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Study of indium tin oxide thin film for separative extended gate ISFET
Materials Chemistry and Physics 70 (2001) 12–16
Study of indium tin oxide thin film for separative extended gate ISFET Li-Te Yin a , Jung-Chuan Chou b,∗ , Wen-Yaw Chung c , Tai-Ping Sun d , Shen-Kan Hsiung c a
b
Department of Bio-Medical Engineering, Chung Yuan Christian University, Chung-Li 320, Taiwan, ROC Institute of Electronic and Information Engineering, National Yunlin University of Science and Technology, Touliu 640, Taiwan, ROC c Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li 320, Taiwan, ROC d Department of Electrical Engineering, National Chi Nan University, Nantou 545, Taiwan, ROC Received 2 March 2000; received in revised form 13 April 2000; accepted 30 May 2000
Abstract In this study, the indium tin oxide (ITO) was used as a sensitive film for H+ ion sensitive field effect transistor (ISFET). The sensitive characteristics of ITO glass structure for separative extended gate ion sensitive field effect transistors (EGFET) were studied. ITO thin film is used for the first time as a H+ ion sensitive film which has a linear pH sensitivity of Nerstern response, about 58 mV/pH, between pH 2 and pH 12. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Indium tin oxide; ISFET; EGFET; pH sensitivity
1. Introduction The ion sensitive field effect transistor (ISFET) is an integrated device composed of a conventional ion selective electrode and a metal oxide semiconductor field effect transistor (MOSFET). The device is similar to the conventional MOSFET except that the metal gate electrode is removed in order to expose the underlying insulator layer to the solution. The gate insulator plays the role of an ion selective electrode [1]. Silicon dioxide (SiO2 ) was first used as a pH-sensitive dielectric for the ISFET [2]. Subsequently, Al2 O2 , Si3 N4 , Ta2 O5 , and SnO2 were used as pH-sensitive dielectrics because of the higher pH response [3–8]. However, an ISFET is a kind of transistor, which works in saline water. Thus, a problem arises from the poor isolation between the device and solution. It is thus very important to develop an ISFET encapsulation process that is compatible with integrated circuit technology. Until now, several fabrication methods for ISFET-based biosensors have been reported. Esashi and Matsuo [9] applied the anisotropic etching technique to make a needle-like ISFET device, which was completely isolated from the water. However, the anisotropic etching is too complex to use in
∗ Corresponding author. Tel.: +886-55342601, ext. 2500; fax: +886-55312029. E-mail address: [email protected] (J.-C. Chou).
this process. Another most frequently used method is based on a silicon-on-sapphire (SOS) structure [10–13]. However, SOS ISFETs have disadvantages, such as an instability of characteristics because of penetration of impurities (for example, Al) from the sapphire substrate, low current sensitivity and high cost of a sapphire wafer. In addition, Poghossian [14,15] solved the ISFET encapsulation problem by Si–SiO2 –Si (SIS) structure. But the silicon bonding process of SIS structure ISFETs is still too complex. An extended gate field effect transistor (EGFET) is another structure to get isolation of FET from the chemical environment, in which a chemically sensitive membrane is deposited on the end of signal line extended from the FET gate electrode [16,17]. This structure has a lot of advantages, such as light insensitivity, simple to passivate and package, the flexibility of shape of the extended gate area, etc. Recently, Chi et al. [18] introduced an improved structure of EGFET which is separated into two parts. One is sensing part with the structure of SnO2 /Al/Si and another is commercial MOSFET, CD4007UB. This structure is suitable for application for a disposable biosensor, because the separative MOSFET is reusable when the sensing part has to interchange. However, the charge could be leakaged from the silicon layer to the conductor layer, Al, in the sensing part, so the critical encapsulation still need for this structure. In addition the SnO2 sensitive film, we have applied Si3 N4 , Al2 O3 and SiO2 to the separative structure, respectively, but all this kind of insulating materials cannot show
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 7 3 - 4
L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
a pH response. By the way, the insulating material may not be a proper sensitive material for separative EGFET. So we were looking for pH-sensitive materials which have lower resistivity. Indium tin oxide (ITO) is popular for their unique electrical and optical properties, i.e. high electrical conductivity and high optical transmittance in the visible region [19]. These qualities have caused them to be widely used as transparent conducting elements for addressing various kinds of alphanumeric displays [20]. In this study, the ITO glass electrode was first used as a pH-sensitivity material for separative EGFET. In this study, we have improved the separative EGFET structure in which the sensing part is ITO glass.
2. Experimental
13
Fig. 2. Measurement circuit of instrumentation with amplifiers AD620.
drift and hysteresis characteristics of the extended gate of the separative EGFET.
2.1. Materials The ITO glasses (the sheet resistance 50–100 /䊐, ITO coating thickness: 230 Å) were granted from Wintek corporation. 2.2. Measurement processes The HP4145B Semiconductor Parameter Analyzer was used to measure the threshold voltage (VT ) of the EGFET in the pH buffer solutions. The measurement system in this study is shown in Fig. 1. The sensing structure and calomel reference electrode were dipped into buffer solution and connected with the gate of commercial MOSFET device, CD4007UB. The VDS is fixed at 0.2 V, where VDS is the voltage between drain and source of MOSFET. The distance between the reference electrode and sensing electrode is about 0.5∼2 cm which will not affect the results during the measurement. A readout circuit based on an instrumentation amplifier AD620, which is shown in Fig. 2, was used to study the
Fig. 1. EGFET measurement system.
