- Email: [email protected]
SO2 gas sensing characteristics of FET- and resistor-type gas sensors having WO3 as sensing material
Solid State Electronics 165 (2020) 107747
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
SO2 gas sensing characteristics of FET- and resistor-type gas sensors having WO3 as sensing material
T
Gyuweon Junga, Yujeong Jeonga, Yoonki Honga, Meile Wub, Seongbin Honga, Wonjun Shina, ⁎ Jinwoo Parka, Dongkyu Janga, Jong-Ho Leea, a
Department of Electrical and Computer Engineering and Inter-University Semiconductor Research Center (ISRC), Seoul National University, Seoul 08826, Republic of Korea b School of Information Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
A R T I C LE I N FO
A B S T R A C T
The review of this paper was arranged by “Jung-Hee Lee”
Sensing characteristics of the SO2 gas are investigated using a horizontal floating-gate FET-type gas sensor. SO2 gas sensing characteristics of the resistor-type gas sensor, a conventional sensor, fabricated on the same wafer are also investigated and compared with the FET-type gas sensor. The 18-nm-thick WO3 deposited using a radio frequency magnetron sputtering method is used as the sensing material. The SO2 gas sensing characteristics are examined while varying the operating temperature, the concentration of the SO2 gas, and the pre-bias (Vpre). The sensing mechanism for detecting the SO2 gas in an FET-type gas sensor is examined. Both sensors are able to detect up to 10 ppm of SO2 gas, and the FET-type sensor shows significantly improved gas response by using the pre-bias scheme.
Keywords: FET-type gas sensor Resistor-type gas sensor Tungsten trioxide (WO3) Sulfur dioxide (SO2) ID drift Pre-bias
1. Introduction Nowadays, as the atmospheric environment becomes more important, the demand for detection of harmful gases in the atmosphere is increasing. Sulfur dioxide (SO2) gas is a typical noxious gas which can causes respiratory illness and acid rain [1]. SO2 gas is mainly produced by the combustion of sulfur-containing materials such as fossil fuels and is easily found in the atmosphere [2]. Numerous researches have been conducted to detect the SO2 gas using various types of gas sensors such as electrochemical type, SAW (surface acoustic wave) type and semiconductor type [3–5]. Electrochemical type gas sensors have the advantage of high selectivity but the life span of the sensing material is relatively short [6]. In case of SAW type gas sensor, while it has the advantage of fast response time, the size of the sensor is large due to the propagation path [4,7,8]. On the contrary, the semiconductor type gas sensors exhibit the superior properties for commercialization, since the durability is reasonable and the manufacturing cost is adequately low. For these reasons, in a recent decade, most of the SO2 gas sensor are examined using the semiconductor type [2,5,9,10]. The semiconductor type gas sensor can be divided into resistor-type and FET-type. So far, nearly all of studies have been conducted using resistor-type gas sensors, because various sensing materials can easily be applied due to their simple structures. However, the resistor type sensors are known to
⁎
Corresponding author. E-mail address: [email protected] (J.-H. Lee).
https://doi.org/10.1016/j.sse.2019.107747
Available online 24 December 2019 0038-1101/ © 2019 Elsevier Ltd. All rights reserved.
be large in size to reduce device to device variation and to obtain enough output current. Even though the conventional FET-type has the advantage of smaller size and lower power than the resistor-type [11], it has limitation that fabrication process is complicated and its sensitivity is low due to the suspended gate structure [12–14]. So our group has introduced the horizontal floating-gate FET-type gas sensor which overcomes the weakness of previous FET-type gas sensors [15,16]. In the process of making the horizontal floating gate FET-type gas sensor, a sensing layer is formed at the final step thus the process become straightforward and various sensing materials can be adopted without contamination [15,16]. The interdigitated structure, which is frequently applied to increase the sensitivity in resistor-type sensors [17], allows the FET-type sensors to have a high sensitivity by having a high coupling ratio (between the control and floating gates) [16]. So far, we have used tin oxide (SnOx), zinc oxide (ZnO), platinumdoped indium oxide nanoparticles (Pt-In2O3), and tungsten disulfide (WS2) as sensing materials and have successfully detected nitrogen dioxide gas (NO2), hydrogen sulfide gas (H2S), carbon monoxide gas (CO), and oxygen gas (O2) [15,16,18–20]. Unfortunately, the response of our previous gas sensors to SO2 gas was so low that it is difficult to detect low concentrations SO2 gas. In order to detect SO2 gas, the 18-nm-thick tungsten trioxide (WO3) film is adopted as a sensing material of gas sensors. The horizontal
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Fig. 1. (a) Top SEM view of the fabricated gas sensor. (b) 2-D cross-sectional schematic diagram cut along a dotted line A-A’ in (a). (c) Surface SEM image of the WO3 layer formed on control-gate (“X” mark in (a)). (d) Result of EDS at the “X” mark in (a).
floating-gate FET-type gas sensor is fabricated on the same wafer as a resistor-type gas sensor with an interdigitated electrode structure. The SO2 gas sensing characteristics of these two sensors are investigated and compared. Both sensors are able to detect up to 10 ppm of SO2 gas, and the FET-type sensor shows significantly improved gas response by using the pre-bias scheme.
The source and drain is formed in a self-aligned manner. The O (10 nm)/N (20 nm)/O (10 nm) passivation layer is consecutively stacked to prevent contamination. After defining contact holes, Ti (30 nm), TiN (20 nm) and Al (100 nm) layers are deposited in sequence by using sputtering process and patterned using a lift-off process to form electrodes for the drain, source, body, and control-gate. In the last step, a lift-off patterning for the sensing material is performed and an 18-nm-thick n-type semiconductor material WO3 is deposited using a radio frequency magnetron sputtering method. The WO3 thin film is grown by using WO3 target in a controlled environment of argon and oxygen gas. Un necessary WO3 is removed together with removing the photoresist for lift-off.
2. Methods 2.1. Device structure and fabrication The structure of the fabricated FET-type sensor is displayed in Fig. 1(a)–(b). As shown in Fig. 1(a), the opposing control and floating gates are interdigitated with each other to increase sensitivity by obtaining a high coupling ratio [15]. As shown in Fig. 1(b), the sensing layer is formed on the part of control-gate and O/N/O passivation layer. The surface SEM image of the sensing material formed on the controlgate is shown in Fig. 1(c). Grains with a diameter of 30 nm on average and grain boundaries are observed. The result of energy dispersive Xray spectroscopy (EDS) at the “X” mark in Fig. 1(a) is presented in Fig. 1(d). The primary energy is set at 6 keV to prevent thinly formed WO3 information from mixing with other material information. In this study, the p-type FET is used since it has less flicker noise than n-type FET [21]. The FET used in this work can be programmed and erased using a floating gate to operate the sensor within the desired operating range [22]. The key device fabrication process steps are explained as follow. The 6” single crystalline n-type Si wafer having (1 0 0) orientation is used as a starting substrate. The local oxidation of silicon (LOCOS) process is used to isolate the devices. The 10-nm-thick gate oxide is grown by a dry oxidation process. The 350-nm-thick in situ doped n+ poly-crystalline Si is deposited and patterned to form the floating-gate.
