS) structures and p-channel radiation sensitive field effect transistors (RADFET) which are capable to function under conditions of high-field tunnel injection of electrons into the dielectric. We demonstrate that under these conditions, the dose sensitivity of the sensor can be significantly raised, and, besides, the intensity of radiation can be monitored in situ on the basis of determining the ionization current arising in the dielectric film. The paper proposes the model allowing to make a quantitative analysis of charge effects taking place in the radiation MOS sensors under concurrent influence of ionization radiation and high-field tunnel injection of electrons. Use of the model allows to properly interpret results of the radiation control. In order to test the designed sensors experimentally, we have utilized γ-rays, α-particle radiation, and proton beams. We have acquired experimental results verifying the enhancement of function capabilities of the radiation MOS sensors when these have been under high-field injection of electrons into the dielectric.

Nanosatellites for Biology in Space: In Situ Measurement of Bacillus subtilis Spore Germination and Growth after 6 Months in Low Earth Orbit on the O/OREOS Mission.

We report here complete 6-month results from the orbiting Space Environment Survivability of Living Organisms (SESLO) experiment. The world’s first and only long-duration live-biology cubesat experiment, SESLO was executed by one of two 10-cm cube-format payloads aboard the 5.5-kg O/OREOS (Organism/Organic Exposure to Orbital Stresses) free-flying nanosatellite, which launched to a 72°-inclination, 650-km Earth orbit in 2010. The SESLO experiment measured the long-term survival, germination, metabolic, and growth responses of Bacillus subtilis spores exposed to microgravity and ionizing radiation including heavy-ion bombardment. A pair of radiation dosimeters (RadFETs, i.e., radiation-sensitive field-effect transistors) within the SESLO payload provided an in-situ dose rate estimate of 6–7.6 mGy/day throughout the mission. Microwells containing samples of dried spores of a wild-type B. subtilis strain and a radiation-sensitive mutant deficient in Non-Homologoous End Joining (NHEJ) were rehydrated after 14, 91, and 181 days in space with nutrient medium containing with the redox dye alamarBlue (aB), which changes color upon reaction with cellular metabolites. Three-color transmitted light intensity measurements of all microwells were telemetered to Earth within days of each 24-hour growth experiment. At 14 and 91 days, spaceflight samples germinated, grew, and metabolized significantly more slowly than matching ground-control samples, as measured both by aB reduction and optical density changes; these rate differences notwithstanding, the final optical density attained was the same in both flight and ground samples. After 181 days in space, spore germination and growth appeared hindered and abnormal. We attribute the differences not to an effect of the space environment per se, as both spaceflight and ground-control samples exhibited the same behavior, but to a pair of ~15-day thermal excursions, after the 91-day measurement and before the 181-day experiment, that peaked above 46 °C in the SESLO payload. Because the payload hardware operated nominally at 181 days, the growth issues point to heat damage, most likely to component(s) of the growth medium (RPMI 1640 containing aB) or to biocompatibility issues caused by heat-accelerated outgassing or leaching of harmful compounds from components of the SESLO hardware and electronics.

Gamma Dose Rate Measurement Using RadFET

Radiation-sensitive field-effect transistors (RadFET) are generally known for the measurement of total ionizing dose (TID) from gamma radiations. The use of an RadFET as a TID sensor is indispensable because of its characteristic dose rate-independent response. The only drawback with the RadFET, which we consider to be important, was its inability to measure the dose rate. We report here a novel methodology to extract an instantaneous dose rate information from the indigenous RadFET, which has been designed to measure an integral dose in the range from 10 mGy to 10 Gy. The methodology has enabled us to achieve a dose rate measurement sensitivity of 1838±9 counts per second (CPS) at 1±0.05 Gy/h for 60Co irradiation. The minimum dose rate that could be determined using the indigenous RadFET is 0.5 mGy/h, which is approximately three orders of magnitude better than reported to date using an RadFET dosimeter.

