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Improving the signal resolution of semiconductor gas sensors to high-concentration gases
Solid State Electronics 162 (2019) 107648
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Improving the signal resolution of semiconductor gas sensors to highconcentration gases ⁎
Xinyuan Zhoua,b, Liping Yanga, Yuzhi Biana,b, Ying Wanga, Ning Hana,b,c, , Yunfa Chena,b,c,
T ⁎
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China c Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal oxide gas sensor Field-effect transistor High-concentration Signal resolution
Detecting high-concentration gases is challenging by metal oxide semiconductor (MOX) gas sensors, because the voltage signal would become saturated. In order to solve this problem, the zooming p + n field-effect transistors (FETs) circuit has been designed, combining an n-type enhancement-mode FET (EMFET) and a p-type depletionmode FET (DMFET). This designed zooming p + n FETs can endow MOX gas sensors with the high signal resolution of ~3.0 V/decade to the 100–2000 ppm (part per million) acetone gas, triple that of MOX gas sensors without FETs (~0.8 V/decade). Meanwhile, this zooming technology is also suitable for detecting other gases at high-concentration, such as 1%–20% LEL (lower explosion limit) methane. The principle of zooming p + n FETs is that with increasing the gas concentration, the suppressing role of the EMFET is firstly induced leading to a reduced signal resolution to the low-concentration target gas; then its suppressing effect becomes saturated and the DMFET starts to switch from ON state to OFF state in the high-concentration target gas, resulting in an amplifying effect herein and thus an enhanced signal resolution. This circuit is promising for the high-concentration gas detection as well as for the multi-functional gas detector design.
1. Introduction Some kinds of gas including acetone and methane should be detected at the high concentration. For example, it is reported that the median time-weighted-average acetone concentration in the cellulose fiber factory is ~1070 ppm (part per million) [1]. The unconsciousness, dizziness, unsteadiness, confusion, and headache are produced by workers who are exposed to > 12 000 ppm acetone for 4 h [2]. Irritation of eyes, nose, and/or throat can result from the acetone gas ranging from ~250 to ~1000 ppm [3–5]. So it is essential to monitor effectively the high-concentration acetone in some special places if the worker’s security and health are taken into consideration. Gas sensors based on the metal oxide semiconductor (MOX) are widely used to detect acetone gas [6–9]. Umar et al. reported that CuO nanosheets exhibited remarkable voltage signals to 10–200 ppm acetone but the voltage signals became saturated in the range of 100–200 ppm [10]. Ag doped ZnO nanoneedles prepared by Al-Hadeethi et al. also exhibited the similar voltage signal saturation [11]. This may be due to the saturation of active sites (oxygen species or surface defects) [12,13]. In all, it’s common that the MOX gas sensors
display the voltage signal saturation exposed to the high-concentration target gas. Hence, it’s necessary to solve the problem to make MOX gas sensors competent to monitor high-concentration target gas. In our previous study, Mn doped ZnO (MZO) nanoparticles [14] show the linear voltage signals to ≤20 ppm acetone but the voltage signals to > 50 ppm become saturated, which is not suitable for detecting the high-concentration acetone. In this paper, an n-type enhancement-mode field-effect transistor (EMFET) is added into the traditional circuit. The voltage signals of MZO sensors to 5–2000 ppm acetone gas are reduced dramatically by the n-type EMFET. Then a ptype DMFET is added to form a new zooming p + n FETs circuit. This zooming p + n FETs shows an interesting feature: the voltage signal resolution to 5–100 ppm acetone gas is reduced while the voltage signal resolution to 100–2000 ppm acetone gas is improved. In short, this zooming p + n FETs shift the effective detection range to higher concentration. The principle of zooming the p + n FETs is the compromise between the suppressing role of the n-type EMFET and the amplifying role of the p-type DMFET. In our published papers [15,16], the single DMFET are used to extend the detection range to lower concentration. For example, the single DMFET 2SK544 is able to amplify the voltage
⁎ Corresponding authors at: State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail addresses: [email protected] (N. Han), [email protected] (Y. Chen).
https://doi.org/10.1016/j.sse.2019.107648 Received 19 June 2019; Received in revised form 12 August 2019; Accepted 2 September 2019 Available online 04 September 2019 0038-1101/ © 2019 Published by Elsevier Ltd.
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where RS,a and RS,g are the resistance of gas sensors in air and in the target gas respectively. VOUT,a and VOUT,g are the output voltage of the conventional circuit in air and in the target gas respectively in Fig. 1. The apparent response of the FET circuit is also calculated in the same way, as derived from (2):
RS, a / RS, g × (RL + RFET , g )/(RL + RFET , a) = (VCC / VOUT , a − 1)/(VCC / VOUT , g − 1)
Therefore, the suppressing factor (SF) can be easily derived by (3) and (4) to be (RL + RFET,g)/(RL + RFET, a) where RFET,a and RFET,g are the resistance of FETs in air and in the target gas respectively, analogous to the magnification factor in the previous report [15]. Obviously, SF is less than one because RFET,g is less than RFET,a.
Fig. 1. Design scheme of the traditional, the n-type FET and the zooming p + n FETs circuits for MOX sensors.
signal resolution to 0.1–5 ppm toluene and thus the lower limit of detection falls to 0.1 ppm [15]. Therefore, the effective detection range of the MOX gas sensors can be adjusted easily by using different modes of FETs according to the practical application.
3. Experimental 3.1. Electrical property measurement Commercial 6670AL (Fairchild, America) [17] is an n-type EMFET. 2SJ44 (Renesas, Japan) [18] and 2SJ104 (Toshiba, Japan) [19] are the p-type DMFETs. All of FETs are purchased from the market. Their output and transfer curves are measured by using the semiconductor characterization system (Keithley 4200SCS, America).