2.2. Gas sensing measurement system The gas sensing characteristics are measured using a semiconductor parameter analyzer (B1500A, Agilent) including waveform generator and fast measurement unit (WGFMU) pulse measurement module. The gases flow into the chamber at a flow rate of 200 SCCM and the gas sensing characteristics are obtained by switching the ambience between the target gas (SO2 gas) and the reference gas (dry air). 3. Results and discussion As mentioned earlier, WO3 thin film is adopted as a sensing material on the horizontal floating-gate FET-type gas sensor and resistor-type gas sensor to detect SO2 gas. In general, both FET and resistor type sensors detect SO2 gas with satisfactory performance. The SO2 gas sensing characteristics are examined while varying the operating temperature, the concentration of the SO2 gas, and the pre-bias (Vpre). The I-V characteristics and sensing characteristics of the FET-type gas sensor are investigated as follows. 2
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Fig. 2. (a) ID-VCG curves of the FET-type gas sensor as a parameter of operating temperature. (b) ID-VDS curves of the gas sensor.
The SO2 gas sensing characteristics of the FET-type sensor using pulse measurement method (Fig. 3(b)) are represented in Figs. 4–7. Under different temperature conditions, a predetermined VrCG is applied to the control-gate to have the same operation current of 465nA. Here, the pre-bias (Vpre) is 0 V except for Fig. 7. The pulse widths of tread and tpre are 30 μs and 5 s, respectively. As shown in Figs. 4, 5 and 7 the pulse measurement method successfully suppresses the ID drift. The response characteristics of the fabricated gas sensor to SO2 gas are measured from 60 °C to 180 °C at an interval of 30 °C (Fig. 4(a)). The SO2 gas is injected for 140 s, reacts with the gas sensor, and then the sensor starts to recover as the atmosphere changes to dry air. As the temperature rises from 60 °C to 150 °C, the value of the current change (ΔID) increases. the amount of current change at 180 °C is reduced as compared with that of 150 °C. This is because the increased desorption amount due to temperature rise is larger than the adsorption amount [25]. Meanwhile, the recovery characteristics gradually improve as the temperature rises. As the concentration of SO2 gas changes from 10 ppm to 125 ppm, the amount of the current change gradually increases (Fig. 4(b)). The SO2 gas response characteristics obtained from the pulse measurements in Fig. 4 are similar, when compared to the DC measurement results in Fig. 3(a) measured at the same temperature (150 °C), same concentration (125 ppm) and different reaction times (150 s). The recovery rate on the other hand, seems to be slower than that of the DC measurement. This is due to the ID drift of the DC measurement in the direction of decreasing the ID. Fig. 5 illustrates the transient response of the FET-type gas sensor, alternating the ambience with SO2 gas and dry air at 180 °C. The sensor repeatedly reacts to 125 ppm SO2 gas for 140 s and then recovers to dry
3.1. I-V characteristics of FET-type sensor The transfer characteristics (ID-VCG curves) of the FET-type gas sensor as a parameter of operating temperature are presented in Fig. 2(a). As the temperature rises, the off-current increases due to increased junction leakage and the on-current is reduced due to phonon scattering, and also the subthreshold swing (SS) increases [23]. Fig. 2(b) shows the ID-VDS curves of the gas sensor as a function of VCG. It is confirmed that the sensor works well as a FET. The channel width and length of the fabricated FET-type sensor are both 2 μm. 3.2. Gas sensing characteristics of FET-type sensor In order to evaluate the SO2 gas sensing capability of the sensor, the transient response is investigated for 125 ppm SO2 gas at 150 °C with DC measurement method (Fig. 3(a)). A VDS of −0.1 V and a VCG of 0.44 V are applied to allow the sensor to operate in the linear region, providing an appropriate current value to cope with oxidizing and reducing gases. After reacting with SO2 gas for 150 s, the sensor is recovered by using dry air. When DC measurement method is used, a constant bias is applied to the control gate for a long time, so charge can be easily trapped at the interface between the sensing material and the O/N/O stack, resulting in ID drift [24]. The drift may not occur depending on the process for forming the sensing material or the composition of the sensing material, and further research is needed. As an alternative to minimizing the drift, the pulse operation scheme shown in Fig. 3(b) is used. Before the read bias is applied, the pre-bias (Vpre) is applied to the control-gate. In the read period (tread), the drain bias (VrDS) is synchronized with the control-gate bias (VrCG).
Fig. 3. (a) Transient response of the FET-type gas sensor for 125 ppm SO2 gas at 150 °C with DC measurement method. (b) Pulse operation scheme for the gas sensor. The VrDS is synchronized with the VrCG during the tread. 3
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Fig. 4. (a) Transient response of the FET-type gas sensor for 125 ppm SO2 gas as a parameter of the operating temperature. (b) Transient response of the gas sensor for SO2 gas at 150 °C as a parameter of the gas concentration. The SO2 gas is injected for 140 s, reacts with the gas sensor, and then the sensor starts to recover as the atmosphere changes to dry air.
Fig. 5. Transient response of the FET-type gas sensor in alternatively changing ambience with SO2 gas (125 ppm, 140 s) and dry air (360 s) at 180 °C.
Fig. 7. Transient response of the fabricated gas sensor for 10 ppm SO2 gas as a parameter of Vpre from −2 V to 1 V.
3.3. SO2 gas sensing mechanism of FET-type sensor The SO2 gas sensing mechanism of the FET-type gas sensor having WO3 as the sensing material is described as follows. The threshold voltage (Vth) of the sensor can be written as [23]:
Vth = ϕms −
Qox + 2ψB − Cox
2qND εSi |2ψB | Cox
(1)
where ϕms, Qox, and Cox represent the work-function difference between the control-gate and the channel, the oxide charge, and the equivalent gate oxide capacitance, respectively. ND, εSi, and ψB stand for the doping concentration in the channel, the dielectric constant of Si, and the surface potential at threshold condition, respectively. Since the SO2 gas is an oxidizing gas that takes electrons from the reaction with WO3 and becomes SO2− [26], the n-type semiconductor WO3 is depleted when the WO3 is exposed to the SO2 gas. Looking closely at this process, when WO3 is exposed to SO2 gas, traps with energy that can trap electrons are generated by the adsorbed SO2 gas molecules. Since the trap energy is lower than the Fermi level of the WO3, electrons move from the WO3 to the adsorbed gas molecules [27]. As a result, the gas molecules are negatively ionized, and the sensing material around them depletes. The SO2 molecules ionized near the interface between the WO3 sensing layer and the O/N/O covering the floating-gate act as equivalent negative oxide charges. The negatively charged molecules generated by the gas reaction along the large interdigitated pattern can have relatively large influences on the small channel region as the oxide
Fig. 6. ID-VCG curves of the sensor as a function of exposed time to 125 ppm SO2 gas. The inset is an enlargement of the circled area for clear comparison.
air for 360 s. Because the sensor does not recover completely within a given recovery time, the minimum and maximum current values increase little by little in one response and recovery cycle, but the overall response and recovery trend is nearly the same. The sensor is considered capable of producing a reproducible gas reaction. Note that if the recovery time is longer, it will recover completely.
