1 Introduction
Nuclear radiation dosimetry can be carried out with different devices such as thermoluminescence dosimeters (TLDs), gas chambers and semiconductor components [1-4]. A detailed survey of radiation dosimetry approaches and instrumentations is expressed in Ref. [1]. The TLDs small and standard, are the most frequently used. Nevertheless, read-out of the TLDs is time-consuming and destructive, and they are not appropriate for distant dosimetry. The semiconductor diodes are small but suffer from two main drawbacks. One is their small dosimetric response and the other is the need of high voltage. In the past decade, OSL (optically stimulated luminescence) dosimetry reappeared with promising advances [5-7], but it requires integration of optical and electronic components in the read-out system, and dosimetry information disappears after read-out.
However, efforts have been made in application of the metal-oxide field-effect transistors (MOSFET) in areas of dosimetry in space crafts, nuclear industry, skin dosimetry, personal dosimetry, clinical control, radiology and radiation therapy [8-14, 2]. MOSFETs are advantageous in their fast readout, accuracy, small volume, low power consumption, the ability to work without bias, suitable sensitivity and reproducibility, low cost, and low radiation attenuation [15, 16].
MOSFETs are used as a switch or an amplifier. As a switch, when the gate voltage is lower than the threshold voltage the transistor turns off, otherwise it turns on. A low conduction channel can be established between the source and drain. The effective resistance of the channel is regulated by the bias voltage on the gate. To create this channel, a positive and a negative gate-source voltage (VGS) need to be applied for the NMOS and the PMOS transistors, respectively. The absolute value of VGS should be greater than the threshold voltage. The channel carriers are electrons for NMOS and holes for PMOS. The VT for the NMOS and PMOS transistors are positive and negative, respectively.
When a MOSFET is exposed to gamma-rays, a shift in the VT takes place. The principle behind the VT shift is the creation of electron-hole pairs by radiation in the oxide layer. This effect leads to a rise in the number of the holes trapped in the bulk and the interface of the SiO2 layer. The VT shift, being proportional to the surface charge density, is proportional to the energy deposition in the oxide. Therefore, ∆VT is a key factor in measuring the absorbed dose (D) by the MOSFET.
L.J. Asensio et al. [17] studied performance of the 3N163 PMOS irradiated in unbiased mode to accumulated doses of 5–58 Gy. In this paper, dosimetry application of commercially available 3N163 and ZVP3306A PMOS transistors is studied in dose range of 1–5 Gy by 662 keV gamma-ray of 137Cs, at 20 mGy/s of dose rate and room temperature, in both biased and unbiased modes. The ∆VT value is considered as a measure of the accumulated radiation dose. The extent of linearity in the threshold voltage shift of the transistors is studied. The results show that the appropriate sensitivity and very good linearity make this type of PMOS an excellent candidate for reliable dosimetry. The sensitivity analysis, as an important dosimetry factor, is performed at different PMOS biases.
2 Experimental section
2.1 MOSFET selection
A MOSFET dosimeter can be made in small sizes because no additional circuit is needed to read its accumulated dose. Also, it keeps the accumulated dose even when it is out of power, and the memorized dose can be read-out after irradiation. Due to its small size and weight, MOSFET finds its dosimetry uses in nuclear industry, space crafts, robot systems and camera.
For p-type MOSFETs, there is a one-to-one relation between VT and D; while for n-type transistors. One threshold voltage value can correspond to two different doses. For this reason, PMOSs are the preferred transistors for dosimetry purposes. Another advantage of PMOSs is that their threshold voltage always changes in one direction.
To investigate the possibility of using MOSFETs available commercially as gamma-ray dosimeter, the first step is choosing appropriate commercial MOSFETs. Most commercial MOSFETs include internal body diodes as parasitic devices. Since the protective diode in reverse bias reduces MOSFETs sensitivity to radiation, these MOSFETs are not suitable for low dose measurement [18]. Another factor affecting the sensitivity of MOSFETs to radiation is thickness of the oxide layer, which is rarely given in the data sheets of the transistor by the manufacturers. Since the absolute maximum gate-substrate voltage (VGB) depends on the SiO2 layer thickness, we look in to account the maximum gate-substrate voltage.