2. Design of electric circuits Fig. 1 schematically shows a chain of electric circuits for the n-type MOX sensors. The electric circuit on the left is a conventional circuit, with the load resistor (RL) and n-type MOX sensor (RS) put in series. After the bias voltage of 5.0 V (VCC) is applied, the partial voltage of the RL is the output voltage (VOUT). Once RS contacts with the reductive gas, RS will decrease and VOUT will increase. The VOUT increment is named as voltage signal (ΔV). Obviously, ΔV depends on the variation of RS. The electric circuit in the middle is an EMFET circuit. It can be seen that drain (D) is connected to the lower potential side of RL, Gate (G) is connected to the higher potential side of RL and source (S) is grounded, which produces a positive gate-source voltage (VGS). The same gas at the same concentration is injected like above, RS decreases and VOUT increases similarly. That will cause the VGS of the EMFET to shift positively, which induces the resistance of FET (RFET) to decrease according to the transfer characteristics of the EMFET. Clearly, the decrease of RFET won’t favor the increase of VOUT and thus suppresses the ΔV. So ΔV in the EMFET circuit will be smaller than that in the conventional circuit. We continue to add a p-type DMFET into the EMFET circuit, as shown on the right part in Fig. 1. Low-concentration reductive gas is injected, RS and RFET will decrease together and the increase of VOUT will be restricted by the suppressing role of the EMFET. Now, the DMFET hasn’t worked. That’s to say, the circuit is equivalent to the EMFET circuit if the concentration is low. Once the concentration reaches a certain level, the VOUT will become large enough to force the DMFET to transform from ON state to OFF state, amplifying the ΔV greatly, just as reported in the previous papers [15,16]. In all, the EMFET is always suppressing ΔV to all concentration of reductive gas while the DMFET only amplifies the ΔV to the reductive gas at the much higher concentration, which realizes a positive shift of the effective detection range. Quantitatively, VOUT in the conventional circuit is calculated as:
VOUT = VCC /(1 + RS / RL)
3.2. Gas-sensing performance measurement MZO nanoparticles and Pd-loaded mesoporous SnO2 (PSO) nanoparticles have been prepared by our laboratory and are the gas sensing materials to detect acetone and methane gas respectively [14,20]. The static gas sensing test system (Hanwei WS-30A, China) [21–22] is utilized to study the sensing property. Certain amount of the acetone liquid is dropped onto an evaporator using a micro-syringe in the test chamber (total volume 18 L) to generate the acetone gas whose concentration varies from 5 ppm to 2000 ppm. This method is also applicable for methane gas. The voltage signal (ΔV) is defined as the increment of output voltage from the air to the reductive gas. The signal resolution is defined as Resolution = ΔV/lgC where C is the concentration of the reductive gas (ppm). The unit of Resolution is V/decade. 4. Results and discussion 4.1. Suppressing effect of EMFET In our previous report [14], acetone sensing performance measurement of MZO materials shows that the optimum temperature is 340 °C and the optimum doping concentration of Mn is 2.2 mol%. In addition, MZO exhibits a better selectivity for acetone than ethanol, formaldehyde and benzene. This is due to the relatively higher surface acidity which favors acetone adsorption and reaction. However, when the acetone concentration is more than 100 ppm, ΔV will be saturated, seen in Fig. 2. The signal resolution will be calculated as ~0.8 V/ decade, meaning ΔV increases by ~0.8 V when the concentration increases from 100 ppm to 1000 ppm. At the same time, the voltage signal saturation also exists in Ag-doped ZnO nanoneedles [11] and CuO nanosheets [10]. Therefore, the signal resolution in the high-concentration range should be improved. Here, we have designed a novel circuit by adding a FET, whose transfer curve is shown in Fig. 3a. From Fig. 3a, this FET is observed to be an n-type FET with the enhancement-mode. Generally, FETs work at room temperature (25 ± 5 °C), whose transfer curves are stable at room temperature. Then RL is set as 0.5, 1.0 and 2.0 MΩ respectively, and the typical ΔV curves with and without EMFET 6670AL are shown in Fig. 3b-d, which illustrates that 6670AL is capable of suppressing ΔV of MZO to acetone gas from 5 ppm to 2000 ppm. Moreover, ΔV in both of circuits increases gradually with increasing concentration. However,
(1)
It is clear that exposed to the reductive gas, RS will decrease, resulting in the increase of VOUT. Similarly, in the proposed EMFET circuit, VOUT is expressed as:
VOUT = VCC /[1 + RS (RL / RFET )]
(2)
Therefore, the increase of VOUT (i.e. ΔV) will be suppressed because both RFET and RS decrease together. From (1), it is easy to derive the intrinsic response of MOX sensors as:
RS, a / RS, g = (VCC / VOUT , a − 1)/(VCC / VOUT , g − 1)
(4)
(3) 2
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Fig. 2. ΔV of MZO sensors to acetone from 5 ppm to 2000 ppm in the traditional circuit with different RL. Inset is the log-scale plot of acetone concentration.
Fig. 4. ΔV suppressing mechanism of 6670AL: (a) Output curves of 6670AL, VGS changes from 0.6 V to 1.5 V (step is 0.1 V); (b) RL + RFET versus current curves of 6670AL loaded with different resistors (0 MΩ–5 MΩ), the values of RL + RFET become stable and are deemed as the fixed load resistors when the current is close to 10 μA.
VOUT = VGS = VDS
Therefore, the working curve of 6670AL is determined by the points where VGS and VDS are equal. Then the output curves of 6670AL at different VGS are measured as shown in Fig. 4a. The point of VGS = VDS = 0.70 V is chosen as a starting point, I will be determined to be 0.055 μA in Fig. 4a. Next, RFET is calculated as 13 MΩ. Then VGS increases by 0.10 V, and the process above is repeated to get another RFET. Finally, all of points can be obtained and are plotted as the black curve in Fig. 4b. As to other circuits with different RL, their working curves are got by adding the corresponding RL into the black curve. Supposing RL is 2.0 MΩ, RFET,a is 13 MΩ according to Fig. 4b and REFT,g is very small, thus the theoretical SF will be calculated as 0.13, which is consistent with the measured SF of 0.11. Supposing RL is zero, RFET always decreases endlessly, which shows the 6670AL alone has an infinite capacity for suppressing ΔV (the signal resolution is about 0.12 V/decade) no matter how high the concentration is, seen in Fig. S2. Besides, it is observed that these curves start to fall quickly with increasing current and then decrease slowly until a fixed value when the resistor is loaded, which explains the signal resolutions in the conventional circuit and EMFET circuit are almost equal in the highconcentration range.
Fig. 3. ΔV suppressing effect of 6670AL: (a) Transfer curve of 6670AL; (b)-(d) ΔV of MZO to acetone from 5 ppm to 2000 ppm in the two kinds of circuits. Table 1 Comparison of sensitivities to acetone from ~100 ppm to 2000 ppm among three kinds of circuits RL (MΩ)
Traditional (V/ decade)
EMFET (V/ decade)
Zooming p + n FETs (V/ decade)
0.5 1 2 5 10 20
1.2 1.2 1.4 1.0 0.8 0.6
1.3 1.3 1.4
2.6 3.0 3.4
it is interesting to find that there is an inflection point for the EMFET circuits. That is, the increase is very slow before the inflection point, after which the change is steep. Meanwhile, the curves of both of circuits are almost parallel with each other after the inflection point. In other words, their signal resolutions to higher concentration acetone gas are almost equal and the related values are listed in Table 1. Other EMFET also suppresses the ΔV shown in Fig. S1. Taking EMFET 6670AL for example, we study how it suppresses ΔV of MZO gas sensors. From Fig. 1, VOUT is expressed as below:
VOUT = VGS = VDS + IRL
(6)
4.2. Zooming p + n FETs After a DMFET 2SJ44 is connected to the fixed load resistor, the ΔV of TGS2602 sensors is enhanced due to the amplifying effect of 2SJ44 [16], whose transfer curve of 2SJ44 is seen in Fig. S3. As mentioned above, the sum of RL and RFET in the high-concentration acetone gas can be deemed to be a fixed resistor in the EMFET circuit, then a DMFET 2SJ44 is added. The ΔV of gas sensors to high-concentration acetone gas is expected to be amplified. Here, 2SJ44 is chosen as a partner of 6670AL in the zooming p + n FETs circuit as shown in Fig. 1. The resultant ΔV curves are seen in Fig. 5a-c. Taking 0.5 MΩ for example, compared to the EMFET circuit, the ΔV in zooming p + n FETs circuit is similarly small when the
(5)
where VDS is the drain-source voltage, I is the circuit current. The case without RL is considered firstly. The Eq. (5) is written below: 3
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Fig. 5. Effect of the zooming p + n FETs: (a)-(c) ΔV comparison of MZO sensors in the n-type FET and zooming p + n FETs circuits; (d) Effective detection range comparison of MZO sensors in the conventional and zooming p + n FETs circuits.