4
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
carriers of the sensing material changes [22,28]. When the negative pre-bias is applied, the number of electrons existing near the boundary of the O/N/O and WO3 is increased, so that the adsorption of SO2 gas at the boundary region is increased [28]. These adsorbed gas molecules near the O/N/O greatly affect the change of the energy band diagram and flat-band voltage of FET, which increases the amount of current change. For example, the response characteristic of 10 ppm SO2 gas with a pre-bias of −1 V is similar to that of 50 ppm SO2 gas with prebias of 0 V in Fig. 4(b), On the other hand, when the positive pre-bias is applied, the electron concentration near the O/N/O is lowered, and SO2 gas is less likely to be adsorbed to the sensing material near the O/N/O. Therefore, the gas sensing characteristic of 10 ppm SO2 gas with 1 V pre-bias is worse than that of 0 V pre-bias. 3.5. Selectivity of the FET-type sensor Fig. 8. Comparison of responses of the FET-type gas sensor to the five different 125 ppm gases at 150 °C.
To check the selectivity of the FET-type sensor, the responses of other gases are investigated (Fig. 8). The concentration of CO2, H2S, NH3, and NO2 gases is fixed at 125 ppm in examining the selectivity. The response is obtained using the following equation,
charges. This can be thought of as a kind of antenna effect. Accordingly, the Qox increases negatively, resulting in a decrease in Vth of the pMOSFET. As a result, ID-VCG curve of the sensor is shifted in a positive direction. In Fig. 6, the ID-VCG curve of the sensor shifts in the positive direction as the exposed time to 125 ppm SO2 gas increases. The ID-VCG curves of the sensor show that the reaction saturates over time. In Fig. 4(a)–(b), it can be also observed that the slope of the ID-t curve gradually decreases with time.
Response(%) = [(Igas/ Iair ) − 1]
(2)
where Iair and Igas are the current in dry air before the reaction and the current when exposed to the target gas for 140 s. The sensor can react to a variety of gases because it does not use a catalyst that can only react selectively with SO2 gas. Although the sensor does not react selectively to SO2 gas, we can identify the specific gas by using multiple sensors and processing data from them [29].
3.4. Effect of pulsed pre-bias to FET-type sensor 3.6. Comparison with resistor-type sensor It has been reported that the use of pre-bias scheme to FET-type sensors can improve gas sensing characteristics [22,28]. The transient response of the fabricated gas sensor for 10 ppm SO2 gas as a parameter of the pre-bias (Vpre) is shown in Fig. 7. It can be observed that the response increases as the negative pre-bias is applied to the controlgate. When the pre-bias of −2 V is applied, the response increased ~ 3.7 times compared to that of 0 V. Applying a negative prebias increases the SO2 gas response and positive pre-bias decreases the SO2 gas response. When the pre-bias is applied, the distribution of
The resistor-type gas sensor is fabricated on the same wafer using the same fabrication process steps as the FET-type gas sensor. The structure of the resistor-type sensor is illustrated in Fig. 9(a)–(b). The two opposing electrodes are interdigitated and the distance between the electrodes is 2 μm. The sensing material WO3 is deposited on the electrodes. The Fig. 9(c) plots the IR-VR curves of the resistor-type sensor as a function of exposed time to 125 ppm SO2 gas. Since the SO2 gas molecules take away the electrons present in WO3 and becomes Fig. 9. Resistor-type gas sensor fabricated on the same chip with the same process as the FET-type gas sensor. (a) Top SEM view of the fabricated resisotrtype gas sensor. (b) 2D cross-sectional schematic diagram cut along a dotted line A-A’ in (a). (c) IR-VR curves of the resisotr-type sensor as a function of exposed time to 125 ppm SO2 gas at 150 °C. (d) Transient response of the resistive gas sensor for SO2 gas at 150 °C as a parameter of the gas concentration.
5
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Atmos Environ 2005;39:6868–74. [2] Lee SC, Hwang BW, Lee SJ, Choi HY, Kim SY, Jung SY, et al. A novel tin oxide-based recoverable thick film SO2 gas sensor promoted with magnesium and vanadium oxides. Sens Actuators, B 2011;160:1328–34. [3] Fergus JW. A review of electrolyte and electrode materials for high temperature electrochemical CO2 and SO2 gas sensors. Sens Actuators, B 2008;134:1034–41. [4] Lee YJ, et al. Development of a SAW gas sensor for monitoring SO2 gas. Sens Actuators, A 1998;64:173–8. [5] Tyagi P, Sharma A, Tomar M, Gupta V. Metal oxide catalyst assisted SnO2 thin film based SO2 gas sensor. Sens Actuators, B 2016;224:282–9. [6] Guth U, Vonau W, Zosel J. Recent developments in electrochemical sensor application and technology–a review. Meas Sci Technol 2009;20:042002. [7] Fox CG, Alder JF. Surface acoustic wave sensors for atmospheric gas monitoring. A review. Analyst 1989;114:997–1004. [8] Devkota J, Ohodnicki P, Greve D. SAW sensors for chemical vapors and gases. Sensors 2017;17:801. [9] Boudiba A, Zhang C, Bittencourt C, Umek P, Olivier MG, Snyders R, et al. SO2 gas sensors based on WO3 nanostructures with different morphologies. Procedia Eng 2012;47:1033–6. [10] Tyagi P, Sharma A, Tomar M, Gupta V. SnO2 thin film sensor having NiO catalyst for detection of SO2 gas with improved response characteristics. Sens Actuators, B 2017;248:998–1005. [11] Capone S, et al. Solid state gas sensors: state of the art and future activities. J Optoelectron Adv Mater 2003;5(5):1335–48. [12] Eisele I, Doll T, Burgmair M. Low power gas detection with FET sensors. Sens Actuators, B 2001;78:19–25. [13] Oprea A, Frerichs HP, Wilbertz C, Lehmann M, Weimar U. Hybrid gas sensor platform based on capacitive coupled field effect transistors: Ammonia and nitrogen dioxide detection. Sens Actuators, B 2007;127:161–7. [14] Burgmair M, Frerichs HP, Zimmer M, Lehmann M, Eisele I. Field effect transducers for work function gas measurements: device improvements and comparison of performance. Sens Actuators, B 2003;95:183–8. [15] Kim CH, et al. A new gas sensor based on MOSFET having a horizontal floating-gate. IEEE Electron Device Lett 2014;35:265–7. [16] Hong Y, et al. Highly selective ZnO gas sensor based on MOSFET having a horizontal floating-gate. Sens Actuators, B 2016;232:653–9. [17] Tamaki J, Hashishin T, Uno Y, Dao DV, Sugiyama S. Ultrahigh-sensitive WO3 nanosensor with interdigitated Au nano-electrode for NO2 detection. Sens Actuators, B 2008;132:234–8. [18] Hong S, et al. Observation of physisorption in a high performance FET-type oxygen gas sensor operating at room temperature. Nanoscale 2018;10:18019–27. [19] Hong S, Hong Y, Jeong Y, Jung G, Shin W, Park J, et al. Improved CO gas detection of Si MOSFET gas sensor with catalytic Pt decoration and pre-bias effect. Sens Actuators, B 2019;300:127040. [20] Jeong YJ, et al. Gas sensing characteristics of the FET-type gas sensor having inkjetprinted WS2 sensing layer. Solid State Electron 2019;153:27–32. [21] Lee JH, Kim SY, Cho IH, Hwang SB, Lee JH. 1/f noise characteristics of sub-100 nm MOS transistors. JSTS 2006;6:38–42. [22] Shin J. et al., “Highly improved response and recovery characteristics of Si FET-type gas sensor using pre-bias, IEEE International. Electron Devices Meeting, pp. 18.1.14, 2016. [23] Taur Y, Ning TH. Fundamentals of modern VLSI devices. Cambridge university press; 2009. p. 72–176. [24] Jeong J. et al., “Suppression of Drift in FET-type Gas Sensor Having WS2 Nanoparticles Using Pulse Measurement,” 17th International Meeting on Chemical Sensors, P1AP.6, 2018. [25] Comini E, Faglia G, Sberveglieri G, editors. Solid state gas sensing. Boston, MA: Springer US; 2009. [26] Shimizu Y, Matsunaga N, Hyodo T, Egashira M. Improvement of SO2 sensing properties of WO3 by noble metal loading. Sens Actuators, B 2001;77:35–40. [27] Wolkenstein T. Electronic processes on semiconductor surfaces during chemisorption. New York: Springer Science & Business Media; 2012. [28] Wu M, Kim CH, Shin J, Hong Y, Jin X, Lee JH. Effect of a pre-bias on the adsorption and desorption of oxidizing gases in FET-type sensor. Sens Actuators, B 2017;245:122–8. [29] Jung G. et al., “Accurate identification of gas type and concentration using DNN reflecting the sensing properties of MOSFET-type gas sensor,” 2019 IEEE International Symposium on Olfaction and Electronic Nose (ISOEN) pp. 1-4, 2019.
