The RADFETs (pMOS dosimeters) were irradiated by ionizing radiation from 60 Co gamma-ray source to dose of 230 Gy(Si) without gate bias, and their reproducibility and sensitivity to radiation were investigated. The completely automatic system containing switching matrix have been used, enabling very reliable measurements of RADFET electrical characteristics. Two sample types from each RADFET chip, with the same gate oxide thickness, but different geometry (channel width and length), were investigated. The samples have shown good reproducibility of the threshold voltage shift during irradiation, i.e., the radiation sensitivity was similar for all samples. Using MG and CP techniques, it has been shown that the slow switching (border) trap density is negligible, but the fixed trap density saturates and the fast switching trap density is linear.

The sensitivity of 100 nm RADFETs with zero gate bias up to dose of 230 Gy(Si)

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 analyzed 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.

General purpose MOSFETs for the dosimetry of electron beams used in intra-operative radiotherapy

A new microcontroller-based RADFET dosimeter reader

Different low-cost commercial general-purpose MOSFETs and a RADFET are subjected to ionizing photon beams for comparison. The main characteristics of the radiation response, such as sensitivity, reproducibility, and short-term fading, have been determined using traditional measurement algorithms and others previously developed by our research group. Several vertical and lateral p-channel transistor structures have been preliminarily analysed and the most promising device, the 3N163, has been fully characterized in different configurations (single and two stacked transistors, unbiased and biased during irradiation) and compared with ad-hoc transistors to detect ionizing radiation (RADFETs). Although all the transistors are reliable for photon dosimetry, the two-stacked configurations with the 3N163 model transistor show high sensitivities to 6 MV photon beams (33 ± 2 mV/Gy for the unbiased mode and 62 ± 3 mV/Gy for the biased mode). These values are similar to the 57.1 ± 0.7 mV/Gy of the analysed RADFET in unbiased mode. The uncertainty of the two-stacked configuration is comparable to the uncertainty obtained with the RADFET, but reproducibility is even better than in the latter. In contrast, RADFETs present the lowest short-term fading among the studied MOSFETs.

Response to ionizing radiation of different biased and stacked pMOS structures

This paper describes the possibility of using an Electrically Programmable Analog Device (EPAD) as a gamma radiation sensor. Zero-biased EPAD has the lowest fading and the highest sensitivity in the 300 Gy dose range. Dynamic bias of the control gate during irradiation was presented for the first time; this method achieved higher sensitivity compared to static-biased EPADs and better linear dependence. Due to the degradation of the transfer characteristics of EPAD during irradiation, a function of the safe operation area has been found that determines the maximum voltage at the control gate for the desired dose, which will not lead to degradation of the transistor. Using an energy band diagram, it was explained why the zero-biased EPAD has higher sensitivity than the static-biased EPAD.

Floating-Gate MOS Transistor with Dynamic Biasing as a Radiation Sensor.

Programmable floating gate MOSFET transistors were tested with gamma radiation with doses up to approximately 100Gy (air equivalent), to evaluate their suitability as dosimeters in radiotherapy. After characterization and programming at different threshold voltages, the devices were irradiated and their Vgs shift with dose monitored in real time. Post-irradiation analysis was carried out to evaluate sensitivity, linearity, reproducibility and voltage threshold annealing. A subsequent re-programming phase followed by characterization was performed to asses their post-irradiation charge restoring capabilities. It was found that up to 73% of the initial maximum threshold voltage could be recovered. A sensitivity of up to 9 mV/Gy with an uncertainty of less than 1%, an excellent linearity up to the maximum programmable threshold voltage and low noise suggest the use of this technology for in vivo dosimetry applications.

https://iopscience.iop.org/article/10.1088/1748-0221/4/02/P02002/pdf

R Bose

Papers

Evaluation of commercial programmable floating gate devices as radiation dosimeters