1. Radiation effects in MOSFETs and RADFET dosimetry


Glenn F. Knoll – Radiation detection and measurement
Review of radiation dosimetry, from basic principles to existing technologies.

A.H. Siedle and L. Adams – Handbook of radiation effects
Review of radiation effects useful for both engineers and researchers. Note section on RADFETs (MOSFET dosimeters), which were invented by A.H. Siedle in the early 1970s.

T.P. Ma and P.V Dressendorfer – Ionizing radiation effects in MOS devices and circuits
Review of radiation effects in MOS devices and circuits. Crucial book for understanding of physical mechanisms of radiation effects in MOS technologies. This is also relevant for understanding RADFET operation.

Scientific articles

T.R. Oldham and F.B. McLean, “Total ionizing dose effects in MOS oxides and devices”, IEEE Trans. Nucl. Sci, 2003
Review of basic mechanisms of radiation effects in MOS oxides. Article contains updates to the large body of work done in this field during the 1980s.

A. Holmes-Siedle, “The space-charge dosimeter: General principles of a new method of radiation detection”, Nuclear Instruments and Methods, 1974.
Paper describing an invention of Anrew Holmes-Siedle – p-channel MOSFET dosimeter or RADFET.

A. Holmes-Siedle, L. Adams, “RADFET: A review of the use of metal-oxide-silicon devices as integrating dosimeters”, Radiation Physics and Chemistry, 1986.
Review of basic operating principles and applications of the RADFET.

A Jaksic, G Ristic, M Pejovic, A Mohammadzadeh, C Sudre, W Lane, “Gamma-ray irradiation and post-irradiation responses of high dose range RADFETs”, Radiation Physics and Chemistry, 2002.
Comprehensive work on basic mechanisms in 100nm RADFETs from Tyndall National Institute.

Marko S Andjelković, Goran S Ristić, and Aleksandar B Jakšić, “Using RADFET for the real-time measurement of gamma radiation dose rate”, Measurement Science and Technology, 2015.
Interesting idea of using a p-n junction in the RADFET structure for measurement of dose rate.

Jiri Hofman ; Aleksandar Jaksic ; Richard Sharp ; Nikola Vasovic ; Jiri Haze, “In-Situ Measurement of Total Ionising Dose Induced Changes in Threshold Voltage and Temperature Coefficients of RADFETs”, IEEE Trans. Nucl. Sci., 2017.
Study of temperature dependence of Tyndall 400nm RADFETs.

S.Kayaa, A.Jaksic, R.Duane, N. Vasovic, E.Yilmaz, “FET-based radiation sensors with Er2O3 gate dielectric”, Nuclear Instruments and Methods in Physics Research – Section B, 2018.
RADFET with high-k dielectric material used as a gate oxide instead of standard silicon dioxide.

Selected UGR articles on RADFETs:

L.J.Asensio, M.A.Carvajal, J.A.López-Villanueva, M.Vilches, A.M.Lallena, A.J.Palma, Evaluation of a low-cost commercial MOSFET as radiation dosimeter. Sensors and Actuators A 125, 288-295, 2006.
Abstract: In this work, a low-power commercial MOS transistor was tested as a gamma radiation dosimeter. Due to the small size of its detector, low cost, reproducibility, minimal power requirements, signal conditioning and data processing, it offers excellent possibilities as a dose monitor in radiotherapy. Sensor irradiation in the unbiased mode was aimed at improving patient comfort and facilitating use. Uncertainties in the results were obtained from an exhaustive dosimetric study, following a full statistical study using Monte Carlo analysis techniques. A procedure to compensate for temperature effects was introduced in order to correct the dosimetric parameter extracted. Excellent linearity and good reproducibility were found in the accumulated threshold voltage shift as a function of the accumulated dose, up to 58 Gy (air equivalent). The uncertainty regarding sensor sensitivity was found to be less than 1% for individual device calibration. When collective calibration was carried out, the uncertainty was around 5%, accounting for a set of 31 devices. In this case, a mean value of 29.2 mV/Gy was obtained. In addition, the angular and dose rate dependencies showed good behaviour. These results suggest that the transistor studied would be an excellent candidate for use as the sensing device of a low-cost measurement system capable of in vivo dosimetry.

