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Oxygen interference mechanism of platinum–FET hydrogen gas sensor
Sensors and Actuators A 136 (2007) 244–248
Oxygen interference mechanism of platinum–FET hydrogen gas sensor T. Yamaguchi a,∗ , T. Kiwa a , K. Tsukada a , K. Yokosawa b a
Department of Electrical and Electronic Engineering, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan b Advanced Research Laboratory, Hitachi Ltd., 1-280 Higashikoigakubo, Kokubunji, Tokyo 185-0014, Japan Received 31 July 2006; received in revised form 16 November 2006; accepted 21 November 2006 Available online 15 December 2006
Abstract This paper reports the effect of oxygen in air on a Pt–FET hydrogen sensor. The output voltage change of the Pt–FET had good linearity to the logarithm of hydrogen concentration in nitrogen. The slope of the linear curve is −26.4 mV/decade, which is close to the calculated value of the Nernstian slope. This indicates the response mechanism of the Pt–FET sensor is based on the catalytic reaction on the Pt surface. However, the output voltage change of the Pt–FET was not proportional to the logarithm of hydrogen concentration in air due to adsorbed oxygen on the Pt surface. This indicates the water formation for the reaction of hydrogen and oxygen on the Pt surface is the main hydrogen response mechanism in air. © 2006 Elsevier B.V. All rights reserved. Keywords: Pt–FET; Hydrogen sensor; Oxygen interference; Water formation
1. Introduction A fuel cell with hydrogen energy is recently expected as a new energy source. However, because hydrogen concentration over 4% has the risk for an ignition, the development of sensors for quickly detecting the leakage of hydrogen has been advanced. At present, various types of sensors have been studied, such as Schottky diode type, field effect transistor (FET) type, etc. [1–6]. The characteristics of these sensors strongly depend on those of catalytic metals, because the principle of them is based on the work function change of catalytic metals. Therefore, it is essential to clarify the hydrogen adsorption mechanism on catalytic metals. FET type sensors using catalytic metals have been developed since 1975 [7]. These sensors have a number of advantages, which are room temperature operation, smaller size compared to the conventional sensors and low power consumption. Recently, we reported the fast response mechanism of the FET with platinum (Pt) gate electrode (Pt–FET) to hydrogen in nitrogen [8]. However, the detection of hydrogen in air is generally required for practical applications, therefore, it is necessary to investigate the hydrogen response mechanism to hydrogen in air in
∗
Corresponding author. Tel.: +81 86 251 8129; fax: +81 86 251 8129. E-mail address: [email protected] (T. Yamaguchi).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.11.026
detail. This paper reports the effect of oxygen gas in air on the Pt–FET. 2. Experimental Fig. 1 shows the gas flow system fabricated to evaluate the response characteristic of the Pt–FET sensor. The Pt–FET sensor was mounted on a ceramic substrate with high thermal conductivity. The operating temperature of the sensor was controlled in range from 30 to 120 ◦ C by a heater under the flow cell. An inlet and an outlet nozzle were fixed at the angle of 45 ◦ C to the substrate normal, therefore, smooth gas exchange was possible. We prepared hydrogen gas with the concentration from 1 to 105 ppm in nitrogen base with one decade interval and oxygen gas with the concentration from 10 to 105 ppm in nitrogen base with two decades interval. A pure oxygen gas was also prepared. The change valve enabled hydrogen concentration to exchange within 1 s. To make hydrogen–oxygen mixture, hydrogen gas in nitrogen base and pure oxygen gas were mixed. A Ta2 O5 FET (Shindengen Kogyo) was used as the base FET. This FET is an n-channel type FET with the size of 15-m long and 340m wide. The gate insulator consists of a 100-nm thick under layer of SiO2 , a 100-nm thick middle layer of Si3 N4 , and a 50nm thick top layer. A 30-nm thick Pt film was sputtered on the gate insulator. A voltage follower circuit was used to measure the work function change of Pt directly, which can be obtained
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Fig. 3. Hydrogen concentration dependence of the output voltage in nitrogen.
