Elsevier

Sensors and Actuators A: Physical

Volume 247, 15 August 2016, Pages 269-276
Sensors and Actuators A: Physical

Dose verification system based on MOS transistor for real-time measurement

https://doi.org/10.1016/j.sna.2016.06.009Get rights and content

Highlights

  • System for real-time dose measurement.

  • Commercial MOSFETs used as dosimeters in biased mode.

  • Discussion about bias voltage selection.

  • Good linearity and repeatability achieved.

  • Resolution high enough to radiotherapy treatment monitoring.

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.

Introduction

Dose monitoring during radiotherapy treatments is highly advisable in order to provide the patient with accurate doses. A dose overestimation can lead to insufficient energy to destroy tumour cells, and an underestimation can lead to damage to healthy organs. Ionization chambers or diodes, as well as systems based on MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistor), have been used for real-time measurement [1], [2], [3], [4], [5], but ionization chambers need to be biased with several hundred volts to collect the charge produced by the radiation, making them less advisable for use with patients. In contrast, pMOSFETs (p-channel MOSFETs) do not need high voltages to work, but diodes must be connected during irradiation to collect the charge generated in the device, whereas MOSFETs can measure without any connections during irradiation (unbiased mode) [6], [7]. The main limitations in the unbiased mode are low sensitivity and reduced linearity at high doses. Efforts to increase the sensitivity of MOSFET dosimeters have involved fabricating them with a special technological process to achieve a thick gate oxide (more than 200 nm), with the resulting devices being known as RADFETs (RADiation Field Effect Transistor) [8], [9], [10].

Certain commercial systems based on RADFETs can be found on the market, such as BMC MOSFETs (TN-502RD-H) (Best Medical Canada) [11], MOSkin (University of Wollongong, Australia) [12], and OneDose [13]. In the first two systems, the RADFETs are connected during irradiation and provide real-time dose measurements. With MOSkin transistors, the RADFET works in the unbiased mode, so the dose is usually measured after an irradiation session instead of in real time as no connections are needed during irradiation [14], [15]. The power consumption and electronics needed for dose measurements using MOSFETs can be very low. In fact, an implantable dosimeter has been developed in an RFID tag [16], and the dose is measured with a wireless link without a battery. The dose is then read after irradiation with an RFID reader.

Another technique to increase sensitivity and linearity is to bias the transistor during irradiation with a positive gate-bulk voltage [17], [18], [19]. The electric field created in the gate oxide reduces the recombination of electro-hole pairs induced by ionization radiation, and the electrons drift to the gate and the holes to the interface between the gate oxide and the bulk. Thus, more charges are preserved and sensitivity is enhanced due to a more efficient positive build-up charge in the oxide.

Our research group developed a dosimetry system that has been used with commercial MOSFET dosimeters and RADFETs without polarization (unbiased mode) [15], [20]. The low sensitivity and limited linearity of commercial transistors is adjusted for using certain compensation and amplification techniques [21], [22]. However, this reader measures the dosimetric parameter (source or threshold voltage, depending on user configuration) before and after irradiation sessions in the off-line mode. This work presents an evolution of our dosimetric system able to measure in real time and the biased mode. The structure of this paper is as follows. After detailing the irradiation setup, the measurement dose system is fully described and tested, adjusting for the voltage drift that appears during readout. Then, the measurement system is applied to measure the dose of two commercial MOSFETs in real time. Finally, the main conclusions are drawn.

Section snippets

Irradiation setup

Irradiations were carried out in a LINAC Artiste (Siemens, Germany) with a radiation field of 10 × 10 cm2 and 15 MV photons. The MOSFETs were placed under 3 cm of solid water as buildup layers in order to ensure electronic equilibrium conditions. To control the LINAC stability, a PTW23332 ionization chamber (Radiation Products Design, Inc.; Albertville, France) was placed under the sensor modules for every irradiation session.

Two models of lateral commercial transistors were studied as dosimeters:

System description

The dose verification system consists of the sensor, the bias module, and the reader unit, which is connected to a personal computer (PC). The whole dosimetry system is placed inside the radiotherapy bunker, with the PC outside (see Fig. 1). The bias module polarizes the MOSFET during irradiation. A USB extension cable connects to a USB port in the bunker, and communication with the reader is via a simple USB B-type cable. A JAVA software application has been developed to control and download

Radiation response

Three transistors per model were irradiated, biasing the CD4007 at 0.85 V and the 3N163 at 0, 1, 10, and 20 V. Obviously, in the case of the 3N163 transistor, the higher the gate-source bias voltage in the irradiation sessions, the higher the drift (see Fig. 7). In this figure, the evolution of VS is depicted for the 3N163 pMOSFET biased to 1, 10, and 20 V. In addition, after each irradiation (4 Gy per session), fast recovery (short-term fading) of the source voltage was observed for 10 and 20 V.