M.S.Martínez-García, F.Simancas, A.J.Palma, A.M.Lallena, J.Banqueria, M.A.Carvajal, General purpose MOSFETs for the dosimetry of electron beams used in intra-operative radiotherapy. Sensors and Actuators A 210, 175-181, 2014.
Abstract: The experimental response of different commercial metal-oxide-semiconductor transistors to electron beams in order to check their capabilities as radiation sensors for intra-operative radiotherapy treatments is studied. The main characteristics of the radiation response, such as sensitivity and reproducibility, have been determined using measuring algorithms previously developed by our research group for photon beams and which allow, among other advantages, the thermal compensation of the devices. Reproducing typical intra-operative radiotherapy treatment sessions, several vertical and lateral p-channel transistors in different configurations (single and two stacked transistors, unbiased and biased during irradiation) have been studied. Non zero temperature coefficients are presented in the analysed vertical transistors (BS250F, ZVP3306 and ZVP4525) and their responses show a linear behaviour with a low dispersion in the results obtained for all the studied devices. Though all of them appear to be reliable for electron dosimetry, the best candidates are the transistors included in the well-known integrated circuit CD4007, due to its higher sensitivity and better thermal compensation. In this case, a sensitivity of 13 ± 1 mV/Gy to 6 MeV electron beams has been measured with two stacked devices in biased mode. Linearity and uncertainty are comparable to that of commercial dosimetry sensors, while sensitivity is smaller.

Juan Román-Raya, Isidoro Ruiz-García, Pablo Escobedo, Alberto J. Palma, Damián Guirado and Miguel A. Carvajal, Light-Dependent Resistors as Dosimetric Sensors in Radiotherapy. Sensors 2020, 20(6), 1568 (Open Access)
Abstract: Safe quality control of radiotherapy treatments lies in reliable dosimetric sensors. Currently, ionization chambers and solid-state diodes along with electrometers as readout systems are accomplishing this task. In this work, we present a well-known and low-cost semiconductor sensor, the light-dependent resistor (LDR), as an alternative to the existing sensing devices for dosimetry. To demonstrate this, a complete characterization of the response to radiation of commercial LDRs has been conducted in terms of sensitivity, reproducibility and thermal correction under different bias voltages. Irradiation sessions have been applied under the common conditions in radiotherapy treatments using a hospital linear accelerator. Moreover, the same electrometer used for the ionization chamber has also been successfully used for LDRs. In comparison with the sensitivity achieved for the ionization chamber (0.2 nC/cGy at 400 V bias voltage), higher sensitivities have been measured for the proposed LDRs, ranging from 0.24 to 1.04 nC/cGy at bias voltages from 30 to 150 V, with a reproducibility uncertainty among samples of around 10%. In addition, LDR temperature dependence has been properly modeled using the simple thermistor model so that an easy thermal drift correction of dose measurements can be applied. Therefore, experimental results show that LDRs can be a reliable alternative to dosimetric sensors with the advantages of low size, affordable cost and the fact that it could be adopted with minimal changes in routine dosimetry quality control since the same readout system is fully compatible.

M.A.Carvajal, M.Vilches, D.Guirado, A.M.Lallena, J.Banqueri, A.J.Palma, Readout techniques for linearity and resolution improvements in MOSFET dosimeters. Sensors and Actuators A 157, 178-184, 2010.
Abstract: Novel measurement methods are presented for improving the linearity and the Signal-to-Noise-Ratio of the radiation response of pMOS transistors used as dosimeters. These methods focus on the modification of the readout process of the dosimetric parameter based, until now, on source voltage measurement at constant drain current. We propose to include an additional drain current to extend the linear range and to pulse the drain currents to reduce the electronic noise level. The response of a low-cost pMOS transistor, the 3N163 of Vishay-Siliconix (USA), has been studied in unbiased mode, without connections between the readout system and the sensor, aiming for patient comfort and greater ease for clinical use. Four different measurement methods have been discussed, depending on the polarization transistor currents during readout: only one DC current, one pulsed current, two DC currents and, finally, two pulsed currents. The standard deviation of the source voltage was measured with one DC and one pulsed current, obtaining a 20% reduction for the pulsed mode compared to the DC mode. A linear range, defined as the accumulated dose that produces 5% of the sensitivity reduction, was measured at 6.8 Gy for only one DC current, and at 10.3 Gy for two DC currents. The linear range was also extended for pulsed currents, from 8.8 Gy with one current to 16.9 Gy with two pulsed currents. The two pulsed current method provides the highest linear range and doubles the value measured with one DC current.

M A Carvajal, A Martínez-Olmos, D P Morales, J A Lopez-Villanueva, A M Lallena and A J Palma, Thermal drift reduction with multiple bias current for MOSFET dosimeters. Physics in Medicine & Biology 56 (12), 3535, 2011.
Abstract: New thermal compensation methods suitable for p-channel MOSFET (pMOS) dosimeters with the usual dose readout procedure based on a constant drain current are presented. Measuring the source–drain voltage shifts for two or three different drain currents and knowing the value of the zero-temperature coefficient drain current, IZTC, the thermal drift of source–drain or threshold voltages can be significantly reduced. Analytical expressions for the thermal compensation have been theoretically deduced on the basis of a linear dependence on temperature of the parameters involved. The proposed thermal modelling has been experimentally proven. These methods have been applied to a group of ten commercial pMOS transistors (3N163). The thermal coefficients of the source–drain voltage and the threshold voltage were reduced from −3.0 mV °C−1, in the worst case, down to −70 µV °C−1. This means a thermal drift of −2.4 mGy °C−1 for the dosimeter. When analysing the thermal drifts of all the studied transistors, in the temperature range from 19 to 36 °C, uncertainty was obtained in the threshold voltage due to a thermal drift of ±9mGy (2 SD), a commonly acceptable value in most radiotherapy treatments. The procedures described herein provide thermal drift reduction comparable to that of other technological or numerical strategies, but can be used in a very simple and low-cost dosimetry sensor.