muir absorption equation of hydrogen is expressed as follows: Fig. 1. Schematic diagram of the gas flow system.
as the gate-source voltage change of the Pt–FET. The hydrogen response characteristic of the Pt–FET depends on the work function change of Pt due to the hydrogen adsorption. Therefore, the Pt–FET response characteristic to hydrogen concentration can be investigated precisely by measuring the work function change of Pt.
dθ = k1 P(H2 )(1 − θ)2 − k2 θ 2 dt
where θ is the surface coverage of hydrogen, P(H2 ) the partial pressure of hydrogen gas, k1 and k2 are constant. The chemical potential of dissociated hydrogen atoms is proportional to ln(θ/1 − θ). Therefore, the following Nernstian equation is obtained at the steady state condition: V = V0 −
3. Results and discussion Fig. 2 shows the hydrogen response characteristic of the Pt–FET in nitrogen. With increasing hydrogen concentration, the response time and the output voltage of the Pt–FET decreased (Fig. 2). The output voltage change of the Pt–FET was proportional to the logarithm of hydrogen concentration (Fig. 3). In addition, the linear slope shifted in the negative direction with increasing temperature. The sensitivity of the Pt–FET calculated from the slope of the linear curve is −26.4 mV/decade at room temperature. The hydrogen response mechanism of the Pt–FET in nitrogen can be explained with the following Laugmuir adsorption equation [8]. Adsorbed hydrogen molecules on the Pt surface dissociate for the catalysis of Pt. Then, the Langu-
Fig. 2. Time response of the Pt–FET hydrogen sensor with hydrogen concentration changed in steps: in nitrogen, flow rate 3 l/min.
(1)
RT ln(P(H2 )) 2F
(2)
where V is the voltage of the Pt–FET, V0 is the constant, R is the gas constant, T is the absolute temperature, and F is the Faraday’s constant. The value of the sensitivity calculated from Eq. (2) is −30 mV/decade at room temperature, which is close to the measured value. In addition, the temperature dependence of the sensitivity plotted in Fig. 3 was well fitted by Eq. (2). These results indicate that the response mechanism of the Pt–FET is based on the catalytic reaction on the Pt surface. As it is reported the Pt–FET can be used as the oxygen sensor, the effect of oxygen gas on the Pt–FET is not negligible [9]. Thus, the oxygen response characteristic of the fabricated Pt–FET was measured. Fig. 4 shows the oxygen concentration dependence of the output voltage change in nitrogen. As oxygen concentration increased, the output voltage shifted in the positive direction and each curve was non-linear in the semi-
Fig. 4. Oxygen concentration dependence of the output voltage in nitrogen.
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Fig. 5. Time response of the Pt–FET hydrogen sensor with hydrogen concentration changed in steps: in air, flow rate 2 l/min. The initial hydrogen concentration (at 0 min) is 800 ppm.
Fig. 7. Temperature dependence of the output voltage in air.