Fig. 10. Response versus SO2 gas concentration of the FET-type sensor and resistor-type sensor.
SO2− [26], the slope of the IR-VR curve decreases as the exposure time to SO2 gas increases. That is to say, as the sensor reacts with SO2 gas over time, the conductivity of the sensor decreases and the resistance increases. The Fig. 9(d) presents the transient response of the resistortype gas sensor for SO2 gas at 150 °C as a parameter of the gas concentration. In Fig. 10, we compare the response of the FET-type sensor and the resistor-type sensor for different concentrations of SO2 gas at 150 °C using the data in Fig. 4(b), Figs. 7 and 9(d). The response is obtained using Eq. (2). In Fig. 10, the resistor-type sensor shows a higher response than that of the FET-type sensor. However, FET-type gas sensor with negative pre-bias (Vpre = -2 V) shows a significant increase in response to 10 ppm SO2 gas, which is higher than that of the resistortype. In our FET-type gas sensors, the pre-bias scheme appears to be very useful for detecting low gas concentrations. 4. Conclusions We have investigated the SO2 gas sensing characteristics of the FETtype and resistor-type gas sensors with an 18-nm-thick WO3 as a sensing layer. Both sensors successfully detected SO2 gas. Since the ID drift was observed when using a DC measurement method, the pulse measurement method was used to thoroughly suppress the ID drift. The SO2 gas sensing characteristics were examined while varying the operating temperature, the concentration of the SO2 gas and the pre-bias. We have shown that applying a pre-bias of −2 V increases the response to 10 ppm of SO2 gas by ~ 3.7 times compared to that of 0 V. Compared to the resistor-type gas sensor fabricated on the same wafer, the FET-type gas sensor with a pre-bias of −2 V showed better sensing characteristics at a concentration of 10 ppm of SO2 gas. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Gyuweon Jung received the B.S. degree with the ‘summa cum laude’ in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2017. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the Inter-University Semiconductor Research Center, SNU. His current research interests include FET-based sensor array and Electronic nose.
Acknowledgement This work was supported by the National Research Foundation of Korea (NRF-2016R1A2B3009361) and the Brain Korea 21 Project in 2019. References [1] Pandey JS, Kumar R, Devotta S. Health risks of NO2, SPM and SO2 in Delhi (India).
6
Solid State Electronics 165 (2020) 107747
G. Jung, et al. Yujeong Jeong received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2017. She is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. She is also with the InterUniversity Semiconductor Research Center, SNU. Her current research interests include FET-based sensor platform design and sensing materials.
Jinwoo Park received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2019. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the InterUniversity Semiconductor Research Center, SNU. His current research interests include FET-based sensor platform fabrication and measurement.
Yoonki Hong received the B.S. degree in Electrical and Computer Engineering from Seoul National University (SNU), Seoul, Korea in 2013. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at SNU. He is also with the Inter-University Semiconductor Research Center, SNU. His current research interests include MOSFET-based gas sensor and humidity sensor.
Dongkyu Jang received the B.S. and M. S. degrees in electrical engineering from Korea University, Seoul, in 2008 and 2010, respectively. In 2010, he joined at Samsung Electronics, where he has been working in the area of DRAM integration. He is currently pursuing the Ph.D. degree with the Department of Electrical and Computer Engineering, Seoul National University, Seoul, South Korea. He is also with the Inter-University Semiconductor Research Center, SNU. His current research interests include pressure sensors and gas sensors.
Meile Wu received the B.S. degree in Electronics Science and Technology from Shenyang University, Shenyang, China in 2012 and M.S. degree in Microelectronics and Solid State Electronic from Shenyang University of Technology, Shenyang, China in 2015. She received the Ph.D. degrees from Seoul National University (SNU), Seoul, Korea in 2019.
Jong-Ho Lee received the B.S. degree from Kyungpook National University, Daegu, Korea, in 1987 and the M.S. and Ph.D. degrees from Seoul National University, Seoul, in 1989 and 1993, respectively, all in Electronic Engineering. In 1993, he worked on advanced BiCMOS process development at ISRC, Seoul National University as an Engineer. In 1994, he was with the School of Electrical Engineering, Wonkwang University, Iksan, Chonpuk, Korea. In 2002, he moved to Kyungpook National University, Daegu, Korea, as a Professor of the School of Electrical Engineering and Computer Science. Since September 2009, he has been a Professor in the School of Electrical and Computer Engineering, Seoul National University, Seoul, Korea. From 1994 to 1998, he was with ETRI as an invited member of technical staff, where he worked on deep submicron MOS devices, device isolation. From August 1998 to July 1999, he was with Massachusetts Institute of Technology, Cambridge, as a postdoctoral fellow, where he was engaged in the research on sub–100 nm double-gate CMOS devices. He has authored or coauthored more than 216 papers published in refereed journals and over 326 conference papers related to his research and has been granted 85 patents in this area. He received 18 awards for excellent research papers and research excellence. He invented bulk FinFET, Saddle FinFET (or bCAT) for DRAM cell, and NAND flash cell string with virtual source/drain, which have been applying for mass production. His research interests include CMOS technology, nonvolatile memory devices, thin film transistors, sensors, neuromorphic technology, and device characterization and modeling.
Seongbin Hong received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2016. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the InterUniversity Semiconductor Research Center, SNU. His current research interests include FET -based sensor platform design and fabrication.
Wonjun Shin received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2017. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the InterUniversity Semiconductor Research Center, SNU. His current research interests include FET-based sensor platform fabrication and measurement.