Based on the VGB information, ZVP3306A and 3N163 transistors are selected, which are shown schematically as an example in Fig. 1. For this transistor maximum VGB is −125 V. The MOSFET is fixed on a dice of 0.3 mm by 0.3 mm surface, covered by a Ni cylinder of 0.4 mm wall thickness.
2.2 Irradiation arrangement
The threshold voltage shift, ΔVT, is frequently used as dosimetry parameter [19], and it is given by:
where A is a coefficient, D is the absorbed dose, and n is degree of linearity. The linearity depends on the voltage applied to the gate during exposure, absorbed radiation dose and thickness of the oxide layer. A linear dependence (n = 1) would be perfect, so the parameter A would define the MOSFET sensitivity [20, 21].
The MOSFET sensitivity denotes changes in the threshold voltage per unit dose:
The sensitivity of a PMOS dosimeter basically is affected by electric field in the oxide layer, radiation energy, the γ-ray beam incidence, packing of the transistor, thickness of the oxide layer, the absorbed dose and process of gate oxide growing.
To increase the radiation sensitivity, the MOSFETs are especially manufactured to have a thick oxide layer and hence greater detection volume [22]. This type of MOSFETs, known as RADFETs, is relatively high in price due to limited production. Evidently, the use of commercial MOSFETs will reduce the cost of a final dosimetry system [23, 24].
The sensitivity of a PMOS transistor is varied by altering either VGS or the gate oxide layer thickness. The sensitivity can be also improved by stacking multiple MOSFETs [25]. In general, the PMOS sensitivity for medical or personal dosimetry applications should be more than that of the nuclear and space industries. In the other words, the absorbed dose in the oxide layer of the PMOS dosimeter is affected by the composition, shape and thickness of the chip package [26-28].
The MOSFET transistors are irradiated in unbiased mode (passive) or biased mode (active). In the passive mode, no voltage is applied on the gate of a transistor and all terminals of the transistor are short-circuited together [29]. In the biased mode, a positive voltage is applied to the gate which leads to a quick charge collection. Consequently, the probability of electron-hole recombination decreases and the probability of charge trapping in the oxide layer increases, hence the increased sensitivity and linearity range for the PMOS dosimetry in the active mode. After irradiation, the variation of threshold voltage can be determined [30, 31].
Twenty MOSFETs of each type were divided in to four groups of 5. They were irradiated to 1–5 Gy at 20 mGy/s and room temperature by 662 keV γ-ray of 137Cs. The γ-ray beam incidence was perpendicular to the oxide layer. Group 1 were irradiated in passive mode, while Groups 2–4 were irradiated in active mode, at 2, 5 and 8 V gate voltage for 3N163, and 5, 8 and 10 V for ZVP3306A. To reduce the dosimetry errors due to temperature variation, the threshold voltage was measured immediately after each irradiation. Then, for each group of MOSFETs, the mean threshold voltage changes were determined and the sensitivity of transistors in each group was calculated.
2.3 Extraction of dosimetry parameter
To obtain satisfactory dosimetry results, particular attention was paid to the absorbed dose reader and its corresponding readout electronics. In saturation region, the characteristic transfer curve of the MOSFET transistor is given by:
where IDS is the drain current and β is a factor which depends on some material constants and the device size and geometry. The IDS and VGS were measured before and after irradiation using an analyser of semiconductor parameters (WQ-4832, Wu Qiang Electronics, LTD,, Hangzhou, China). The block diagram of a hardware module is shown in Fig. 2.
As illustrated in Fig. 3, the threshold voltages (VT and VT0) were obtained on the VGS axis by extrapolating the linear region of the IDS1/2-VGS curves. To minimize drift effects, the VT was measured immediately after each irradiation.
3 Results and discussion
Figure 4 shows that the absolute value of the threshold voltage shifts after irradiation. The threshold voltages were measured before and after each irradiation, and the characteristic curves were obtained from the measurements.
As can be seen from the Fig. 4, the absolute value of the VT in every gate bias voltage increases with accumulated dose. At 0–5 Gy, the threshold voltage shift is directly related to the gate bias voltage. A linear relationship between the threshold voltage shift and the absorbed dose is preserved over the studied dose range. The linear relationship proves that the threshold voltage is an adaptable parameter for radiation dosimetry.