Fig. 6. Universality of the zooming p + n FETs: (a)-(c) ΔV comparison of PSO sensors in the n-type FET and zooming p + n FETs circuits; (d) Comparison of the effective detection range of PSO sensors in the conventional and zooming p + n FETs circuits.
concentration is less than 200 ppm. But the ΔV in latter circuit is much higher than that in the former circuit with the acetone concentration in the range of 200–2000 ppm. Meanwhile, the signal resolution to ≥200 ppm acetone gas in the zooming p + n FETs circuit is higher than that in the EMFET circuit, and related data are listed in Table 1. More importantly, the effective detection range can be tuned to higher detection only by choosing a smaller RL as shown in Fig. 5d. In all, this zooming p + n FETs circuit can be used to respond effectively to the acetone concentration up to 1000 ppm and 2000 ppm with the large ΔV and the excellent signal resolution. What’s more, it is observed from Fig. 3b-d that the ΔV in the conventional circuits doesn’t become saturated with the acetone concentration up to 1000 ppm and 2000 ppm, because the load resistors are very small (e.g. 1.0 MΩ). The sensor resistance in air is measured to be 90 MΩ, then output voltage in air (VOUT,a) is 0.05 V, very close to zero. The small VOUT,a caused by small RL is not beneficial to the practical application of gas sensors. Even so, the smaller RL provides a reference to understand more vividly how the EMFET works. Therefore, without FETs, the appropriate RL should be set as ~10 MΩ and thus VOUT,a is about 0.5 V, the medium output voltage. Then the ΔV becomes saturated and the signal resolution is as low as ~0.8 V/decade in ≥100 ppm acetone gas in Fig. 2, less than one third of that (3.0 V/decade) in the zooming p + n FETs circuit. Furthermore, from Fig. 5d, for the zooming p + n FETs circuits, the signal resolution to < 100 ppm acetone is small (~0.2 V/decade) while that to ≥100 ppm acetone is high (~3.0 V/ decade), meeting the needs of high-concentration detection. Inflammable gases are particularly prone to the signal saturation especially when their concentration is close to the lower explosion limit (LEL). In order to extend the hydrogen’s detection range to the higher concentration, Okuyama et al. increased the content of Ni in the Pd/Ni electrode in the Pd/Ni-Al2O3-Al diode (i.e. metal-insulator-metal diode) [23]. As for methane detection, the LEL of methane is known to be approximately 5% [24–26]. In the case of methane apparatus indicating up to the lower flammable limit only, the alarm set point (or the lowest, where there are two or more set points) is recommended to be in the range of 1%–25% LEL to avoid the sudden explosion. Our laboratory has fabricated the PSO nanoparticles [20]. Pd dosage is optimized as 1 wt% and the working temperature is determined to be 300 °C. The excellent gas-sensing properties of PSO result from the spillover effect of catalytic Pd and heterostructure sensitization of PdO-SnO2. PSO sensors can monitor accurately the methane lower than 1% LEL, after
which the responses are saturated. In order to monitor accurately the high-concentration methane, the zooming p + n FETs circuit are employed, as shown in Fig. 6a-c. Taking 0.2 MΩ for example, the ΔV in the zooming p + n FETs circuit is initially lower than that in the conventional circuit, then exceed the latter with the methane concentration increasing from 0.2% LEL to 20% LEL. What’s more, the zooming p + n FETs circuit has the highest signal resolution of 1.7 V/decade to the methane in the range of 1%–20% LEL among three types of circuits. Meanwhile, if the effective detection range needs to be shifted to much higher concentration, just the smaller RL is chosen as shown in Fig. 6d. Moreover, it is obvious that the response time and recovery time are not influenced by FETs in Fig. S4. Finally, this methane measurement demonstrates the universality of this type of zooming p + n FETs circuit. 4.3. Mechanism of zooming p + n FETs The synergetic [16], coupling [27] or interlocking [28] p + n DMFETs are developed in our previous reports, where both p-type and n-type DMFETs can amplify the ΔV together. The interlocking p + n DMFETs cause the MZO sensors to display a transilient ΔV of ~4.0 V to ≥2 ppm acetone. It’s due to the resistance jump from ON state to OFF state of both p-type and n-type DMFETs. Howbeit, in the zooming p + n FETs circuit, the p-type DMFET has the amplifying effect because the DMFET’s resistance increases from small to large with its gate-source voltage increasing according to Fig. S5c. On the other hand, from Fig. 4b, the n-type EMFET is opposite, leading to its suppressing effect. In all, the n-type EMFET is responsible for suppressing the ΔV at low gas concentration, while the p-type DMFET is responsible for amplifying at higher gas concentration. Fig. 7 shows the working curves of the zooming p + n FETs circuit. It is obvious that they are assemblies of the curves of the n-type EMFET (Fig. 4b) and the curves of the p-type DMFET (Fig. S5). According to Fig. 1, the VOUT can be expressed as below:
VOUT = IRL + VDS (n) − VDS (p)
(7)
The term of (IRL + VDS(n)) is just the formula (5), referring to the suppressing role of EMFET 6670AL. The term of VDS(p) refers to the role of DMFET 2SJ44. Whether or not 2SJ44 can play the amplifying role depends on the concentration of target gas. Thus, three stages are taken into consideration in terms of the concentration. RL is chosen to be 0.5, 4
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that of the conventional circuit. Finally, MOX gas sensors can be used indoors and outdoors to monitor dangerous gas. Based on this, if they are needed to capture the weak signals to the lower-concentration gas, DMFETs will be qualified. On the other hand, when it comes to sensitively detecting the higherconcentration gas, EMFETs should be taken into consideration. Briefly, selecting different modes of FETs is a simple way of changing the effective detection range of gas sensor. It should be noted that the addition of FETs can alter the signals of gas sensors but cannot alter their selectivity, depending on the material itself. 5. Conclusion The EMFET is able to strongly suppress the signals of the MOX sensors. The mechanism is deemed to be the increase of VGS inducing the sharp decrease of the EMFET resistance. The designed zooming p + n FETs can cause the MOX gas sensors to show weak resolution (~0.2 V/decade) to < 100 ppm acetone, but strong signal resolution (~3 V/decade) to ≥100 ppm acetone, thus the effective detection range of the MOX sensors is extended to the higher concentration. Meanwhile, this zooming technology is also used to detect 1%–20% LEL methane. The working mechanism of zooming p + n FETs is that the ntype EMFET firstly works and plays the suppressing role. The amplifying role of the p-type DMFET depends entirely on the concentration of the target gas. The p-type EMFET will be driven to amplify only if the gas concentration is high enough. The zooming p + n FETs provide more opportunities for MOX gas sensors to detect high-concentration target gas. More importantly, people can choose the appropriate mode of FETs to alter the effective detection range of the MOX sensors flexibly according to the actual needs.
Fig. 7. Working mechanism of zooming p + n FETs: The solid are the working curves of 6670AL while the dashed are the working curves of 2SJ44, it can be seen that 6670AL firstly works and suppresses the ΔV until the output voltage is large enough to cause 2SJ44 to start to work and amplify the ΔV.
1 and 2 MΩ respectively in Fig. 5. Each RL has a critical concentration of the acetone gas. For instance, 0.5 and 1 MΩ correspond to ~100 ppm while 2 MΩ corresponds to ~50 ppm, seen in Fig. 5a-c. Obviously, the slope of curve is very low before the critical concentration, after which the slope is steep. For simplicity, RL of 0.5 MΩ is chosen as an example to elaborate the working mechanism of zooming p + n FETs.