7
Contents lists available at ScienceDirect
Solid State Electronics journal homepage: www.elsevier.com/locate/sse
SO2 gas sensing characteristics of FET- and resistor-type gas sensors having WO3 as sensing material
T
Gyuweon Junga, Yujeong Jeonga, Yoonki Honga, Meile Wub, Seongbin Honga, Wonjun Shina, ⁎ Jinwoo Parka, Dongkyu Janga, Jong-Ho Leea, a
Department of Electrical and Computer Engineering and Inter-University Semiconductor Research Center (ISRC), Seoul National University, Seoul 08826, Republic of Korea b School of Information Science and Engineering, Shenyang University of Technology, Shenyang 110870, China
A R T I C LE I N FO
A B S T R A C T
The review of this paper was arranged by “Jung-Hee Lee”
Sensing characteristics of the SO2 gas are investigated using a horizontal floating-gate FET-type gas sensor. SO2 gas sensing characteristics of the resistor-type gas sensor, a conventional sensor, fabricated on the same wafer are also investigated and compared with the FET-type gas sensor. The 18-nm-thick WO3 deposited using a radio frequency magnetron sputtering method is used as the sensing material. The SO2 gas sensing characteristics are examined while varying the operating temperature, the concentration of the SO2 gas, and the pre-bias (Vpre). The sensing mechanism for detecting the SO2 gas in an FET-type gas sensor is examined. Both sensors are able to detect up to 10 ppm of SO2 gas, and the FET-type sensor shows significantly improved gas response by using the pre-bias scheme.
Keywords: FET-type gas sensor Resistor-type gas sensor Tungsten trioxide (WO3) Sulfur dioxide (SO2) ID drift Pre-bias
1. Introduction Nowadays, as the atmospheric environment becomes more important, the demand for detection of harmful gases in the atmosphere is increasing. Sulfur dioxide (SO2) gas is a typical noxious gas which can causes respiratory illness and acid rain [1]. SO2 gas is mainly produced by the combustion of sulfur-containing materials such as fossil fuels and is easily found in the atmosphere [2]. Numerous researches have been conducted to detect the SO2 gas using various types of gas sensors such as electrochemical type, SAW (surface acoustic wave) type and semiconductor type [3–5]. Electrochemical type gas sensors have the advantage of high selectivity but the life span of the sensing material is relatively short [6]. In case of SAW type gas sensor, while it has the advantage of fast response time, the size of the sensor is large due to the propagation path [4,7,8]. On the contrary, the semiconductor type gas sensors exhibit the superior properties for commercialization, since the durability is reasonable and the manufacturing cost is adequately low. For these reasons, in a recent decade, most of the SO2 gas sensor are examined using the semiconductor type [2,5,9,10]. The semiconductor type gas sensor can be divided into resistor-type and FET-type. So far, nearly all of studies have been conducted using resistor-type gas sensors, because various sensing materials can easily be applied due to their simple structures. However, the resistor type sensors are known to
⁎
Corresponding author. E-mail address: [email protected] (J.-H. Lee).
https://doi.org/10.1016/j.sse.2019.107747
Available online 24 December 2019 0038-1101/ © 2019 Elsevier Ltd. All rights reserved.
be large in size to reduce device to device variation and to obtain enough output current. Even though the conventional FET-type has the advantage of smaller size and lower power than the resistor-type [11], it has limitation that fabrication process is complicated and its sensitivity is low due to the suspended gate structure [12–14]. So our group has introduced the horizontal floating-gate FET-type gas sensor which overcomes the weakness of previous FET-type gas sensors [15,16]. In the process of making the horizontal floating gate FET-type gas sensor, a sensing layer is formed at the final step thus the process become straightforward and various sensing materials can be adopted without contamination [15,16]. The interdigitated structure, which is frequently applied to increase the sensitivity in resistor-type sensors [17], allows the FET-type sensors to have a high sensitivity by having a high coupling ratio (between the control and floating gates) [16]. So far, we have used tin oxide (SnOx), zinc oxide (ZnO), platinumdoped indium oxide nanoparticles (Pt-In2O3), and tungsten disulfide (WS2) as sensing materials and have successfully detected nitrogen dioxide gas (NO2), hydrogen sulfide gas (H2S), carbon monoxide gas (CO), and oxygen gas (O2) [15,16,18–20]. Unfortunately, the response of our previous gas sensors to SO2 gas was so low that it is difficult to detect low concentrations SO2 gas. In order to detect SO2 gas, the 18-nm-thick tungsten trioxide (WO3) film is adopted as a sensing material of gas sensors. The horizontal
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Fig. 1. (a) Top SEM view of the fabricated gas sensor. (b) 2-D cross-sectional schematic diagram cut along a dotted line A-A’ in (a). (c) Surface SEM image of the WO3 layer formed on control-gate (“X” mark in (a)). (d) Result of EDS at the “X” mark in (a).
floating-gate FET-type gas sensor is fabricated on the same wafer as a resistor-type gas sensor with an interdigitated electrode structure. The SO2 gas sensing characteristics of these two sensors are investigated and compared. Both sensors are able to detect up to 10 ppm of SO2 gas, and the FET-type sensor shows significantly improved gas response by using the pre-bias scheme.
The source and drain is formed in a self-aligned manner. The O (10 nm)/N (20 nm)/O (10 nm) passivation layer is consecutively stacked to prevent contamination. After defining contact holes, Ti (30 nm), TiN (20 nm) and Al (100 nm) layers are deposited in sequence by using sputtering process and patterned using a lift-off process to form electrodes for the drain, source, body, and control-gate. In the last step, a lift-off patterning for the sensing material is performed and an 18-nm-thick n-type semiconductor material WO3 is deposited using a radio frequency magnetron sputtering method. The WO3 thin film is grown by using WO3 target in a controlled environment of argon and oxygen gas. Un necessary WO3 is removed together with removing the photoresist for lift-off.
2. Methods 2.1. Device structure and fabrication The structure of the fabricated FET-type sensor is displayed in Fig. 1(a)–(b). As shown in Fig. 1(a), the opposing control and floating gates are interdigitated with each other to increase sensitivity by obtaining a high coupling ratio [15]. As shown in Fig. 1(b), the sensing layer is formed on the part of control-gate and O/N/O passivation layer. The surface SEM image of the sensing material formed on the controlgate is shown in Fig. 1(c). Grains with a diameter of 30 nm on average and grain boundaries are observed. The result of energy dispersive Xray spectroscopy (EDS) at the “X” mark in Fig. 1(a) is presented in Fig. 1(d). The primary energy is set at 6 keV to prevent thinly formed WO3 information from mixing with other material information. In this study, the p-type FET is used since it has less flicker noise than n-type FET [21]. The FET used in this work can be programmed and erased using a floating gate to operate the sensor within the desired operating range [22]. The key device fabrication process steps are explained as follow. The 6” single crystalline n-type Si wafer having (1 0 0) orientation is used as a starting substrate. The local oxidation of silicon (LOCOS) process is used to isolate the devices. The 10-nm-thick gate oxide is grown by a dry oxidation process. The 350-nm-thick in situ doped n+ poly-crystalline Si is deposited and patterned to form the floating-gate.