Performance Assessment of a New Radiation Dosimeter Based on Carbon Nanotube Field-Effect Transistor: A Quantum Simulation Study

In this paper, the performance of a new nanoscale radiation sensitive field-effect transistor (RADFET) based on carbon nanotube (CNT) is investigated using an atomistic simulation approach. The proposed radiation nanosensor uses the radiation-induced interface trapped charges as a sensing mechanism, and it is endowed with the coaxial gate and the CNT-based sensitive channel. The quantum simulations are carried out using the non-equilibrium Green’s function formalism with self-consistent electrostatics assuming a ballistic transport. The threshold voltage shift is considered as a sensing metric. A comprehensive RADFET simulation that includes the electrostatic potential, charge density, $I_{\mathrm {DS}}$ – $V_{\mathrm {GS}}$ characteristics, and sensitivity behavior, has been performed. In addition, a parametric study has been done in order to explore the impact of change in the dosimeter geometrical parameters on the RADFET sensitivity. It has been found that the proposed radiation sensor is effective and its sensitivity can be further improved using thicker silicon dioxide layer and longer CNT diameters. The obtained results make the proposed CNT-based RADFET as a promising candidate for high-performance radiation sensing applications, especially for in vivo dosimetry, where the development of new nanodosimeters is in dire need.

A novel graphene field-effect transistor for radiation sensing application with improved sensitivity: Proposal and analysis

In this paper, a new radiation sensitive field-effect Transistor (RADFET) dosimeter design based on armchair-edge graphene nanoribbon (AGNR), for high performance low-dose monitoring applications, is proposed through a quantum simulation study. The simulation approach used to investigate the proposed nanoscale RADFET is based on solving the Schrödinger equation using the mode space (MS) non-equilibrium Green’s function (NEGF) formalism coupled self-consistently with a two dimensional (2D) Poisson equation under the ballistic limits. The responsiveness of the proposed RADFET to the modulation of radiation-induced trapped charge densities is reflected via the threshold voltage, which is considered as a sensing parameter. The dosimeter behavior is investigated, and the impact of variation in physical and geometrical parameters on the dosimeter sensitivity is also studied. In comparison to other RADFETs designs, the proposed radiation sensor provides higher sensitivity and better scalability, which are the main requirements for futuristic dosimeters. The obtained results make the suggested RADFET dosimeter as a viable and attractive replacement to silicon-based MOS dosimeters.

Comparative study of MOSFET response to photon and electron beams in reference conditions

A comparative study of radiation-sensitive field effect transistors (RADFETs) response to photon and electron beams has been carried out in reference conditions. Both types of beams, routinely used in clinical radiotherapy, have been applied to RADFETs manufactured by Tyndall National Institute. All tests were carried out on RADFETs of several gate oxide thicknesses, sizes and technological processes. Experimental results showed very similar behaviour in terms of sensitivity, linearity and short-term post-irradiation fading with dose ranges typical of clinical radiation treatments. Influence of the temperature has been minimized by biasing RADFETs at zero temperature coefficient drain current during readout process. This novel experimental study shows that analysed RADFETs can be suitable to be used for dose verification in radiation therapies where electron beams are used in intra-operative radiotherapy (IORT).

Using RADFET for the real-time measurement of gamma radiation dose rate

RADFETs (RADiation sensitive Field Effect Transistors) are integrating ionizing radiation dosimeters operating on the principle of conversion of radiation-induced threshold voltage shift into absorbed dose. However, one of the major drawbacks of RADFETs is the inability to provide the information on the dose rate in real-time using the conventional absorbed dose measurement technique. The real-time monitoring of dose rate and absorbed dose can be achieved with the current mode dosimeters such as PN and PIN diodes/photodiodes, but these dosimeters have some limitations as absorbed dose meters and hence they are often not a suitable replacement for RADFETs. In that sense, this paper investigates the possibility of using the RADFET as a real-time dose rate meter so that it could be applied for simultaneous online measurement of the dose rate and absorbed dose. A RADFET sample, manufactured by Tyndall National Institute, Cork, Ireland, was tested as a dose rate meter under gamma irradiation from a Co-60 source. The RADFET was configured as a PN junction, such that the drain, gate and source terminals were grounded, while the radiation-induced current was measured at the bulk terminal, whereby the bulk was successively biased with 0 , 10 , 20 and 30 V. In zero-bias mode the radiation-induced current was unstable, but in the biased mode the current response was stable for the investigated dose rates from 0.65 to 32.1 Gy h−1 and up to the total absorbed dose of 25 Gy. The current increased with the dose rate in accordance with the power law, whereas the sensitivity of the current read-out was linear with respect to the applied bias voltage. Comparison with previously analyzed PIN photodiodes has shown that the investigated RADFET is competitive with PIN photodiodes as a gamma radiation dose rate meter and therefore has the potential to be employed for the real-time monitoring of the dose rate and absorbed dose.