M.A.Carvajal, F.Simancas, D.Guirado, M.Vilches, A.M.Lallena, A.J.Palma, A compact and low cost dosimetry system based on MOSFET for in vivo radiotherapy. Sensors and Actuators A 182, 146-152, 2012.
Abstract: A compact and low-cost dosimetry system based on MOSFET for in vivo radiotherapy has been developed and characterized. This electronic system consists of a wireless sensor module containing a general-purpose MOSFET and a microcontrolled reader unit. Various methods, developed in previous works, to extend the linear range and reduce noise and thermal drift have been implemented and tested in the present system. A complete dose response characterization of the system has been carried out, including thermal drift, fading effect, linear range, sensitivity, and angular dependence for a 60Co source and for 6 MV and 18 MV photon beams provided by a linear accelerator. As a result, a procedure to use the system has been built, including the recalibration conditions to enhance the dosimeter life. The system provides suitable values for monitoring in vivo radiotherapy treatments in a reliable way. Although the present system has specifications that are comparable to those of some available commercial dosimetry systems, also based on MOSFET, the sensors here considered have a larger range of use.

M.A.Carvajal, M.S.Martínez-García, D.Guirado, J.Banqueria, A.JPalma, Dose verification system based on MOS transistor for real-time measurement. Sensors and Actuators A 247, 269-276, 2016.
Abstract: This work presents a dosimetry system based on MOSFET sensors for real-time dose monitoring. MOS transistors were biased during irradiation, and the response of lateral, general-purpose 3N163 and CD4007 transistors were characterized with a 15-MV photon beam provided by a linear accelerator. The electronic circuitry to condition the sensor output and the measurement algorithm are described in depth. Due to the real-time measurement mode, the dosimetric parameter (the source voltage) showed drift. This drift depends on the bias voltage applied between the gate and the bulk terminals alternately during irradiation and readout. It can be minimized by applying a 1-V bias voltage for the 3N163 transistor and 0.85 V for the CD4007 during readout. The CD4007 transistor showed an average sensitivity of (7.8 ± 0.4) mV/Gy and the 3N163 an average of (26.4 ± 0.8) mV/Gy. The low uncertainty and acceptable sensitivity yields a resolution of 0.8 and 1.5 cGy for the CD4007 and 3N163 transistors respectively for unirradiated devices, which increases to 3 cGy and 2 cGy after 16 Gy of cumulative dose, respectively.

M.A.Carvajal, P.Escobedo, M.Jiménez-Melguizo, M.S.Martínez-García, F.Martínez-Martí, A.Martínez-Olmos, A.J.Palma, A compact dosimetric system for MOSFETs based on passive NFC tag and smartphone. Sensors and Actuators A 267, 82-89, 2017.
Abstract: In this work we describe and evaluate a dosimetric system based on an NFC (Near Field Communication) tag and a smartphone that uses commercial MOSFETs as radiation sensors. The tag is designed with commercial integrated circuits and the smartphone is the power source of the tag configured as a readout unit, user interface and storage unit. The NFC tag is supplied wirelessly by the smartphone via NFC, using a home-made structure to align the tag coil and smartphone coil in order to achieve a good inductive coupling. In this case, the commercial DMOS transistor ZVP3306 is used as dosimeter in unbiased mode, connected to the tag before and after each irradiation session to perform the sensor reading. An evaluation of the dosimetric system has been carried out irradiating three transistors with photon beam of 6 MV up to 20 Gy. The average sensitivity found is (4.75 ± 0.15) mV/Gy, which is in good agreement with the results found with our previously developed dosimetric system. Therefore, this miniaturised dosimetric system can be considered as a promising and low cost electronic architecture to be used for dosimetry control in radio-therapy treatments.


RAP2019-TYN: Detailed characterisation of Tyndall RADFETs for commercial applications in various fields.
Review of Tyndall work on commercial RADFETs with an emphasis on device characterisation.

RAD2012-UGR: Some advances in dose measurement with MOSFET for portable instrumentation.
A general view of the UGR work in this topic until 2012, including multiple current readout methods and readout systems.

RAD2015-UGR: Multiple Current method applied to characterization of RADFETs.
The multiple current method explained in detailed for RADFET readout.

RAD2018-UGR: MOSFET probe for Intra Operative Radiotherapy.
Complete readout system and probe for Intra Operative Radiotherapy using Tyndall and commercial DMOS.