2.2. Gas sensing measurement system The gas sensing characteristics are measured using a semiconductor parameter analyzer (B1500A, Agilent) including waveform generator and fast measurement unit (WGFMU) pulse measurement module. The gases flow into the chamber at a flow rate of 200 SCCM and the gas sensing characteristics are obtained by switching the ambience between the target gas (SO2 gas) and the reference gas (dry air). 3. Results and discussion As mentioned earlier, WO3 thin film is adopted as a sensing material on the horizontal floating-gate FET-type gas sensor and resistor-type gas sensor to detect SO2 gas. In general, both FET and resistor type sensors detect SO2 gas with satisfactory performance. The SO2 gas sensing characteristics are examined while varying the operating temperature, the concentration of the SO2 gas, and the pre-bias (Vpre). The I-V characteristics and sensing characteristics of the FET-type gas sensor are investigated as follows. 2
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Fig. 2. (a) ID-VCG curves of the FET-type gas sensor as a parameter of operating temperature. (b) ID-VDS curves of the gas sensor.
The SO2 gas sensing characteristics of the FET-type sensor using pulse measurement method (Fig. 3(b)) are represented in Figs. 4–7. Under different temperature conditions, a predetermined VrCG is applied to the control-gate to have the same operation current of 465nA. Here, the pre-bias (Vpre) is 0 V except for Fig. 7. The pulse widths of tread and tpre are 30 μs and 5 s, respectively. As shown in Figs. 4, 5 and 7 the pulse measurement method successfully suppresses the ID drift. The response characteristics of the fabricated gas sensor to SO2 gas are measured from 60 °C to 180 °C at an interval of 30 °C (Fig. 4(a)). The SO2 gas is injected for 140 s, reacts with the gas sensor, and then the sensor starts to recover as the atmosphere changes to dry air. As the temperature rises from 60 °C to 150 °C, the value of the current change (ΔID) increases. the amount of current change at 180 °C is reduced as compared with that of 150 °C. This is because the increased desorption amount due to temperature rise is larger than the adsorption amount [25]. Meanwhile, the recovery characteristics gradually improve as the temperature rises. As the concentration of SO2 gas changes from 10 ppm to 125 ppm, the amount of the current change gradually increases (Fig. 4(b)). The SO2 gas response characteristics obtained from the pulse measurements in Fig. 4 are similar, when compared to the DC measurement results in Fig. 3(a) measured at the same temperature (150 °C), same concentration (125 ppm) and different reaction times (150 s). The recovery rate on the other hand, seems to be slower than that of the DC measurement. This is due to the ID drift of the DC measurement in the direction of decreasing the ID. Fig. 5 illustrates the transient response of the FET-type gas sensor, alternating the ambience with SO2 gas and dry air at 180 °C. The sensor repeatedly reacts to 125 ppm SO2 gas for 140 s and then recovers to dry
3.1. I-V characteristics of FET-type sensor The transfer characteristics (ID-VCG curves) of the FET-type gas sensor as a parameter of operating temperature are presented in Fig. 2(a). As the temperature rises, the off-current increases due to increased junction leakage and the on-current is reduced due to phonon scattering, and also the subthreshold swing (SS) increases [23]. Fig. 2(b) shows the ID-VDS curves of the gas sensor as a function of VCG. It is confirmed that the sensor works well as a FET. The channel width and length of the fabricated FET-type sensor are both 2 μm. 3.2. Gas sensing characteristics of FET-type sensor In order to evaluate the SO2 gas sensing capability of the sensor, the transient response is investigated for 125 ppm SO2 gas at 150 °C with DC measurement method (Fig. 3(a)). A VDS of −0.1 V and a VCG of 0.44 V are applied to allow the sensor to operate in the linear region, providing an appropriate current value to cope with oxidizing and reducing gases. After reacting with SO2 gas for 150 s, the sensor is recovered by using dry air. When DC measurement method is used, a constant bias is applied to the control gate for a long time, so charge can be easily trapped at the interface between the sensing material and the O/N/O stack, resulting in ID drift [24]. The drift may not occur depending on the process for forming the sensing material or the composition of the sensing material, and further research is needed. As an alternative to minimizing the drift, the pulse operation scheme shown in Fig. 3(b) is used. Before the read bias is applied, the pre-bias (Vpre) is applied to the control-gate. In the read period (tread), the drain bias (VrDS) is synchronized with the control-gate bias (VrCG).
Fig. 3. (a) Transient response of the FET-type gas sensor for 125 ppm SO2 gas at 150 °C with DC measurement method. (b) Pulse operation scheme for the gas sensor. The VrDS is synchronized with the VrCG during the tread. 3
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Fig. 4. (a) Transient response of the FET-type gas sensor for 125 ppm SO2 gas as a parameter of the operating temperature. (b) Transient response of the gas sensor for SO2 gas at 150 °C as a parameter of the gas concentration. The SO2 gas is injected for 140 s, reacts with the gas sensor, and then the sensor starts to recover as the atmosphere changes to dry air.
Fig. 5. Transient response of the FET-type gas sensor in alternatively changing ambience with SO2 gas (125 ppm, 140 s) and dry air (360 s) at 180 °C.
Fig. 7. Transient response of the fabricated gas sensor for 10 ppm SO2 gas as a parameter of Vpre from −2 V to 1 V.
3.3. SO2 gas sensing mechanism of FET-type sensor The SO2 gas sensing mechanism of the FET-type gas sensor having WO3 as the sensing material is described as follows. The threshold voltage (Vth) of the sensor can be written as [23]:
Vth = ϕms −
Qox + 2ψB − Cox
2qND εSi |2ψB | Cox
(1)
where ϕms, Qox, and Cox represent the work-function difference between the control-gate and the channel, the oxide charge, and the equivalent gate oxide capacitance, respectively. ND, εSi, and ψB stand for the doping concentration in the channel, the dielectric constant of Si, and the surface potential at threshold condition, respectively. Since the SO2 gas is an oxidizing gas that takes electrons from the reaction with WO3 and becomes SO2− [26], the n-type semiconductor WO3 is depleted when the WO3 is exposed to the SO2 gas. Looking closely at this process, when WO3 is exposed to SO2 gas, traps with energy that can trap electrons are generated by the adsorbed SO2 gas molecules. Since the trap energy is lower than the Fermi level of the WO3, electrons move from the WO3 to the adsorbed gas molecules [27]. As a result, the gas molecules are negatively ionized, and the sensing material around them depletes. The SO2 molecules ionized near the interface between the WO3 sensing layer and the O/N/O covering the floating-gate act as equivalent negative oxide charges. The negatively charged molecules generated by the gas reaction along the large interdigitated pattern can have relatively large influences on the small channel region as the oxide
Fig. 6. ID-VCG curves of the sensor as a function of exposed time to 125 ppm SO2 gas. The inset is an enlargement of the circled area for clear comparison.
air for 360 s. Because the sensor does not recover completely within a given recovery time, the minimum and maximum current values increase little by little in one response and recovery cycle, but the overall response and recovery trend is nearly the same. The sensor is considered capable of producing a reproducible gas reaction. Note that if the recovery time is longer, it will recover completely.