RadFET Dosimeters in the Belt: the Van Allen Probes on Day 365

Van Allen Probes A and B, launched more than a year ago (in August 2012), carried 16 p-channel metal-oxide-semiconductor Radiation-sensitive Field Effect Transistors (RadFET)s into an orbit designed by NASA to probe the heart of the trapped-radiation belts. Nearly 350 days of in situ measurements from the Engineering Radiation Monitor (ERM) (1) demonstrated strong variations of dose rates with time, (2) revealed a critical correlation between the ERM RadFET dosimeters and the ERM Faraday cup data on charged particles, and (3) permitted the mapping of the belts by measuring variation with orbit altitude. This paper provides an update on early results given in a NSREC2012 paper along with details and discussion of the RadFET dosimetry data analyzed .

Sensitivity of RADFET for gamma and X-ray doses used in medicine

In this paper, the results of radiation sensitive field effect transistors (Al-gate p-channel metal-oxide-semiconductor field effect transistors) sensitivity to gamma and X-ray irradiation are presented. Radiation fields were created using 60Co source for three dose ranges (0-1 Gy, 0-5 Gy, and 0-50 Gy), as well as X-ray unit of 280 kVp spectrum for a single dose range from 0 to 5 Gy. The sensitivity was characterized by the threshold voltage shift, determined from reader circuit measurements, as a function of absorbed radiation dose. It was shown that for the three dose ranges of gamma radiation, as well as for the X-ray range from 0 Gy to 5 Gy there is approximately a linear dependence between threshold voltage shift DVT and radiation dose D. The application of positive bias of +5 V at the RADFET gate during irradiation, for these ranges of gamma radiation, also for X-ray dose range, leads to the increase in DVT and also, approximately a linear dependence between DVT and D, is established. Moreover, it was shown that the sensitivity of RADFET is much higher in the case of X-ray irradiation then in the case of gamma-ray irradiation for the same dose range.

The gamma-ray irradiation sensitivity and dosimetric information instability of RADFET dosimeter

The gamma-ray irradiation sensitivity to radiation dose range from 0.5 Gy to 5 Gy and post-irradiation annealing at room and elevated temperatures have been studied for p-channel metal-oxide-semiconductor field effect transistors (also known as radiation sensitive field effect transistors or pMOS dosimeters) with gate oxide thicknesses of 400 nm and 1 mm. The gate biases during the irradiation were 0 and 5 V and 5 V during the annealing. The radiation and the post-irradiation sensitivity were followed by measuring the threshold voltage shift, which was determined by using transfer characteristics in saturation and reader circuit characteristics. The dependence of threshold voltage shift DVT on absorbed radiation dose D and annealing time was assessed. The results show that there is a linear dependence between DVT and D during irradiation, so that the sensitivity can be defined as DVT/D for the investigated dose interval. The annealing of irradiated metal-oxide-semiconductor field effect transistors at different temperatures ranging from room temperature up to 150?C was performed to monitor the dosimetric information loss. The results indicated that the dosimeters information is saved up to 600 hours at room temperature, whereas the annealing at 150?C leads to the complete loss of dosimetric information in the same period of time. The mechanisms responsible for the threshold voltage shift during the irradiation and the later annealing have been discussed also.