• The first stage is that the concentration is zero, i.e. in the air. R •
•
FET,a
of the 6670AL is 13 MΩ, 2SJ44 is at ON state and its resistance is neglectable. Hence, the baseline of the zooming p + n FETs circuit in clean air is described as VOUT ≈ 0.7 V. Injecting ≤100 ppm acetone is the second stage. 2SJ44 is still at ON state, but resistance of 6670AL starts to decrease from 13 MΩ and the output voltage undergoes a restricted increase due to the suppressing role of 6670AL. Therefore, the zooming p + n FETs circuit is equivalent to the EMFET circuit, namely VOUT ≈ IRL + VDS(n), whose curve overlaps the curve of the EMFET circuit (red solid curve) as shown in Fig. 7. Injecting > 100 ppm acetone is the third stage. The resistance of 6670AL decreases firstly and it also plays the suppressing role, along the red solid curve in Fig. 7. Even so, the output voltage still increases and finally is large enough to drive the 2SJ44 to work. Once the 2SJ44 works, the resistance of 2SJ44 will increase dramatically, and so do will the output voltage, along the red dashed curve in Fig. 7. Notably, when 2SJ44 works, the suppressing role of 6670AL remains but is neglectable compared to the amplifying role of the 2SJ44, because the slopes of 2SJ44 are much steeper than those of 6670AL. The large slopes of 2SJ44 cause the zooming p + n FETs circuit to possess the highest signal resolution among three types of circuits.
Acknowledgment This research was financially supported by the National Key R&D Program of China (2016YFC0207100), the National Natural Science Foundation of China (51602314), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05C146). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.sse.2019.107648. References [1] Hasen H, Wilbur SB. Toxicological profile for acetone. Atlanta: U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry; 1994. [2] Ross DS. Acute acetone intoxication involving eight male workers. Ann Occup Hygiene 1973;16:73–5. [3] Mitran E, Callender T, Orha B, Dragnea P, Botezatu G. Neurotoxicity associated with occupational exposure to acetone, methyl ethyl ketone, and cyclohexanone. Environ Res 1997;73:181–8. [4] Dick RB, Setzer JV, Taylor BJ, Shukla R. Neurobehavioural effects of short duration exposures to acetone and methyl ethyl ketone. Br J Ind Med 1989;46:111–21. [5] Satoh T, Omae K, Nakashima H, Takebayashi T, Matsumura H, Kawai T, et al. Relationship between acetone exposure concentration and health effects in acetate fiber plant workers. Int Arch Occup Environ Health 1996;68:147–53. [6] Cho H-J, Kim S-J, Choi S-J, Jang J-S, Kim I-D. Facile synthetic method of catalystloaded ZnO nanofibers composite sensor arrays using bio-inspired protein cages for pattern recognition of exhaled breath. Sens Actuators B 2017;243:166–75. [7] Zhou X, Wang A, Wang Y, Bian L, Yang Z, Bian Y, et al. Crystal-defect-dependent gas-sensing mechanism of the single ZnO nanowire sensors. ACS Sensors 2018;3:2385–93. [8] Li X, Zhou X, Guo H, Wang C, Liu J, Sun P, et al. Design of Au@ZnO yolk-shell nanospheres with enhanced gas sensing properties. ACS Appl Mater Interfaces 2014;6:18661–7. [9] Moon HG, Jung Y, Jun D, Park JH, Chang YW, Park HH, et al. Hollow Pt-functionalized SnO2 hemipill network formation using a bacterial skeleton for the noninvasive diagnosis of diabetes. ACS Sensors 2018;3:661–9. [10] Umar A, Alshahrani AA, Algarni H, Kumar R. CuO nanosheets as potential scaffolds for gas sensing applications. Sens Actuators B 2017;250:24–31.
Anyhow, it can be simply deemed as that with increasing the gas concentration, the 6670AL firstly plays the suppressing role leading to a reduced signal resolution to the low-concentration target gas; then its suppressing effect becomes saturated and the 2SJ44 DMFET starts to switch from ON state to OFF state in the high-concentration target gas. Thus, the amplifying effect is induced and the signal resolution is enhanced. Particularly, voltage signal would remain unchangeable if the amplifying role is offset completely by the suppressing role, i.e. the intersection points of the black curve and blue curve in Fig. S6. Many similar intersection points appear in Fig. 6a-c. After the intersection point, the voltage signal of the zooming p + n FETs circuit outnumbers 5
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Yuzhi Bian graduated from Beijing University of Technology and now is a Second-year graduate student in University of Chinese Academy of Sciences, his major is materials science.
[11] Al-Hadeethi Y, Umar A, Ibrahim AA, Al-Heniti SH, Kumar R, Baskoutas S, et al. Synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles. Ceram Int 2017;43:6765–70. [12] Göpel W. Reactions of oxygen with ZnO-1010-surfaces. J Vacuum Sci Technol 1978;15:1298–310. [13] Iwamoto M, Yoda Y, Egashira M, Seiyama T. Study of metal oxide catalysts by temperature programmed desorption. 1. Chemisorption of oxygen on nickel oxide. J Phys Chem 1976;80:1989–94. [14] Wang JX, Yang J, Han N, Zhou XY, Gong SY, Yang JF, et al. Highly sensitive and selective ethanol and acetone gas sensors based on modified ZnO nanomaterials. Mater Des 2017;121:69–76. [15] Zhou X, Wang Y, Wang J, Xie Z, Wu X, Han N, et al. Amplifying the signal of metal oxide gas sensors for low concentration gas detection. IEEE Sens J 2017;17:2841–7. [16] Zhou X, Wang Y, Wang Z, Yang L, Wu X, Han N, et al. Synergetic p+n field-effect transistor circuits for ppb-level xylene detection. IEEE Sens J 2018;18:3875–82. [17] Fairchild 6670AL. Available online: http://pdf-file.ic37.com/pdf3/FAIRCHILD/ FDB66_datasheet_975979/159950/FDB66_datasheet.pdf (accesses on 13 May 2019). [18] Renesas 2SJ144. Available online: https://4donline.ihs.com/images/VipMasterIC/ IC/NECE/NECED039/NECED039-4-5.pdf?hkey= EF798316E3902B6ED9A73243A3159BB0 (accesses on 13 May 2019). [19] Toshiba 2SJ104. Available online: http://pdf-file.ic37.com/pdf5/TOSHIBA/ 2SJ104_datasheet_642222/104797/2SJ104_datasheet.pdf (accesses on 13 May 2019). [20] Yang L, Wang Z, Zhou X, Wu X, Han N, Chen Y. Synthesis of Pd-loaded mesoporous SnO2 hollow spheres for highly sensitive and stable methane gas sensors. RSC Adv 2018;8:24268–75. [21] Qin N, Wang XH, Xiang Q, Xu JQ. A biomimetic nest-like ZnO: Controllable synthesis and enhanced ethanol response. Sens Actuators B 2014;191:770–8. [22] Guan Y, Wang DW, Zhou X, Sun P, Wang HY, Ma J, et al. Hydrothermal preparation and gas sensing properties of Zn-doped SnO2 hierarchical architectures. Sens Actuators B 2014;191:45–52. [23] Okuyama S, Umemoto K, Okuyama K, Ohshima S, Matsushita K. Pd/Ni-Al2O3-Al tunnel diode as high-concentration-hydrogen gas sensor. Jpn J Appl Phys 1997;36:1228–32. [24] Smedt GD, Corte FD, Notele R, Berghmans J. Comparison of two standard test methods for determining explosion limits of gases at atmospheric conditions. J Hazard Mater 1999;70:105–13. [25] Vuong NM, Hieu NM, Hieu HN, Yi H, Kim D, Han Y-S, et al. Ni2O3-decorated SnO2 particulate films for methane gas sensors. Sens Actuators B 2014;192:327–33. [26] Arts JHE, Mojet J, van Gemert LJ, Emmen HH, Lammers JHCM, Marquart J, et al. An analysis of human response to the irritancy of acetone vapors. Crit Rev Toxicol 2002;32:43–66. [27] Zhou X, Yang L, Bian Y, Ma X, Han N, Chen Y. Coupling p+n field-effect transistor circuits for low concentration methane gas detection. Sensors 2018;18:787. [28] Zhou X, Wang J, Wang Z, Bian Y, Wang Y, Han N, et al. Transilient response to acetone gas using the interlocking p+n field-effect transistor circuit. Sensors 1914;2018:18.