4
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
carriers of the sensing material changes [22,28]. When the negative pre-bias is applied, the number of electrons existing near the boundary of the O/N/O and WO3 is increased, so that the adsorption of SO2 gas at the boundary region is increased [28]. These adsorbed gas molecules near the O/N/O greatly affect the change of the energy band diagram and flat-band voltage of FET, which increases the amount of current change. For example, the response characteristic of 10 ppm SO2 gas with a pre-bias of −1 V is similar to that of 50 ppm SO2 gas with prebias of 0 V in Fig. 4(b), On the other hand, when the positive pre-bias is applied, the electron concentration near the O/N/O is lowered, and SO2 gas is less likely to be adsorbed to the sensing material near the O/N/O. Therefore, the gas sensing characteristic of 10 ppm SO2 gas with 1 V pre-bias is worse than that of 0 V pre-bias. 3.5. Selectivity of the FET-type sensor Fig. 8. Comparison of responses of the FET-type gas sensor to the five different 125 ppm gases at 150 °C.
To check the selectivity of the FET-type sensor, the responses of other gases are investigated (Fig. 8). The concentration of CO2, H2S, NH3, and NO2 gases is fixed at 125 ppm in examining the selectivity. The response is obtained using the following equation,
charges. This can be thought of as a kind of antenna effect. Accordingly, the Qox increases negatively, resulting in a decrease in Vth of the pMOSFET. As a result, ID-VCG curve of the sensor is shifted in a positive direction. In Fig. 6, the ID-VCG curve of the sensor shifts in the positive direction as the exposed time to 125 ppm SO2 gas increases. The ID-VCG curves of the sensor show that the reaction saturates over time. In Fig. 4(a)–(b), it can be also observed that the slope of the ID-t curve gradually decreases with time.
Response(%) = [(Igas/ Iair ) − 1]
(2)
where Iair and Igas are the current in dry air before the reaction and the current when exposed to the target gas for 140 s. The sensor can react to a variety of gases because it does not use a catalyst that can only react selectively with SO2 gas. Although the sensor does not react selectively to SO2 gas, we can identify the specific gas by using multiple sensors and processing data from them [29].
3.4. Effect of pulsed pre-bias to FET-type sensor 3.6. Comparison with resistor-type sensor It has been reported that the use of pre-bias scheme to FET-type sensors can improve gas sensing characteristics [22,28]. The transient response of the fabricated gas sensor for 10 ppm SO2 gas as a parameter of the pre-bias (Vpre) is shown in Fig. 7. It can be observed that the response increases as the negative pre-bias is applied to the controlgate. When the pre-bias of −2 V is applied, the response increased ~ 3.7 times compared to that of 0 V. Applying a negative prebias increases the SO2 gas response and positive pre-bias decreases the SO2 gas response. When the pre-bias is applied, the distribution of
The resistor-type gas sensor is fabricated on the same wafer using the same fabrication process steps as the FET-type gas sensor. The structure of the resistor-type sensor is illustrated in Fig. 9(a)–(b). The two opposing electrodes are interdigitated and the distance between the electrodes is 2 μm. The sensing material WO3 is deposited on the electrodes. The Fig. 9(c) plots the IR-VR curves of the resistor-type sensor as a function of exposed time to 125 ppm SO2 gas. Since the SO2 gas molecules take away the electrons present in WO3 and becomes Fig. 9. Resistor-type gas sensor fabricated on the same chip with the same process as the FET-type gas sensor. (a) Top SEM view of the fabricated resisotrtype gas sensor. (b) 2D cross-sectional schematic diagram cut along a dotted line A-A’ in (a). (c) IR-VR curves of the resisotr-type sensor as a function of exposed time to 125 ppm SO2 gas at 150 °C. (d) Transient response of the resistive gas sensor for SO2 gas at 150 °C as a parameter of the gas concentration.
5
Solid State Electronics 165 (2020) 107747
G. Jung, et al.
Atmos Environ 2005;39:6868–74. [2] Lee SC, Hwang BW, Lee SJ, Choi HY, Kim SY, Jung SY, et al. A novel tin oxide-based recoverable thick film SO2 gas sensor promoted with magnesium and vanadium oxides. Sens Actuators, B 2011;160:1328–34. [3] Fergus JW. A review of electrolyte and electrode materials for high temperature electrochemical CO2 and SO2 gas sensors. Sens Actuators, B 2008;134:1034–41. [4] Lee YJ, et al. Development of a SAW gas sensor for monitoring SO2 gas. Sens Actuators, A 1998;64:173–8. [5] Tyagi P, Sharma A, Tomar M, Gupta V. Metal oxide catalyst assisted SnO2 thin film based SO2 gas sensor. Sens Actuators, B 2016;224:282–9. [6] Guth U, Vonau W, Zosel J. Recent developments in electrochemical sensor application and technology–a review. Meas Sci Technol 2009;20:042002. [7] Fox CG, Alder JF. Surface acoustic wave sensors for atmospheric gas monitoring. A review. Analyst 1989;114:997–1004. [8] Devkota J, Ohodnicki P, Greve D. SAW sensors for chemical vapors and gases. Sensors 2017;17:801. [9] Boudiba A, Zhang C, Bittencourt C, Umek P, Olivier MG, Snyders R, et al. SO2 gas sensors based on WO3 nanostructures with different morphologies. Procedia Eng 2012;47:1033–6. [10] Tyagi P, Sharma A, Tomar M, Gupta V. SnO2 thin film sensor having NiO catalyst for detection of SO2 gas with improved response characteristics. Sens Actuators, B 2017;248:998–1005. [11] Capone S, et al. Solid state gas sensors: state of the art and future activities. J Optoelectron Adv Mater 2003;5(5):1335–48. [12] Eisele I, Doll T, Burgmair M. Low power gas detection with FET sensors. Sens Actuators, B 2001;78:19–25. [13] Oprea A, Frerichs HP, Wilbertz C, Lehmann M, Weimar U. Hybrid gas sensor platform based on capacitive coupled field effect transistors: Ammonia and nitrogen dioxide detection. Sens Actuators, B 2007;127:161–7. [14] Burgmair M, Frerichs HP, Zimmer M, Lehmann M, Eisele I. Field effect transducers for work function gas measurements: device improvements and comparison of performance. Sens Actuators, B 2003;95:183–8. [15] Kim CH, et al. A new gas sensor based on MOSFET having a horizontal floating-gate. IEEE Electron Device Lett 2014;35:265–7. [16] Hong Y, et al. Highly selective ZnO gas sensor based on MOSFET having a horizontal floating-gate. Sens Actuators, B 2016;232:653–9. [17] Tamaki J, Hashishin T, Uno Y, Dao DV, Sugiyama S. Ultrahigh-sensitive WO3 nanosensor with interdigitated Au nano-electrode for NO2 detection. Sens Actuators, B 2008;132:234–8. [18] Hong S, et al. Observation of physisorption in a high performance FET-type oxygen gas sensor operating at room temperature. Nanoscale 2018;10:18019–27. [19] Hong S, Hong Y, Jeong Y, Jung G, Shin W, Park J, et al. Improved CO gas detection of Si MOSFET gas sensor with catalytic Pt decoration and pre-bias effect. Sens Actuators, B 2019;300:127040. [20] Jeong YJ, et al. Gas sensing characteristics of the FET-type gas sensor having inkjetprinted WS2 sensing layer. Solid State Electron 2019;153:27–32. [21] Lee JH, Kim SY, Cho IH, Hwang SB, Lee JH. 1/f noise characteristics of sub-100 nm MOS transistors. JSTS 2006;6:38–42. [22] Shin J. et al., “Highly improved response and recovery characteristics of Si FET-type gas sensor using pre-bias, IEEE International. Electron Devices Meeting, pp. 18.1.14, 2016. [23] Taur Y, Ning TH. Fundamentals of modern VLSI devices. Cambridge university press; 2009. p. 72–176. [24] Jeong J. et al., “Suppression of Drift in FET-type Gas Sensor Having WS2 Nanoparticles Using Pulse Measurement,” 17th International Meeting on Chemical Sensors, P1AP.6, 2018. [25] Comini E, Faglia G, Sberveglieri G, editors. Solid state gas sensing. Boston, MA: Springer US; 2009. [26] Shimizu Y, Matsunaga N, Hyodo T, Egashira M. Improvement of SO2 sensing properties of WO3 by noble metal loading. Sens Actuators, B 2001;77:35–40. [27] Wolkenstein T. Electronic processes on semiconductor surfaces during chemisorption. New York: Springer Science & Business Media; 2012. [28] Wu M, Kim CH, Shin J, Hong Y, Jin X, Lee JH. Effect of a pre-bias on the adsorption and desorption of oxidizing gases in FET-type sensor. Sens Actuators, B 2017;245:122–8. [29] Jung G. et al., “Accurate identification of gas type and concentration using DNN reflecting the sensing properties of MOSFET-type gas sensor,” 2019 IEEE International Symposium on Olfaction and Electronic Nose (ISOEN) pp. 1-4, 2019.