logarithmic plot. Additionally, the voltage change in the range from 10 ppm to 0.1% comes to be large at over 90 ◦ C, which is the different response characteristic from the Nernstian slope. Although it is well known that not only hydrogen molecules but also oxygen molecules dissociate on the Pt surface, these results indicate that the adsorption mechanism of the oxygen is different from that of simple dissociative adsorption. However, on the hydrogen response of the Pt–FET in air, it is thought that the effect of dissociated oxygen atoms is larger than that of other adsorbed oxygen, because they are activated. Therefore, only the effect of dissociated oxygen was considered in the hydrogen response of the Pt–FET in air. To evaluate the effect of oxygen in air, the hydrogen response of the Pt–FET in air was measured. Hydrogen gas in air base was made by mixing hydrogen gas in nitrogen base and pure oxygen gas to measure of the hydrogen response in air. Because of the mixture, the hydrogen concentration in mixed gas decreased by 80% compared to that in nitrogen. Fig. 5 shows the time response of the Pt–FET to hydrogen gas in air. As hydrogen concentration increased, the response time and the output voltage decreased as with the hydrogen response in nitrogen. As shown in Fig. 6, however, the output voltage change of the Pt–FET was not proportional to the logarithm of hydrogen concentration in
air, which could not be explained by the Nernstian equation. This result indicates that oxygen in air affects the hydrogen response. In air, it is thought a number of oxygen atoms are adsorbed on the Pt surface. These oxygen atoms form water molecules with hydrogen atoms (because these dissociated atoms are activated). As the result, the number of oxygen atoms on the Pt surface is reduced, which seems to be the main hydrogen response mechanism in air. To investigate the water formation on the Pt surface, the temperature dependence of the Pt–FET in air was measured as shown in Fig. 7. With increasing temperature, the response curve shifted in the positive direction, (which was different from Figs. 2 and 4). Additionally, the response time rapidly decreased with increasing temperature from 30 to 70 ◦ C (Fig. 8). It suggests that the water formation rate increases by increasing temperature. The temperature dependence of the sensitivity obtained by each curve in Fig. 7 was also different from the Nernstian slope. These results indicate the hydrogen response mechanism of the Pt–FET in air is related to the water formation on the Pt surface. The output voltage change of the Pt–FET was not proportional to the logarithm of hydrogen concentration in air and the temperature dependence in air was different from that in nitrogen. To analyze these phenomena precisely, the relationship between hydrogen concentration and the voltage change was considered. It is assumed that the chemical potential of
Fig. 6. Hydrogen concentration dependence of the output voltage in air at room temperature.
Fig. 8. Temperature dependence of the response time to hydrogen concentration changed from 8 → 80 ppm.
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was not proportional to the logarithm of hydrogen concentration in air, which is due to the effect of oxygen in air. From the Langumuir adsorption equation, it was shown that the hydrogen response of the Pt–FET in air was due to the water formation on the Pt surface. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization. References Fig. 9. V−1 vs. P(H2 )−1/2 .
adsorbed oxygen φ is expressed by the following equation: φ = −
μNS θ ε
(3)
where μ is the dipole moment, ε the dielectric constant, NS the density of adsorption sites on the Pt surface, and θ is the coverage of oxygen atoms. It is assumed that the number of hydrogen atoms reacting to oxygen atoms is expressed by NS θ with the hydrogen coverage θ. Then, the number of reduced oxygen atoms n is n =
NS θ 2
(4)
From Eqs. (3) and (4), the output voltage change V is represented as V = −
μNS θ 2ε
(5)
Using Eqs. (1) and (5), the following equation is obtained at the steady state condition: 1 1 1 P(H2 )−1/2 − =− V KVmax Vmax
(6)