Ying Wang received the bachelor’s degree in optics from Beijing Jiaotong University in 2010. She received the Ph.D. degree in physical electronics with the College of Electronics, Peking University.
Ning Han received the Ph.D. degree in chemical engineering from the Institute of Process Engineering, CAS, in 2010. He was a Post-Doctoral Fellow with the Department of Physics and Materials Science, City University of Hong Kong, from 2010 to 2014, and has been a Professor with IPE CAS via One Hundred Talents Plan since 2014. He works on investigations of preparation, structure, and property of semiconductors, and has developed highly sensitive and selective gas sensing materials. He has authored over 60 articles.
Yunfa Chen received his Ph.D. in Material Science at Université Louis Pasteur Strasbourg (ULP), France in 1993. He is a professor of the Graduate University of Chinese Academy of Sciences, and Research Professor and Vice Director of Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are preparation and assembly of nanoparticles, functional materials, organic–inorganic composite materials and layered materials. And he is also interested in industrial application of nanomaterials.
Xinyuan Zhou received the B.S. degree in materials chemistry from Taiyuan University of Technology in 2014. He is currently pursuing the Ph.D. degree in materials engineering at the Institute of Process Engineering, CAS. His research focuses on the metal oxide semiconductor gas sensors.
Liping Yang received the Ph.D. degree in physical chemistry from Institute of Chemistry, Chinese Academy of Sciences. Currently, she is engaging postdoctoral study in the Institute of Process Engineering, Chinese Academy of Sciences.
6
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Solid State Electronics journal homepage: www.elsevier.com/locate/sse
Improving the signal resolution of semiconductor gas sensors to highconcentration gases ⁎
Xinyuan Zhoua,b, Liping Yanga, Yuzhi Biana,b, Ying Wanga, Ning Hana,b,c, , Yunfa Chena,b,c,
T ⁎
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China c Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Metal oxide gas sensor Field-effect transistor High-concentration Signal resolution
Detecting high-concentration gases is challenging by metal oxide semiconductor (MOX) gas sensors, because the voltage signal would become saturated. In order to solve this problem, the zooming p + n field-effect transistors (FETs) circuit has been designed, combining an n-type enhancement-mode FET (EMFET) and a p-type depletionmode FET (DMFET). This designed zooming p + n FETs can endow MOX gas sensors with the high signal resolution of ~3.0 V/decade to the 100–2000 ppm (part per million) acetone gas, triple that of MOX gas sensors without FETs (~0.8 V/decade). Meanwhile, this zooming technology is also suitable for detecting other gases at high-concentration, such as 1%–20% LEL (lower explosion limit) methane. The principle of zooming p + n FETs is that with increasing the gas concentration, the suppressing role of the EMFET is firstly induced leading to a reduced signal resolution to the low-concentration target gas; then its suppressing effect becomes saturated and the DMFET starts to switch from ON state to OFF state in the high-concentration target gas, resulting in an amplifying effect herein and thus an enhanced signal resolution. This circuit is promising for the high-concentration gas detection as well as for the multi-functional gas detector design.
1. Introduction Some kinds of gas including acetone and methane should be detected at the high concentration. For example, it is reported that the median time-weighted-average acetone concentration in the cellulose fiber factory is ~1070 ppm (part per million) [1]. The unconsciousness, dizziness, unsteadiness, confusion, and headache are produced by workers who are exposed to > 12 000 ppm acetone for 4 h [2]. Irritation of eyes, nose, and/or throat can result from the acetone gas ranging from ~250 to ~1000 ppm [3–5]. So it is essential to monitor effectively the high-concentration acetone in some special places if the worker’s security and health are taken into consideration. Gas sensors based on the metal oxide semiconductor (MOX) are widely used to detect acetone gas [6–9]. Umar et al. reported that CuO nanosheets exhibited remarkable voltage signals to 10–200 ppm acetone but the voltage signals became saturated in the range of 100–200 ppm [10]. Ag doped ZnO nanoneedles prepared by Al-Hadeethi et al. also exhibited the similar voltage signal saturation [11]. This may be due to the saturation of active sites (oxygen species or surface defects) [12,13]. In all, it’s common that the MOX gas sensors
display the voltage signal saturation exposed to the high-concentration target gas. Hence, it’s necessary to solve the problem to make MOX gas sensors competent to monitor high-concentration target gas. In our previous study, Mn doped ZnO (MZO) nanoparticles [14] show the linear voltage signals to ≤20 ppm acetone but the voltage signals to > 50 ppm become saturated, which is not suitable for detecting the high-concentration acetone. In this paper, an n-type enhancement-mode field-effect transistor (EMFET) is added into the traditional circuit. The voltage signals of MZO sensors to 5–2000 ppm acetone gas are reduced dramatically by the n-type EMFET. Then a ptype DMFET is added to form a new zooming p + n FETs circuit. This zooming p + n FETs shows an interesting feature: the voltage signal resolution to 5–100 ppm acetone gas is reduced while the voltage signal resolution to 100–2000 ppm acetone gas is improved. In short, this zooming p + n FETs shift the effective detection range to higher concentration. The principle of zooming the p + n FETs is the compromise between the suppressing role of the n-type EMFET and the amplifying role of the p-type DMFET. In our published papers [15,16], the single DMFET are used to extend the detection range to lower concentration. For example, the single DMFET 2SK544 is able to amplify the voltage
⁎ Corresponding authors at: State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail addresses: [email protected] (N. Han), [email protected] (Y. Chen).
https://doi.org/10.1016/j.sse.2019.107648 Received 19 June 2019; Received in revised form 12 August 2019; Accepted 2 September 2019 Available online 04 September 2019 0038-1101/ © 2019 Published by Elsevier Ltd.
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where RS,a and RS,g are the resistance of gas sensors in air and in the target gas respectively. VOUT,a and VOUT,g are the output voltage of the conventional circuit in air and in the target gas respectively in Fig. 1. The apparent response of the FET circuit is also calculated in the same way, as derived from (2):
RS, a / RS, g × (RL + RFET , g )/(RL + RFET , a) = (VCC / VOUT , a − 1)/(VCC / VOUT , g − 1)
Therefore, the suppressing factor (SF) can be easily derived by (3) and (4) to be (RL + RFET,g)/(RL + RFET, a) where RFET,a and RFET,g are the resistance of FETs in air and in the target gas respectively, analogous to the magnification factor in the previous report [15]. Obviously, SF is less than one because RFET,g is less than RFET,a.
Fig. 1. Design scheme of the traditional, the n-type FET and the zooming p + n FETs circuits for MOX sensors.
signal resolution to 0.1–5 ppm toluene and thus the lower limit of detection falls to 0.1 ppm [15]. Therefore, the effective detection range of the MOX gas sensors can be adjusted easily by using different modes of FETs according to the practical application.
3. Experimental 3.1. Electrical property measurement Commercial 6670AL (Fairchild, America) [17] is an n-type EMFET. 2SJ44 (Renesas, Japan) [18] and 2SJ104 (Toshiba, Japan) [19] are the p-type DMFETs. All of FETs are purchased from the market. Their output and transfer curves are measured by using the semiconductor characterization system (Keithley 4200SCS, America).