Fig. 10. Response versus SO2 gas concentration of the FET-type sensor and resistor-type sensor.
SO2− [26], the slope of the IR-VR curve decreases as the exposure time to SO2 gas increases. That is to say, as the sensor reacts with SO2 gas over time, the conductivity of the sensor decreases and the resistance increases. The Fig. 9(d) presents the transient response of the resistortype gas sensor for SO2 gas at 150 °C as a parameter of the gas concentration. In Fig. 10, we compare the response of the FET-type sensor and the resistor-type sensor for different concentrations of SO2 gas at 150 °C using the data in Fig. 4(b), Figs. 7 and 9(d). The response is obtained using Eq. (2). In Fig. 10, the resistor-type sensor shows a higher response than that of the FET-type sensor. However, FET-type gas sensor with negative pre-bias (Vpre = -2 V) shows a significant increase in response to 10 ppm SO2 gas, which is higher than that of the resistortype. In our FET-type gas sensors, the pre-bias scheme appears to be very useful for detecting low gas concentrations. 4. Conclusions We have investigated the SO2 gas sensing characteristics of the FETtype and resistor-type gas sensors with an 18-nm-thick WO3 as a sensing layer. Both sensors successfully detected SO2 gas. Since the ID drift was observed when using a DC measurement method, the pulse measurement method was used to thoroughly suppress the ID drift. The SO2 gas sensing characteristics were examined while varying the operating temperature, the concentration of the SO2 gas and the pre-bias. We have shown that applying a pre-bias of −2 V increases the response to 10 ppm of SO2 gas by ~ 3.7 times compared to that of 0 V. Compared to the resistor-type gas sensor fabricated on the same wafer, the FET-type gas sensor with a pre-bias of −2 V showed better sensing characteristics at a concentration of 10 ppm of SO2 gas. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Gyuweon Jung received the B.S. degree with the ‘summa cum laude’ in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2017. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the Inter-University Semiconductor Research Center, SNU. His current research interests include FET-based sensor array and Electronic nose.
Acknowledgement This work was supported by the National Research Foundation of Korea (NRF-2016R1A2B3009361) and the Brain Korea 21 Project in 2019. References [1] Pandey JS, Kumar R, Devotta S. Health risks of NO2, SPM and SO2 in Delhi (India).
6
Solid State Electronics 165 (2020) 107747
G. Jung, et al. Yujeong Jeong received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2017. She is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. She is also with the InterUniversity Semiconductor Research Center, SNU. Her current research interests include FET-based sensor platform design and sensing materials.
Jinwoo Park received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2019. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the InterUniversity Semiconductor Research Center, SNU. His current research interests include FET-based sensor platform fabrication and measurement.
Yoonki Hong received the B.S. degree in Electrical and Computer Engineering from Seoul National University (SNU), Seoul, Korea in 2013. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at SNU. He is also with the Inter-University Semiconductor Research Center, SNU. His current research interests include MOSFET-based gas sensor and humidity sensor.
Dongkyu Jang received the B.S. and M. S. degrees in electrical engineering from Korea University, Seoul, in 2008 and 2010, respectively. In 2010, he joined at Samsung Electronics, where he has been working in the area of DRAM integration. He is currently pursuing the Ph.D. degree with the Department of Electrical and Computer Engineering, Seoul National University, Seoul, South Korea. He is also with the Inter-University Semiconductor Research Center, SNU. His current research interests include pressure sensors and gas sensors.
Meile Wu received the B.S. degree in Electronics Science and Technology from Shenyang University, Shenyang, China in 2012 and M.S. degree in Microelectronics and Solid State Electronic from Shenyang University of Technology, Shenyang, China in 2015. She received the Ph.D. degrees from Seoul National University (SNU), Seoul, Korea in 2019.
Jong-Ho Lee received the B.S. degree from Kyungpook National University, Daegu, Korea, in 1987 and the M.S. and Ph.D. degrees from Seoul National University, Seoul, in 1989 and 1993, respectively, all in Electronic Engineering. In 1993, he worked on advanced BiCMOS process development at ISRC, Seoul National University as an Engineer. In 1994, he was with the School of Electrical Engineering, Wonkwang University, Iksan, Chonpuk, Korea. In 2002, he moved to Kyungpook National University, Daegu, Korea, as a Professor of the School of Electrical Engineering and Computer Science. Since September 2009, he has been a Professor in the School of Electrical and Computer Engineering, Seoul National University, Seoul, Korea. From 1994 to 1998, he was with ETRI as an invited member of technical staff, where he worked on deep submicron MOS devices, device isolation. From August 1998 to July 1999, he was with Massachusetts Institute of Technology, Cambridge, as a postdoctoral fellow, where he was engaged in the research on sub–100 nm double-gate CMOS devices. He has authored or coauthored more than 216 papers published in refereed journals and over 326 conference papers related to his research and has been granted 85 patents in this area. He received 18 awards for excellent research papers and research excellence. He invented bulk FinFET, Saddle FinFET (or bCAT) for DRAM cell, and NAND flash cell string with virtual source/drain, which have been applying for mass production. His research interests include CMOS technology, nonvolatile memory devices, thin film transistors, sensors, neuromorphic technology, and device characterization and modeling.
Seongbin Hong received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2016. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the InterUniversity Semiconductor Research Center, SNU. His current research interests include FET -based sensor platform design and fabrication.
Wonjun Shin received the B.S. degree in Electrical and Computer Engineering from Seoul National University, Seoul, Korea in 2017. He is currently working toward a combined master’s and doctorate program in Department of Electrical and Computer Engineering at Seoul National University (SNU), Seoul, Korea. He is also with the InterUniversity Semiconductor Research Center, SNU. His current research interests include FET-based sensor platform fabrication and measurement.
7