where Vmax = μNS /2ε, and K is constant. From Eq. (6), V−1 ∝ P(H2 )−1/2 is obtained. Although Eq. (6) is similar to the equation reported for the Pd-gate FET hydrogen sensor, it is different that K is independent of oxygen concentration [10]. Fig. 9 shows the V−1 versus P(H2 )−1/2 of the measured data for the Pt–FET, and which was well fitted by Eq. (6). This result indicates that the hydrogen response mechanism of the Pt–FET in air is based on the work function change of Pt due to the water formation on the Pt surface. 4. Conclusion In this study, the oxygen interference mechanism for the hydrogen response of the Pt–FET was investigated. The output voltage change of the Pt–FET had good linearity to the logarithm of hydrogen concentration in nitrogen and the response mechanism was based on the catalytic reaction of hydrogen on the Pt surface. However, the output voltage change of the Pt–FET
[1] I. Eisele, T. Doll, M. Burgmair, Low power detection with FET sensors, Sens. Actuators B: Chem. 78 (2001) 19–25. [2] C.K. Kim, J.H. Lee, Y.H. Lee, N.I. Cho, D.J. Kim, A study on a platinum–silicon carbide Schottky diode as a hydrogen gas senor, Sens. Actuators B: Chem. 66 (2000) 116–118. [3] K. Scarnagl, A. Karthigeyan, M. Burgmair, M. Zimmer, T. Doll, I. Eisele, Low temperature hydrogen detection at high concentrations: comparison of platinum and iridium, Sens. Actuators B: Chem. 80 (2001) 163–168. [4] M. Burgmair, H.-P. Frerichs, M. Zimmer, M. Lemann, I. Eisele, Field effect transducers for work function gas measurements: device improvements and comparison of performance, Sens. Actuators B: Chem. 95 (2003) 183– 185. [5] A. Salomonsson, M. Eriksson, H. Dannetun, Hydrogen interaction with platinum and palladium metal–insulator–semiconductor devices, J. Appl. Phys. 98 (2005) 014505. [6] Ch. Wilbertz, H.-P. Frerichs, I. Freund, M. Lemann, Suspended-gate- and lundstrom-FET integrated on a CMOS-chip, Sens. Actuators A: Phys. 123124 (2005) 2–6. [7] I. Lundstr¨om, S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogensensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55–57. [8] K. Tsukada, T. Kiwa, T. Yamaguchi, S. Migitaka, Y. Goto, K. Yokosawa, A study of fast response characteristics for hydrogen sensing with platinum FET sensor, Sens. Actuators B: Chem. 114 (2006) 158–163. [9] Y. Miyahara, K. Tsukada, H. Miyagi, Field-effect transistor using a solid electrolyte as a new oxygen sensor, J. Appl. Phys. 63 (1988) 2431–2434. [10] I. Lundstr¨om, S. Shivaraman, C. Svensson, A hydrogen-sensitive Pd-gate MOS transistor, J. Appl. Phys. 46 (1975) 3876–3880.
Biographies T. Yamaguchi was born in Hyogo, Japan. He received his BS degree from Okayama University in 2005. He is now a PhD student at Okayama University. His subject includes microgas sensor. T. Kiwa was born in Nara, Japan in 1976. He received his Dr. Eng. Degree from Osaka University in 2003. After that he worked for 1 year as a JSPS fellow at the Research Center for Sueprconductor Photonics, Osaka University, where he was involved in the development of terahertz and superconductor devices. Currently, he is a Lecturer of Department of Electrical and Electronic Engineering, Okayama University. His research interests include chemical sensors, magnetometric sensors, and terahertz devices. K. Tsukada was born in Kumamoto, Japan in 1954. He received his Dr. Eng. and the Ph. Dr. degrees from Tsukuba University in 1990, and 2001, respectively. He joined the Central Research Laboratory, Hitachi Ltd. in 1982, where he was involved in the study of integrated solid-state chemical sensor for blood analyses. He was with the Superconducting Sensor Laboratory from 1991 to 1996. He was involved in the research and development of SQUID’s and multichannel SQUID system. He was with the Central Research Laboratory, Hitachi Ltd. from 1996 to 2003. He was a Project Leader of the SQUID application research group. He is presently a Professor of Department of Electrical and Electronic Engineering, Okayama University. He is involved in the research of gas sensor and superconducting sensor devices, and their applications.
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K. Yokosawa was born in Sendai, Japan, in 1962. He received his BS and MS degrees in physics from the Faculty of Science, Hokkaido University in 1984 and 1986, respectively, and the PhD degree in systems and information engineering from the Faculty of Engineering, Hokkaido University in 1998. He joined the Central Research Laboratory, Hitachi Ltd. in 1986. Since then he has been engaged in the research and development of SQUIDs and biomagnetometer sys-
tems, and ultrasonic diagnostic probes. He has been with the Advanced Research Laboratory, Hitachi Ltd., since 2004, where he is the leader of a national project to develop a hydrogen gas detection system. His research interests and activities cover sensors and sensing systems using superconductor, dielectronics, and semiconductor.