2. Design of electric circuits Fig. 1 schematically shows a chain of electric circuits for the n-type MOX sensors. The electric circuit on the left is a conventional circuit, with the load resistor (RL) and n-type MOX sensor (RS) put in series. After the bias voltage of 5.0 V (VCC) is applied, the partial voltage of the RL is the output voltage (VOUT). Once RS contacts with the reductive gas, RS will decrease and VOUT will increase. The VOUT increment is named as voltage signal (ΔV). Obviously, ΔV depends on the variation of RS. The electric circuit in the middle is an EMFET circuit. It can be seen that drain (D) is connected to the lower potential side of RL, Gate (G) is connected to the higher potential side of RL and source (S) is grounded, which produces a positive gate-source voltage (VGS). The same gas at the same concentration is injected like above, RS decreases and VOUT increases similarly. That will cause the VGS of the EMFET to shift positively, which induces the resistance of FET (RFET) to decrease according to the transfer characteristics of the EMFET. Clearly, the decrease of RFET won’t favor the increase of VOUT and thus suppresses the ΔV. So ΔV in the EMFET circuit will be smaller than that in the conventional circuit. We continue to add a p-type DMFET into the EMFET circuit, as shown on the right part in Fig. 1. Low-concentration reductive gas is injected, RS and RFET will decrease together and the increase of VOUT will be restricted by the suppressing role of the EMFET. Now, the DMFET hasn’t worked. That’s to say, the circuit is equivalent to the EMFET circuit if the concentration is low. Once the concentration reaches a certain level, the VOUT will become large enough to force the DMFET to transform from ON state to OFF state, amplifying the ΔV greatly, just as reported in the previous papers [15,16]. In all, the EMFET is always suppressing ΔV to all concentration of reductive gas while the DMFET only amplifies the ΔV to the reductive gas at the much higher concentration, which realizes a positive shift of the effective detection range. Quantitatively, VOUT in the conventional circuit is calculated as:
VOUT = VCC /(1 + RS / RL)
3.2. Gas-sensing performance measurement MZO nanoparticles and Pd-loaded mesoporous SnO2 (PSO) nanoparticles have been prepared by our laboratory and are the gas sensing materials to detect acetone and methane gas respectively [14,20]. The static gas sensing test system (Hanwei WS-30A, China) [21–22] is utilized to study the sensing property. Certain amount of the acetone liquid is dropped onto an evaporator using a micro-syringe in the test chamber (total volume 18 L) to generate the acetone gas whose concentration varies from 5 ppm to 2000 ppm. This method is also applicable for methane gas. The voltage signal (ΔV) is defined as the increment of output voltage from the air to the reductive gas. The signal resolution is defined as Resolution = ΔV/lgC where C is the concentration of the reductive gas (ppm). The unit of Resolution is V/decade. 4. Results and discussion 4.1. Suppressing effect of EMFET In our previous report [14], acetone sensing performance measurement of MZO materials shows that the optimum temperature is 340 °C and the optimum doping concentration of Mn is 2.2 mol%. In addition, MZO exhibits a better selectivity for acetone than ethanol, formaldehyde and benzene. This is due to the relatively higher surface acidity which favors acetone adsorption and reaction. However, when the acetone concentration is more than 100 ppm, ΔV will be saturated, seen in Fig. 2. The signal resolution will be calculated as ~0.8 V/ decade, meaning ΔV increases by ~0.8 V when the concentration increases from 100 ppm to 1000 ppm. At the same time, the voltage signal saturation also exists in Ag-doped ZnO nanoneedles [11] and CuO nanosheets [10]. Therefore, the signal resolution in the high-concentration range should be improved. Here, we have designed a novel circuit by adding a FET, whose transfer curve is shown in Fig. 3a. From Fig. 3a, this FET is observed to be an n-type FET with the enhancement-mode. Generally, FETs work at room temperature (25 ± 5 °C), whose transfer curves are stable at room temperature. Then RL is set as 0.5, 1.0 and 2.0 MΩ respectively, and the typical ΔV curves with and without EMFET 6670AL are shown in Fig. 3b-d, which illustrates that 6670AL is capable of suppressing ΔV of MZO to acetone gas from 5 ppm to 2000 ppm. Moreover, ΔV in both of circuits increases gradually with increasing concentration. However,
(1)
It is clear that exposed to the reductive gas, RS will decrease, resulting in the increase of VOUT. Similarly, in the proposed EMFET circuit, VOUT is expressed as:
VOUT = VCC /[1 + RS (RL / RFET )]
(2)
Therefore, the increase of VOUT (i.e. ΔV) will be suppressed because both RFET and RS decrease together. From (1), it is easy to derive the intrinsic response of MOX sensors as:
RS, a / RS, g = (VCC / VOUT , a − 1)/(VCC / VOUT , g − 1)
(4)
(3) 2
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Fig. 2. ΔV of MZO sensors to acetone from 5 ppm to 2000 ppm in the traditional circuit with different RL. Inset is the log-scale plot of acetone concentration.
Fig. 4. ΔV suppressing mechanism of 6670AL: (a) Output curves of 6670AL, VGS changes from 0.6 V to 1.5 V (step is 0.1 V); (b) RL + RFET versus current curves of 6670AL loaded with different resistors (0 MΩ–5 MΩ), the values of RL + RFET become stable and are deemed as the fixed load resistors when the current is close to 10 μA.
VOUT = VGS = VDS
Therefore, the working curve of 6670AL is determined by the points where VGS and VDS are equal. Then the output curves of 6670AL at different VGS are measured as shown in Fig. 4a. The point of VGS = VDS = 0.70 V is chosen as a starting point, I will be determined to be 0.055 μA in Fig. 4a. Next, RFET is calculated as 13 MΩ. Then VGS increases by 0.10 V, and the process above is repeated to get another RFET. Finally, all of points can be obtained and are plotted as the black curve in Fig. 4b. As to other circuits with different RL, their working curves are got by adding the corresponding RL into the black curve. Supposing RL is 2.0 MΩ, RFET,a is 13 MΩ according to Fig. 4b and REFT,g is very small, thus the theoretical SF will be calculated as 0.13, which is consistent with the measured SF of 0.11. Supposing RL is zero, RFET always decreases endlessly, which shows the 6670AL alone has an infinite capacity for suppressing ΔV (the signal resolution is about 0.12 V/decade) no matter how high the concentration is, seen in Fig. S2. Besides, it is observed that these curves start to fall quickly with increasing current and then decrease slowly until a fixed value when the resistor is loaded, which explains the signal resolutions in the conventional circuit and EMFET circuit are almost equal in the highconcentration range.