The fixed oxide trap modelling during isothermal and isochronal annealing of irradiated RADFETs

The density of fixed traps (FTs), ΔNft (cm−2), during isothermal annealing of irradiated radiation-sensitive field-effect transistors (RADFETs) from room temperature to 200 °C has been modelled. The modelling has shown that a simple, two-defect model (2DM), containing two defect types: one defect type for positively charged FTs (PCFTs), and another for negatively charged FTs (NCFTs), is applicable for the temperatures up to 100 °C. However, the three-defect model (3DM), which includes additional PCFT type, should be used for temperatures from 100 to 200 °C. The models describing the behaviour of NCFTs during isothermal annealing have also been analysed. The activation energies of , and , for two PCFT types and one NCFT type, respectively, have been found. In addition, isochronal unannealed curves have been well fitted using logistic function. The modelling of isochronal annealing has also shown that one PCFT type up to 100 °C, and two PCFT types for 100–200 °C, respectively, are active, and additional defect(s) should be included for the temperatures higher than 200 °C. The isochronal annealing, which is significantly less time consuming than isothermal annealing, could be very good tool for the revealing of defects created by various stress types.

Radiation-sensitive field effect transistor response to gamma-ray irradiation

The influence of gate bias during gamma-ray irradiation on the threshold voltage shift of radiation sensitive p-channel MOSFETs determined on the basis of transfer characteristics in saturation has been investigated. It has been shown that for the gate bias during the irradiation of 5 V and 10 V the sensitivity of these transistors can be presented as the threshold voltage shift and the absorbed irradiation dose ratio. On the bases of the subthreshold characteristics and transfer characteristics in saturation using the midgap technique we have determined the densities of radiation induced oxide traps and interface traps responsible for the threshold voltage shift. In addition, the charge pumping technique was used to determine the energy density of true interface traps. It has been shown that radiation-induced oxide traps have dominant role on threshold voltage shift, especially for gate biases during the irradiation of 5 V and 10 V.

Enhanced low dose rate effect of the radiation-sensitive field effect transistors developed by the National Microelectronics Research Centre

An enhanced low dose rate effect (ELDRE) was observed in the radiation-sensitive field effect transistors (RADFETs) developed by the National Microelectronics Research Centre (NMRC) for a low total accumulated dose of less than ∼45 Gy in silicon, or 45 Gy(Si). The effect was seen to persist even after∼8 days of annealing at room temperature, implying that it was not a transient effect but rather a permanent one. On the other hand, the ELDRE was seen to disappear at high total doses of above ∼90 Gy(Si).

Total radiation dose at geostationary orbit

Active semiconductor components in satellites are sensitive to accumulated ionization radiation dose. Radiation dose and shielding estimations for electronic components are usually carried out using NASA models of space radiation particle flux. Accurate measurement of accumulated dose during the life of a satellite is essential for optimizing radiation shielding design for electronic components. Dosimeters were designed using radiation sensitive field-effect transistors (RADFETs) and accumulated dose at geostationary orbit was measured. Radiation dose as measured by these dosimeters with spherical aluminum shields are presented and compared with NASA model doses.

Temperature effects and long term fading of implanted and unimplanted gate oxide RADFETs

Key aspects of continuous operation of Radiation Sensitive Field Effect Transistors (RADFETs) as radiation monitors in space missions are addressed. The effect of possible temperature fluctuations and long-term fading on threshold-voltage measurement of Implanted and Unimplanted gate oxide RADFETs were studied. Evidence for temperature coefficient changes following irradiation and annealing cycles is presented. In addition, fading of unimplanted gate oxide RADFETs is shown to be significantly lower than that of Implanted ones.

Stacked RADFETs for increased radiation sensitivity

Hitherto, pMOS Radiation Sensitive Field Effect Transistors (RADFETs) have not been able to detect doses in the milli-rad range, which are required for low dose clinical/personnel applications. This paper reports on further investigation of a design approach, where RADFETs are connected in a stacked sequence so that increased radiation sensitivity is obtained. The radiation sensitivity obtained for 40 stacked RADFETs is approximately 220 times the single RADFET sensitivity. This enables radiation sensitivities in the milli-rad range to be measured. Theoretical equations governing the threshold voltage of a MOS device as a function of bulk-source voltage are used to theoretically evaluate the output voltage of the stacked structure. Measurement and theory are found to agree closely in this analysis. % drift and % fading of the single RADFET, as a function of total radiation induced shift in V/sub T/, is similar to that of the stacked structure.