Oxygen interference mechanism of platinum–FET hydrogen gas sensor T. Yamaguchi a,∗ , T. Kiwa a , K. Tsukada a , K. Yokosawa b a
Department of Electrical and Electronic Engineering, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan b Advanced Research Laboratory, Hitachi Ltd., 1-280 Higashikoigakubo, Kokubunji, Tokyo 185-0014, Japan Received 31 July 2006; received in revised form 16 November 2006; accepted 21 November 2006 Available online 15 December 2006
Abstract This paper reports the effect of oxygen in air on a Pt–FET hydrogen sensor. The output voltage change of the Pt–FET had good linearity to the logarithm of hydrogen concentration in nitrogen. The slope of the linear curve is −26.4 mV/decade, which is close to the calculated value of the Nernstian slope. This indicates the response mechanism of the Pt–FET sensor is based on the catalytic reaction on the Pt surface. However, the output voltage change of the Pt–FET was not proportional to the logarithm of hydrogen concentration in air due to adsorbed oxygen on the Pt surface. This indicates the water formation for the reaction of hydrogen and oxygen on the Pt surface is the main hydrogen response mechanism in air. © 2006 Elsevier B.V. All rights reserved. Keywords: Pt–FET; Hydrogen sensor; Oxygen interference; Water formation
1. Introduction A fuel cell with hydrogen energy is recently expected as a new energy source. However, because hydrogen concentration over 4% has the risk for an ignition, the development of sensors for quickly detecting the leakage of hydrogen has been advanced. At present, various types of sensors have been studied, such as Schottky diode type, field effect transistor (FET) type, etc. [1–6]. The characteristics of these sensors strongly depend on those of catalytic metals, because the principle of them is based on the work function change of catalytic metals. Therefore, it is essential to clarify the hydrogen adsorption mechanism on catalytic metals. FET type sensors using catalytic metals have been developed since 1975 [7]. These sensors have a number of advantages, which are room temperature operation, smaller size compared to the conventional sensors and low power consumption. Recently, we reported the fast response mechanism of the FET with platinum (Pt) gate electrode (Pt–FET) to hydrogen in nitrogen [8]. However, the detection of hydrogen in air is generally required for practical applications, therefore, it is necessary to investigate the hydrogen response mechanism to hydrogen in air in
∗
Corresponding author. Tel.: +81 86 251 8129; fax: +81 86 251 8129. E-mail address: [email protected] (T. Yamaguchi).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.11.026
detail. This paper reports the effect of oxygen gas in air on the Pt–FET. 2. Experimental Fig. 1 shows the gas flow system fabricated to evaluate the response characteristic of the Pt–FET sensor. The Pt–FET sensor was mounted on a ceramic substrate with high thermal conductivity. The operating temperature of the sensor was controlled in range from 30 to 120 ◦ C by a heater under the flow cell. An inlet and an outlet nozzle were fixed at the angle of 45 ◦ C to the substrate normal, therefore, smooth gas exchange was possible. We prepared hydrogen gas with the concentration from 1 to 105 ppm in nitrogen base with one decade interval and oxygen gas with the concentration from 10 to 105 ppm in nitrogen base with two decades interval. A pure oxygen gas was also prepared. The change valve enabled hydrogen concentration to exchange within 1 s. To make hydrogen–oxygen mixture, hydrogen gas in nitrogen base and pure oxygen gas were mixed. A Ta2 O5 FET (Shindengen Kogyo) was used as the base FET. This FET is an n-channel type FET with the size of 15-m long and 340m wide. The gate insulator consists of a 100-nm thick under layer of SiO2 , a 100-nm thick middle layer of Si3 N4 , and a 50nm thick top layer. A 30-nm thick Pt film was sputtered on the gate insulator. A voltage follower circuit was used to measure the work function change of Pt directly, which can be obtained
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245
Fig. 3. Hydrogen concentration dependence of the output voltage in nitrogen.
muir absorption equation of hydrogen is expressed as follows: Fig. 1. Schematic diagram of the gas flow system.