Conclusions

In this work, a dosimetry system capable of real-time measurements is presented and characterized. Two commercial lateral transistors (CD4007 and 3N163) are tested as dosimeters, obtaining very acceptable results. The transistors were biased during irradiation in order to increase linearity and sensitivity. However, this biasing can cause voltage drift due to changes in the interface state charge if it is not carefully chosen. We have experimentally verified this phenomenon, and a theoretical

Acknowledgements

The authors acknowledge to the Servicio de Radiofísica of the Hospital Universitario San Cecilio (Granada, Spain) for permitting us to use its Installations. This work was funded by the Spanish Government, under project FPA2012-31993 and FPA2015-67694, also by the Junta de Andalucía, under projects P09-FQM-0534, and FQM-0220. These projects have been partially supported by European Regional Development Funds (ERDF).

Miguel A. Carvajal Rodríguez was born in 1977 in Granada (Spain). He received the MSc degrees in Physics in 2000 and the MSc degree in Electronic Engineering in 2002, both from the University of Granada and the PhD degree in Electronic Engineering from the University of Granada in 2007 about the development a dosimeter system based on commercial MOSFETs. Currently he works as tenured Professor at the University of Granada. His research interests include the effects of irradiation and

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    Miguel A. Carvajal Rodríguez was born in 1977 in Granada (Spain). He received the MSc degrees in Physics in 2000 and the MSc degree in Electronic Engineering in 2002, both from the University of Granada and the PhD degree in Electronic Engineering from the University of Granada in 2007 about the development a dosimeter system based on commercial MOSFETs. Currently he works as tenured Professor at the University of Granada. His research interests include the effects of irradiation and post-irradiation in MOSFET transistors, RFID tags with sensor capabilities, gas sensor and electrochemiluminescent sensors, and their applications to handheld instrumentation.

    M. Sofía Martínez García was born in 1985 in Granada (Spain). She received the B.S and M.Sc. degree in Telecommunications and Electronics Engineering in 2006 and 2009 respectively, and her PhD degree in 2014 from University of Granada (Granada, Spain). In addition, the M.Sc. degree in Telemedicine and Bioengineering in 2011 from Technical University of Madrid (Madrid, Spain). Currently, she works as assistant professor intern at the Autonomous University of Madrid. Her current research interests include the effects of irradiation and post-irradiation in MOSFET transistors and design of electronic instrumentations for biomedical applications.

    Damián Guirado was born in Murcia (Spain) in 1967, obtained his MSc degree in Physics in 1993 and his PhD degree in 2012 both at the University of Granada (Spain). He works at the Universitary Hospital “San Cecilio” in Granada as Medical Physics Specialist, and is interested in various topics related to medical physics, such as ionizing radiation dosimetry and metrology, radiation therapy and radiobiology.

    Jesús Banqueri was born in 1965 in Jaen (Spain). He received the BS and MS c degrees in physics in 1988 and the PhD degree in 1994 from the University of Granada, Granada, Spain. He is currently an associate professor at the University of Granada. Since 1989, he has been working in modelling and characterization of MOS transistors in the whole range of temperature with emphasis in the study of the degradation of electron mobility and other parameters as a consequence of high electric field. From 2000 in the interdisciplinary group ECsens, his current research interests are devoted to design, development and fabrication of sensors and portable electronic instrumentation for environmental, biomedical and food analysis and monitoring. Recently we are working in integrated sensors with processing electronics using MEMS and CMOS technologies.

    Alberto J. Palma was born in 1968 in Granada (Spain). He received the BS and MSc degrees in Physics (Electronics) in 1991 and the PhD degree in 1995 from the University of Granada, Granada, Spain. He is currently full professor at the University of Granada. Since 1992, he has been working on trapping of carriers in different electronic devices including characterisation and simulation of capture cross sections, random telegraph noise, and generation-recombination noise. Since 2000, in the interdisciplinary group ECsens, his current research interests are devoted to design, develop and fabrication of sensors and portable electronic instrumentation for environmental monitoring, biomedical and food analysis.

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