Fig. 3. ΔV suppressing effect of 6670AL: (a) Transfer curve of 6670AL; (b)-(d) ΔV of MZO to acetone from 5 ppm to 2000 ppm in the two kinds of circuits. Table 1 Comparison of sensitivities to acetone from ~100 ppm to 2000 ppm among three kinds of circuits RL (MΩ)
Traditional (V/ decade)
EMFET (V/ decade)
Zooming p + n FETs (V/ decade)
0.5 1 2 5 10 20
1.2 1.2 1.4 1.0 0.8 0.6
1.3 1.3 1.4
2.6 3.0 3.4
it is interesting to find that there is an inflection point for the EMFET circuits. That is, the increase is very slow before the inflection point, after which the change is steep. Meanwhile, the curves of both of circuits are almost parallel with each other after the inflection point. In other words, their signal resolutions to higher concentration acetone gas are almost equal and the related values are listed in Table 1. Other EMFET also suppresses the ΔV shown in Fig. S1. Taking EMFET 6670AL for example, we study how it suppresses ΔV of MZO gas sensors. From Fig. 1, VOUT is expressed as below:
VOUT = VGS = VDS + IRL
(6)
4.2. Zooming p + n FETs After a DMFET 2SJ44 is connected to the fixed load resistor, the ΔV of TGS2602 sensors is enhanced due to the amplifying effect of 2SJ44 [16], whose transfer curve of 2SJ44 is seen in Fig. S3. As mentioned above, the sum of RL and RFET in the high-concentration acetone gas can be deemed to be a fixed resistor in the EMFET circuit, then a DMFET 2SJ44 is added. The ΔV of gas sensors to high-concentration acetone gas is expected to be amplified. Here, 2SJ44 is chosen as a partner of 6670AL in the zooming p + n FETs circuit as shown in Fig. 1. The resultant ΔV curves are seen in Fig. 5a-c. Taking 0.5 MΩ for example, compared to the EMFET circuit, the ΔV in zooming p + n FETs circuit is similarly small when the
(5)
where VDS is the drain-source voltage, I is the circuit current. The case without RL is considered firstly. The Eq. (5) is written below: 3
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Fig. 5. Effect of the zooming p + n FETs: (a)-(c) ΔV comparison of MZO sensors in the n-type FET and zooming p + n FETs circuits; (d) Effective detection range comparison of MZO sensors in the conventional and zooming p + n FETs circuits.
Fig. 6. Universality of the zooming p + n FETs: (a)-(c) ΔV comparison of PSO sensors in the n-type FET and zooming p + n FETs circuits; (d) Comparison of the effective detection range of PSO sensors in the conventional and zooming p + n FETs circuits.
concentration is less than 200 ppm. But the ΔV in latter circuit is much higher than that in the former circuit with the acetone concentration in the range of 200–2000 ppm. Meanwhile, the signal resolution to ≥200 ppm acetone gas in the zooming p + n FETs circuit is higher than that in the EMFET circuit, and related data are listed in Table 1. More importantly, the effective detection range can be tuned to higher detection only by choosing a smaller RL as shown in Fig. 5d. In all, this zooming p + n FETs circuit can be used to respond effectively to the acetone concentration up to 1000 ppm and 2000 ppm with the large ΔV and the excellent signal resolution. What’s more, it is observed from Fig. 3b-d that the ΔV in the conventional circuits doesn’t become saturated with the acetone concentration up to 1000 ppm and 2000 ppm, because the load resistors are very small (e.g. 1.0 MΩ). The sensor resistance in air is measured to be 90 MΩ, then output voltage in air (VOUT,a) is 0.05 V, very close to zero. The small VOUT,a caused by small RL is not beneficial to the practical application of gas sensors. Even so, the smaller RL provides a reference to understand more vividly how the EMFET works. Therefore, without FETs, the appropriate RL should be set as ~10 MΩ and thus VOUT,a is about 0.5 V, the medium output voltage. Then the ΔV becomes saturated and the signal resolution is as low as ~0.8 V/decade in ≥100 ppm acetone gas in Fig. 2, less than one third of that (3.0 V/decade) in the zooming p + n FETs circuit. Furthermore, from Fig. 5d, for the zooming p + n FETs circuits, the signal resolution to < 100 ppm acetone is small (~0.2 V/decade) while that to ≥100 ppm acetone is high (~3.0 V/ decade), meeting the needs of high-concentration detection. Inflammable gases are particularly prone to the signal saturation especially when their concentration is close to the lower explosion limit (LEL). In order to extend the hydrogen’s detection range to the higher concentration, Okuyama et al. increased the content of Ni in the Pd/Ni electrode in the Pd/Ni-Al2O3-Al diode (i.e. metal-insulator-metal diode) [23]. As for methane detection, the LEL of methane is known to be approximately 5% [24–26]. In the case of methane apparatus indicating up to the lower flammable limit only, the alarm set point (or the lowest, where there are two or more set points) is recommended to be in the range of 1%–25% LEL to avoid the sudden explosion. Our laboratory has fabricated the PSO nanoparticles [20]. Pd dosage is optimized as 1 wt% and the working temperature is determined to be 300 °C. The excellent gas-sensing properties of PSO result from the spillover effect of catalytic Pd and heterostructure sensitization of PdO-SnO2. PSO sensors can monitor accurately the methane lower than 1% LEL, after
which the responses are saturated. In order to monitor accurately the high-concentration methane, the zooming p + n FETs circuit are employed, as shown in Fig. 6a-c. Taking 0.2 MΩ for example, the ΔV in the zooming p + n FETs circuit is initially lower than that in the conventional circuit, then exceed the latter with the methane concentration increasing from 0.2% LEL to 20% LEL. What’s more, the zooming p + n FETs circuit has the highest signal resolution of 1.7 V/decade to the methane in the range of 1%–20% LEL among three types of circuits. Meanwhile, if the effective detection range needs to be shifted to much higher concentration, just the smaller RL is chosen as shown in Fig. 6d. Moreover, it is obvious that the response time and recovery time are not influenced by FETs in Fig. S4. Finally, this methane measurement demonstrates the universality of this type of zooming p + n FETs circuit. 4.3. Mechanism of zooming p + n FETs The synergetic [16], coupling [27] or interlocking [28] p + n DMFETs are developed in our previous reports, where both p-type and n-type DMFETs can amplify the ΔV together. The interlocking p + n DMFETs cause the MZO sensors to display a transilient ΔV of ~4.0 V to ≥2 ppm acetone. It’s due to the resistance jump from ON state to OFF state of both p-type and n-type DMFETs. Howbeit, in the zooming p + n FETs circuit, the p-type DMFET has the amplifying effect because the DMFET’s resistance increases from small to large with its gate-source voltage increasing according to Fig. S5c. On the other hand, from Fig. 4b, the n-type EMFET is opposite, leading to its suppressing effect. In all, the n-type EMFET is responsible for suppressing the ΔV at low gas concentration, while the p-type DMFET is responsible for amplifying at higher gas concentration. Fig. 7 shows the working curves of the zooming p + n FETs circuit. It is obvious that they are assemblies of the curves of the n-type EMFET (Fig. 4b) and the curves of the p-type DMFET (Fig. S5). According to Fig. 1, the VOUT can be expressed as below:
VOUT = IRL + VDS (n) − VDS (p)
(7)
The term of (IRL + VDS(n)) is just the formula (5), referring to the suppressing role of EMFET 6670AL. The term of VDS(p) refers to the role of DMFET 2SJ44. Whether or not 2SJ44 can play the amplifying role depends on the concentration of target gas. Thus, three stages are taken into consideration in terms of the concentration. RL is chosen to be 0.5, 4
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that of the conventional circuit. Finally, MOX gas sensors can be used indoors and outdoors to monitor dangerous gas. Based on this, if they are needed to capture the weak signals to the lower-concentration gas, DMFETs will be qualified. On the other hand, when it comes to sensitively detecting the higherconcentration gas, EMFETs should be taken into consideration. Briefly, selecting different modes of FETs is a simple way of changing the effective detection range of gas sensor. It should be noted that the addition of FETs can alter the signals of gas sensors but cannot alter their selectivity, depending on the material itself. 5. Conclusion The EMFET is able to strongly suppress the signals of the MOX sensors. The mechanism is deemed to be the increase of VGS inducing the sharp decrease of the EMFET resistance. The designed zooming p + n FETs can cause the MOX gas sensors to show weak resolution (~0.2 V/decade) to < 100 ppm acetone, but strong signal resolution (~3 V/decade) to ≥100 ppm acetone, thus the effective detection range of the MOX sensors is extended to the higher concentration. Meanwhile, this zooming technology is also used to detect 1%–20% LEL methane. The working mechanism of zooming p + n FETs is that the ntype EMFET firstly works and plays the suppressing role. The amplifying role of the p-type DMFET depends entirely on the concentration of the target gas. The p-type EMFET will be driven to amplify only if the gas concentration is high enough. The zooming p + n FETs provide more opportunities for MOX gas sensors to detect high-concentration target gas. More importantly, people can choose the appropriate mode of FETs to alter the effective detection range of the MOX sensors flexibly according to the actual needs.