as the gate-source voltage change of the Pt–FET. The hydrogen response characteristic of the Pt–FET depends on the work function change of Pt due to the hydrogen adsorption. Therefore, the Pt–FET response characteristic to hydrogen concentration can be investigated precisely by measuring the work function change of Pt.
dθ = k1 P(H2 )(1 − θ)2 − k2 θ 2 dt
where θ is the surface coverage of hydrogen, P(H2 ) the partial pressure of hydrogen gas, k1 and k2 are constant. The chemical potential of dissociated hydrogen atoms is proportional to ln(θ/1 − θ). Therefore, the following Nernstian equation is obtained at the steady state condition: V = V0 −
3. Results and discussion Fig. 2 shows the hydrogen response characteristic of the Pt–FET in nitrogen. With increasing hydrogen concentration, the response time and the output voltage of the Pt–FET decreased (Fig. 2). The output voltage change of the Pt–FET was proportional to the logarithm of hydrogen concentration (Fig. 3). In addition, the linear slope shifted in the negative direction with increasing temperature. The sensitivity of the Pt–FET calculated from the slope of the linear curve is −26.4 mV/decade at room temperature. The hydrogen response mechanism of the Pt–FET in nitrogen can be explained with the following Laugmuir adsorption equation [8]. Adsorbed hydrogen molecules on the Pt surface dissociate for the catalysis of Pt. Then, the Langu-
Fig. 2. Time response of the Pt–FET hydrogen sensor with hydrogen concentration changed in steps: in nitrogen, flow rate 3 l/min.
(1)
RT ln(P(H2 )) 2F
(2)
where V is the voltage of the Pt–FET, V0 is the constant, R is the gas constant, T is the absolute temperature, and F is the Faraday’s constant. The value of the sensitivity calculated from Eq. (2) is −30 mV/decade at room temperature, which is close to the measured value. In addition, the temperature dependence of the sensitivity plotted in Fig. 3 was well fitted by Eq. (2). These results indicate that the response mechanism of the Pt–FET is based on the catalytic reaction on the Pt surface. As it is reported the Pt–FET can be used as the oxygen sensor, the effect of oxygen gas on the Pt–FET is not negligible [9]. Thus, the oxygen response characteristic of the fabricated Pt–FET was measured. Fig. 4 shows the oxygen concentration dependence of the output voltage change in nitrogen. As oxygen concentration increased, the output voltage shifted in the positive direction and each curve was non-linear in the semi-
Fig. 4. Oxygen concentration dependence of the output voltage in nitrogen.
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T. Yamaguchi et al. / Sensors and Actuators A 136 (2007) 244–248
Fig. 5. Time response of the Pt–FET hydrogen sensor with hydrogen concentration changed in steps: in air, flow rate 2 l/min. The initial hydrogen concentration (at 0 min) is 800 ppm.
Fig. 7. Temperature dependence of the output voltage in air.