The linear relation between ΔVT and D can be clarified by the reality that the density of interface states increases the density of interface states and reduces the holes mobility through Coulomb interactions in the conduction channel. The VT shift can be increased by the high dose irradiation. The essential reason behind the VT shift should be the shifted flat-band voltage that comes from the interface trapped charges [32]. In Fig. 4, the slope of each line represents dosimetry sensitivity of the PMOS, which increases with the gate bias voltage and remains constant with accumulated dose. A similar response of RADFETs was reported in gamma irradiation from several tenths to several hundreds of Gy [20, 21, 33-36].
The bias dependent sensitivity of the MOSFET can be explained in terms of electron-hole pairs: a higher bias voltage means a stronger electric field in the oxide layer, hence the reduced recombination probability of electron-hole pairs formed by the radiation. And in a stronger oxide electric field, electrons can easily leave the oxide and be absorbed by the gate layer which gives rise to the interface trapping. This effect, in turn, causes an increase in the threshold voltage shift. Using linear behaviour between threshold voltage shift and radiation dose for this type of commercial MOSFETs, low price and good dosimeters can be constructed.
In the unbiased mode, the difference between work functions of the gate and substrate is the main reason of the electric field in the oxide layer. Therefore, in the unbiased mode, probability of electron-hole recombination is higher than that of the biased mode. During irradiation in the biased mode, a large number of the holes escape from the primary recombination, which cause the oxide layer trapping.
Figure 5 shows mean sensitivity values for the transistors in four groups of bias voltage. As the sensitivity increases with the VGS, due to increased interface trapped charges. The sensitivity is greater in biased mode than unbiased mode.
Figure 6 shows typical values of the sensitivity verses the dose, from one of the MOSFETs in biased mode (VGS = 5 V for 3N163 and VGS = 8 V for ZVP3306A) and in unbiased mode. The remaining transistors show similar behavior. As shown in Fig.7, the obtained sensitivities are of good linearity and low uncertainty.
4 Conclusion
In this paper commercial MOSFETs were irradiated in biased mode (active) and unbiased mode (passive). Linear response of the evaluated MOSFETs to gamma radiation in the dose range of 1 to 5 Gy was obtained for both biased and unbiased modes. It was observed that the absolute value of the threshold voltage increases with increasing accumulated dose and the sensitivity of the PMOS increases with increasing gate bias.
Results have shown that the analyzed MOSFETs may be used as a dosimeter but requires further investigation for a particular use. Due to low cost and complexity, compared to the RADFET dosimeters developing of PMOS dosimeters would be more beneficial. These results suggest that, using appropriate methods of irradiation and suitable electronics for readout, these transistors can be used for dosimetry in various areas, which significantly reduces the cost of the final system.
Radio chromic film as a radiotherapy surface-dose detector
. Phys. Med. Biol. 41, 1073-1078 (1996). doi: 10.1088/0031-9155/41/6/011Improved spatial resolution by MOSFET dosimetry of an X-ray micro beam
. Med. Phys. 27, 239-244 (2000). doi: 10.1118/1.598866A miniature MOSFET radiation dosimeter probe
. Med. Phys. 21, 1721-1728 (1994). doi: 10.1118/1.597214Characterization of a 63 MeV proton beam with optically stimulated luminescent films
.Characterization of an integrated sensor using optically stimulated luminescence for in flight dosimetry
.Electronic dosimetry based on solid state detectors
. Nucl. Instrum. Methods. B. 184, 158-189 (2001). doi: 10.1016/S0168-583X(01)00711-XCalibration of a MOSFET detection system for 6-MV in vivo dosimetry