Fig. 7. Working mechanism of zooming p + n FETs: The solid are the working curves of 6670AL while the dashed are the working curves of 2SJ44, it can be seen that 6670AL firstly works and suppresses the ΔV until the output voltage is large enough to cause 2SJ44 to start to work and amplify the ΔV.
1 and 2 MΩ respectively in Fig. 5. Each RL has a critical concentration of the acetone gas. For instance, 0.5 and 1 MΩ correspond to ~100 ppm while 2 MΩ corresponds to ~50 ppm, seen in Fig. 5a-c. Obviously, the slope of curve is very low before the critical concentration, after which the slope is steep. For simplicity, RL of 0.5 MΩ is chosen as an example to elaborate the working mechanism of zooming p + n FETs.
• The first stage is that the concentration is zero, i.e. in the air. R •
•
FET,a
of the 6670AL is 13 MΩ, 2SJ44 is at ON state and its resistance is neglectable. Hence, the baseline of the zooming p + n FETs circuit in clean air is described as VOUT ≈ 0.7 V. Injecting ≤100 ppm acetone is the second stage. 2SJ44 is still at ON state, but resistance of 6670AL starts to decrease from 13 MΩ and the output voltage undergoes a restricted increase due to the suppressing role of 6670AL. Therefore, the zooming p + n FETs circuit is equivalent to the EMFET circuit, namely VOUT ≈ IRL + VDS(n), whose curve overlaps the curve of the EMFET circuit (red solid curve) as shown in Fig. 7. Injecting > 100 ppm acetone is the third stage. The resistance of 6670AL decreases firstly and it also plays the suppressing role, along the red solid curve in Fig. 7. Even so, the output voltage still increases and finally is large enough to drive the 2SJ44 to work. Once the 2SJ44 works, the resistance of 2SJ44 will increase dramatically, and so do will the output voltage, along the red dashed curve in Fig. 7. Notably, when 2SJ44 works, the suppressing role of 6670AL remains but is neglectable compared to the amplifying role of the 2SJ44, because the slopes of 2SJ44 are much steeper than those of 6670AL. The large slopes of 2SJ44 cause the zooming p + n FETs circuit to possess the highest signal resolution among three types of circuits.
Acknowledgment This research was financially supported by the National Key R&D Program of China (2016YFC0207100), the National Natural Science Foundation of China (51602314), Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05C146). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.sse.2019.107648. References [1] Hasen H, Wilbur SB. Toxicological profile for acetone. Atlanta: U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry; 1994. [2] Ross DS. Acute acetone intoxication involving eight male workers. Ann Occup Hygiene 1973;16:73–5. [3] Mitran E, Callender T, Orha B, Dragnea P, Botezatu G. Neurotoxicity associated with occupational exposure to acetone, methyl ethyl ketone, and cyclohexanone. Environ Res 1997;73:181–8. [4] Dick RB, Setzer JV, Taylor BJ, Shukla R. Neurobehavioural effects of short duration exposures to acetone and methyl ethyl ketone. Br J Ind Med 1989;46:111–21. [5] Satoh T, Omae K, Nakashima H, Takebayashi T, Matsumura H, Kawai T, et al. Relationship between acetone exposure concentration and health effects in acetate fiber plant workers. Int Arch Occup Environ Health 1996;68:147–53. [6] Cho H-J, Kim S-J, Choi S-J, Jang J-S, Kim I-D. Facile synthetic method of catalystloaded ZnO nanofibers composite sensor arrays using bio-inspired protein cages for pattern recognition of exhaled breath. Sens Actuators B 2017;243:166–75. [7] Zhou X, Wang A, Wang Y, Bian L, Yang Z, Bian Y, et al. Crystal-defect-dependent gas-sensing mechanism of the single ZnO nanowire sensors. ACS Sensors 2018;3:2385–93. [8] Li X, Zhou X, Guo H, Wang C, Liu J, Sun P, et al. Design of Au@ZnO yolk-shell nanospheres with enhanced gas sensing properties. ACS Appl Mater Interfaces 2014;6:18661–7. [9] Moon HG, Jung Y, Jun D, Park JH, Chang YW, Park HH, et al. Hollow Pt-functionalized SnO2 hemipill network formation using a bacterial skeleton for the noninvasive diagnosis of diabetes. ACS Sensors 2018;3:661–9. [10] Umar A, Alshahrani AA, Algarni H, Kumar R. CuO nanosheets as potential scaffolds for gas sensing applications. Sens Actuators B 2017;250:24–31.
Anyhow, it can be simply deemed as that with increasing the gas concentration, the 6670AL firstly plays the suppressing role leading to a reduced signal resolution to the low-concentration target gas; then its suppressing effect becomes saturated and the 2SJ44 DMFET starts to switch from ON state to OFF state in the high-concentration target gas. Thus, the amplifying effect is induced and the signal resolution is enhanced. Particularly, voltage signal would remain unchangeable if the amplifying role is offset completely by the suppressing role, i.e. the intersection points of the black curve and blue curve in Fig. S6. Many similar intersection points appear in Fig. 6a-c. After the intersection point, the voltage signal of the zooming p + n FETs circuit outnumbers 5
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Yuzhi Bian graduated from Beijing University of Technology and now is a Second-year graduate student in University of Chinese Academy of Sciences, his major is materials science.
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Ying Wang received the bachelor’s degree in optics from Beijing Jiaotong University in 2010. She received the Ph.D. degree in physical electronics with the College of Electronics, Peking University.
Ning Han received the Ph.D. degree in chemical engineering from the Institute of Process Engineering, CAS, in 2010. He was a Post-Doctoral Fellow with the Department of Physics and Materials Science, City University of Hong Kong, from 2010 to 2014, and has been a Professor with IPE CAS via One Hundred Talents Plan since 2014. He works on investigations of preparation, structure, and property of semiconductors, and has developed highly sensitive and selective gas sensing materials. He has authored over 60 articles.
Yunfa Chen received his Ph.D. in Material Science at Université Louis Pasteur Strasbourg (ULP), France in 1993. He is a professor of the Graduate University of Chinese Academy of Sciences, and Research Professor and Vice Director of Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are preparation and assembly of nanoparticles, functional materials, organic–inorganic composite materials and layered materials. And he is also interested in industrial application of nanomaterials.
Xinyuan Zhou received the B.S. degree in materials chemistry from Taiyuan University of Technology in 2014. He is currently pursuing the Ph.D. degree in materials engineering at the Institute of Process Engineering, CAS. His research focuses on the metal oxide semiconductor gas sensors.
Liping Yang received the Ph.D. degree in physical chemistry from Institute of Chemistry, Chinese Academy of Sciences. Currently, she is engaging postdoctoral study in the Institute of Process Engineering, Chinese Academy of Sciences.
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