logarithmic plot. Additionally, the voltage change in the range from 10 ppm to 0.1% comes to be large at over 90 ◦ C, which is the different response characteristic from the Nernstian slope. Although it is well known that not only hydrogen molecules but also oxygen molecules dissociate on the Pt surface, these results indicate that the adsorption mechanism of the oxygen is different from that of simple dissociative adsorption. However, on the hydrogen response of the Pt–FET in air, it is thought that the effect of dissociated oxygen atoms is larger than that of other adsorbed oxygen, because they are activated. Therefore, only the effect of dissociated oxygen was considered in the hydrogen response of the Pt–FET in air. To evaluate the effect of oxygen in air, the hydrogen response of the Pt–FET in air was measured. Hydrogen gas in air base was made by mixing hydrogen gas in nitrogen base and pure oxygen gas to measure of the hydrogen response in air. Because of the mixture, the hydrogen concentration in mixed gas decreased by 80% compared to that in nitrogen. Fig. 5 shows the time response of the Pt–FET to hydrogen gas in air. As hydrogen concentration increased, the response time and the output voltage decreased as with the hydrogen response in nitrogen. As shown in Fig. 6, however, the output voltage change of the Pt–FET was not proportional to the logarithm of hydrogen concentration in
air, which could not be explained by the Nernstian equation. This result indicates that oxygen in air affects the hydrogen response. In air, it is thought a number of oxygen atoms are adsorbed on the Pt surface. These oxygen atoms form water molecules with hydrogen atoms (because these dissociated atoms are activated). As the result, the number of oxygen atoms on the Pt surface is reduced, which seems to be the main hydrogen response mechanism in air. To investigate the water formation on the Pt surface, the temperature dependence of the Pt–FET in air was measured as shown in Fig. 7. With increasing temperature, the response curve shifted in the positive direction, (which was different from Figs. 2 and 4). Additionally, the response time rapidly decreased with increasing temperature from 30 to 70 ◦ C (Fig. 8). It suggests that the water formation rate increases by increasing temperature. The temperature dependence of the sensitivity obtained by each curve in Fig. 7 was also different from the Nernstian slope. These results indicate the hydrogen response mechanism of the Pt–FET in air is related to the water formation on the Pt surface. The output voltage change of the Pt–FET was not proportional to the logarithm of hydrogen concentration in air and the temperature dependence in air was different from that in nitrogen. To analyze these phenomena precisely, the relationship between hydrogen concentration and the voltage change was considered. It is assumed that the chemical potential of
Fig. 6. Hydrogen concentration dependence of the output voltage in air at room temperature.
Fig. 8. Temperature dependence of the response time to hydrogen concentration changed from 8 → 80 ppm.
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was not proportional to the logarithm of hydrogen concentration in air, which is due to the effect of oxygen in air. From the Langumuir adsorption equation, it was shown that the hydrogen response of the Pt–FET in air was due to the water formation on the Pt surface. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization. References Fig. 9. V−1 vs. P(H2 )−1/2 .
adsorbed oxygen φ is expressed by the following equation: φ = −
μNS θ ε
(3)
where μ is the dipole moment, ε the dielectric constant, NS the density of adsorption sites on the Pt surface, and θ is the coverage of oxygen atoms. It is assumed that the number of hydrogen atoms reacting to oxygen atoms is expressed by NS θ with the hydrogen coverage θ. Then, the number of reduced oxygen atoms n is n =
NS θ 2
(4)
From Eqs. (3) and (4), the output voltage change V is represented as V = −
μNS θ 2ε
(5)
Using Eqs. (1) and (5), the following equation is obtained at the steady state condition: 1 1 1 P(H2 )−1/2 − =− V KVmax Vmax
(6)