. Int. J. Radiat. Oncol. Biol. Phys. 40, 987-993 (1998). doi: 10.1016/S0360-3016(97)00894-8MOSFET dosimetry on modern radiation oncology modalities
. Radiat. Prot. Dosimetry. 101, 393-398 (2002). doi: 10.1093/oxfordjournals.rpd.a006009Skin dosimetry with new MOSFET detectors
. Radiat. Meas. 43, 929-932 (2008). doi: 10.1016/j.radmeas.2007.12.052Development of a RADFET linear array for intra cavitary in vivo dosimetry during external beam radiotherapy and brachytherapy
. IEEE Trans. Nucl. Sci. 51, 1420-1426 (2004). doi: 10.1109/TNS.2004.832570Evaluation of pre calibration implantable MOSFET radiation dosimeters for megavoltage photons beams
. Med. Phys. 32, 3346-3349 (2005). doi: 10.1118/1.2065447Feasibility study of using MOSFET detectors for in vivo dosimetry during permanent low-dose-rate prostate implants
. Radiother. Oncol. 80, 296-301 (2006). doi: 10.1016/j.radonc.2006.07.008An implantable radiation dosimeter for use in external beam radiation therapy
. Med. Phys. 31, 2658-2671 (2004). doi: 10.1118/1.1778809Evaluation of a dual bias dual Metal-Oxide-Silicon Semiconductor Field-Effect Transistor detector as radiation dosimeter
. Med. Phys. 21, 567-572 (1994). doi: 10.1118/1.597314MOS ionizing radiation dosimeters: from low to high dose measurement
. Radiat. Phys. and Chem. 61, 511-513 (2001). doi: 10.1016/S0969-806X(01)00317-6Evaluation of a low-cost commercial mosfet as radiation dosimeter
. Sens. Actuators A. 125, 288-295 (2006). doi: 10.1016/j.sna.2005.08.020Application of p-channel power VMOSFET as a high radiation doses sensor
. IEEE Trans. Nucl. Sci. 62, 1905-1910 (2015). doi: 10.1109/TNS.2015.2456211PMOS transistors for dosimetric application
. Electro. Lett. 29, 1644-1646 (1993). doi: 10.1049/el:19931095Contribution of fixed oxide traps to sensitivity of pMOS dosimeters during gamma ray irradiation and annealing at room and elevated temperature
. Sens. Actuators A. 174, 85-90 (2012). doi: 10.1016/j.sna.2011.12.011Development of the radiation sensitivity of PMOS dosimeters
. IEEE Trans. Nucl. Sci. NS. 39, 342-346 (1992). doi: 10.1109/23.277514Readout techniques for linearity and resolution improvements in MOSFET dosimeters
. Sens. Actuators A. Physical. 157, 178-184 (2010). doi: 10.1016/j.sna.2009.11.034Hole traps and trivalent silicon centers in metal/oxide/silicon devices
. J. App. Phys. 55, 3495-3499 (1984). doi: 10.1063/1.332937Strategies for millirad sensitivity in PMOS dosimeters
. IEEE Trans. Nuc. Sci. 45, 1475-1480 (1997). doi: 10.1109/RADECS.1997.698911Sensitivity and fading of pMOS dosimeters with thick gate oxide
. Sens. Actuators A. 51, 153-158 (1996). doi: 10.1016/0924-4247(95)01211-7Electrical properties of MOS radiation dosimeters
. Revue De Physique Appliquee. 21, 283-287 (1986). doi: 10.1051/rphysap:01986002104028300Build-up modification of commercial diodes for entrance dose measurementsin higher energy’ photon beams
. Radiother. Oncol. 51, 249-256 (1999). doi: 10.1016/S0167-8140(99)00058-4Reversibility of trapped hole annealing
. IEEE Trans. Nucl. Sci. 35, 1186-1191 (1988) doi: 10.1109/23.25437Direct Reading Dosimeter
, (1991).Dosimetric evaluation of a new design MOSFET in vivo dosimeter
. Med. Phys. 32, 110-117 (2005). doi: 10.1118/1.1827771A Novel quasi-3-D interface trapped charge induced threshold voltage model for quadruple-gate MOSFETs, including equivalent number of gates (ENG)
. IEEE Trans. on Electron Devices. 61, 1615-1618 (2014). doi: 10.1109/TED.2014.2312922Gamma-ray irradiation and post-irradiation responses of high dose range RADFETs
. IEEE Trans. Nucl. Sci. 49, 1356-1363 (2002). doi: 10.1109/TNS.2002.1039667Radiation sensitive field effect transistor response to gamma-ray irradiation
. Nucl. Tech. Rad. Protec. 26, 25-31 (2011). doi: 10.2298/NTRP1101025PTemperature effects and long term fading of implanted and unimplanted gate oxide RADFETs
. IEEE Trans. Nucl. Sci. 51, 2917-2921 (2004). doi: 10.1109/TNS.2004.835065Gamma-ray irradiation and post-irradiation at room and elevated temperature response of pMOS dosimeters with thick gate oxides
. Nucl. Tech. Rad. Protec. 26, 261-265 (2011). doi: 10.2298/NTRP1103261P