where Vmax = μNS /2ε, and K is constant. From Eq. (6), V−1 ∝ P(H2 )−1/2 is obtained. Although Eq. (6) is similar to the equation reported for the Pd-gate FET hydrogen sensor, it is different that K is independent of oxygen concentration [10]. Fig. 9 shows the V−1 versus P(H2 )−1/2 of the measured data for the Pt–FET, and which was well fitted by Eq. (6). This result indicates that the hydrogen response mechanism of the Pt–FET in air is based on the work function change of Pt due to the water formation on the Pt surface. 4. Conclusion In this study, the oxygen interference mechanism for the hydrogen response of the Pt–FET was investigated. The output voltage change of the Pt–FET had good linearity to the logarithm of hydrogen concentration in nitrogen and the response mechanism was based on the catalytic reaction of hydrogen on the Pt surface. However, the output voltage change of the Pt–FET
[1] I. Eisele, T. Doll, M. Burgmair, Low power detection with FET sensors, Sens. Actuators B: Chem. 78 (2001) 19–25. [2] C.K. Kim, J.H. Lee, Y.H. Lee, N.I. Cho, D.J. Kim, A study on a platinum–silicon carbide Schottky diode as a hydrogen gas senor, Sens. Actuators B: Chem. 66 (2000) 116–118. [3] K. Scarnagl, A. Karthigeyan, M. Burgmair, M. Zimmer, T. Doll, I. Eisele, Low temperature hydrogen detection at high concentrations: comparison of platinum and iridium, Sens. Actuators B: Chem. 80 (2001) 163–168. [4] M. Burgmair, H.-P. Frerichs, M. Zimmer, M. Lemann, I. Eisele, Field effect transducers for work function gas measurements: device improvements and comparison of performance, Sens. Actuators B: Chem. 95 (2003) 183– 185. [5] A. Salomonsson, M. Eriksson, H. Dannetun, Hydrogen interaction with platinum and palladium metal–insulator–semiconductor devices, J. Appl. Phys. 98 (2005) 014505. [6] Ch. Wilbertz, H.-P. Frerichs, I. Freund, M. Lemann, Suspended-gate- and lundstrom-FET integrated on a CMOS-chip, Sens. Actuators A: Phys. 123124 (2005) 2–6. [7] I. Lundstr¨om, S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogensensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55–57. [8] K. Tsukada, T. Kiwa, T. Yamaguchi, S. Migitaka, Y. Goto, K. Yokosawa, A study of fast response characteristics for hydrogen sensing with platinum FET sensor, Sens. Actuators B: Chem. 114 (2006) 158–163. [9] Y. Miyahara, K. Tsukada, H. Miyagi, Field-effect transistor using a solid electrolyte as a new oxygen sensor, J. Appl. Phys. 63 (1988) 2431–2434. [10] I. Lundstr¨om, S. Shivaraman, C. Svensson, A hydrogen-sensitive Pd-gate MOS transistor, J. Appl. Phys. 46 (1975) 3876–3880.
Biographies T. Yamaguchi was born in Hyogo, Japan. He received his BS degree from Okayama University in 2005. He is now a PhD student at Okayama University. His subject includes microgas sensor. T. Kiwa was born in Nara, Japan in 1976. He received his Dr. Eng. Degree from Osaka University in 2003. After that he worked for 1 year as a JSPS fellow at the Research Center for Sueprconductor Photonics, Osaka University, where he was involved in the development of terahertz and superconductor devices. Currently, he is a Lecturer of Department of Electrical and Electronic Engineering, Okayama University. His research interests include chemical sensors, magnetometric sensors, and terahertz devices. K. Tsukada was born in Kumamoto, Japan in 1954. He received his Dr. Eng. and the Ph. Dr. degrees from Tsukuba University in 1990, and 2001, respectively. He joined the Central Research Laboratory, Hitachi Ltd. in 1982, where he was involved in the study of integrated solid-state chemical sensor for blood analyses. He was with the Superconducting Sensor Laboratory from 1991 to 1996. He was involved in the research and development of SQUID’s and multichannel SQUID system. He was with the Central Research Laboratory, Hitachi Ltd. from 1996 to 2003. He was a Project Leader of the SQUID application research group. He is presently a Professor of Department of Electrical and Electronic Engineering, Okayama University. He is involved in the research of gas sensor and superconducting sensor devices, and their applications.
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T. Yamaguchi et al. / Sensors and Actuators A 136 (2007) 244–248
K. Yokosawa was born in Sendai, Japan, in 1962. He received his BS and MS degrees in physics from the Faculty of Science, Hokkaido University in 1984 and 1986, respectively, and the PhD degree in systems and information engineering from the Faculty of Engineering, Hokkaido University in 1998. He joined the Central Research Laboratory, Hitachi Ltd. in 1986. Since then he has been engaged in the research and development of SQUIDs and biomagnetometer sys-
tems, and ultrasonic diagnostic probes. He has been with the Advanced Research Laboratory, Hitachi Ltd., since 2004, where he is the leader of a national project to develop a hydrogen gas detection system. His research interests and activities cover sensors and sensing systems using superconductor, dielectronics, and semiconductor.