3. Results and discussion The ITO thin film is a well-known material used as a conductor of high optical transmittance, but a novel material used as a pH-ISFET sensitive film. In this paper the fundamental characteristic of ITO glass electrode that applied as a separative EGFET sensing gate was studied. 3.1. pH sensitivity of ITO gate ISFET The transconductance gives the same peak value in a concentration range between pH 2 and pH 12 as shown in Fig. 3 under the same temperature (25◦ C). The slop of the same ID versus Vref can be obtained around the maximum transconductance. This slop has a concept with Vref that is the threshold voltage of ISFET. The pH sensitivity of the ISFET was investigated through a shift in the threshold voltage of an ISFET sensor. The result shows that the ITO gate separative
Fig. 3. I–V characteristics of ITO sensitive film EGFET which has a contact window of 10 × 10 mm2 .
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L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
Fig. 4. Connection diagram of AD620.
EGFET sensor has a linear pH sensitivity of approximately 58 mV/pH in a concentration range between pH 2 and pH 12. In this study, the instrumentation amplifier AD620 was used as a readout circuit which is shown in Fig. 2. The connection diagram of AD620 is shown in Fig. 4. ITO was connected to one of the input terminal that is terminal 2 or 3 and the other was connected to ground, the output of which is shown in Fig. 4. Fig. 5(a) and (b) show the results of the ITO which were connected to terminal 2 and 3 of AD520, respectively, and both the results show the pH-sensitivity about 58 mV pH−1 . In more base solution which means that more negative ions (OH− ) will be accumulated on ITO sensitive film. So in Fig. 5(a) which connect to negative input terminal of AD620 shows a positive slop of pH sensitivity, and Fig. 5(b) shows a negative slop. 3.2. Hysteresis of ITO sensing gate EGFET
Fig. 5. Output voltage versus pH value of the ITO sensing gate connected with instrumentation amplifier AD620: (a) ITO connected to terminal 2 and (b) ITO connected to terminal 3.
In the study of hysteresis of the ITO sensing gate EGFET sensor, the ITO was connected to terminal 2 of instrumentation amplifier AD620. To evaluate the hysteresis of the EGFET, we measured the output offset voltage after solution change such as pH 7 → pH 4 → pH 7 and pH 7 → pH 10 → pH 7. The results show that the hysteresis of the ITO sensing gate EGFET is about 9.8 mV as shown in Fig. 6. Table 1 shows the sensitivity and hysteresis comparison with SiO2 , SnO2 and a-WO3 thin films ISFET sensors [1,7,21]. As can be seen from the table, ITO sensing gate EGFET has a better pH-sensitivity characteristic than other thin films and normal hysteresis characteristic in comparison with others. 3.3. The contact area effect of ITO sensing gate EGFET In this study, we have determined that the structure of ITO gate EGFET has a pH sensitivity of about 55 mV pH−1 as the contact window between the sensitive layer and buffer solution is beyond 6 mm2 . However the pH sensitivity is
Fig. 6. Hysteresis characteristic of ITO sensing gate EGFET.
L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
15
Table 1 pH sensitivity and hysteresis characteristics of various ISFETs
(mV pH−1 )
pH-Sensitivity Hysteresis (mV)
SiO2 [1]
SnO2 [7]
a-WO3 [21]
ITO
25∼35, pH>7; 37∼48, pH<7 Unstable
55∼58 2.5a
50∼58, pH<7 12.5b
54∼58 9.8a
Measured the output offset voltage after solution exchanges of pH pH 7 → pH 4 → pH 7 and pH 7 → pH 10 → pH 7. Measured the output offset voltage after solution exchanges of pH 4 → pH 7 → pH 4 and pH 4 → pH 1 → pH 4 (a-WO3 can work only in acid environment). a
b
References
Fig. 7. Relation between contact area and pH-sensitivity of ITO glass structure.
decreased as the contact window is smaller than 6 mm2 , the result of which is shown in Fig. 7. This phenomenal may be caused by that the voltage sharing effect sets off a reducing of MOSFET output signal, the pH sensitivity of the small contact window on the ITO gate EGFET becomes lower.
4. Conclusions ITO can be used as a pH-sensitive film of EGFET, and it has a linear sensitivity about 58 mV pH−1 in a pH concentration ranging from pH 2 to pH 12. The instrumentation amplifier AD620 can be used as a readout circuit for separative gate EGFET. The ITO gate EGFET has the hysteresis characteristic of 9.8 mV after solution change such as pH pH 7 → pH 4 → pH 7 and pH 7 → pH 10 → pH 7. The pH sensitivity of ITO sensing gate EGFET is decreased as the contact window is smaller than 6 mm2 .
Acknowledgements This work was supported by National Science Council, the Republic of China under the contracts NSC88-2215-E033006.
[1] T. Matsuo, M. Esashi, Methods of ISFET fabrication, Sensors and Actuators 1 (1981) 77–96. [2] P. Bergveld, Development of an ion sensitive solid-state device for neurophysiological measurements, IEEE Trans. Biomed. Eng. 17 (1970) 70–71. [3] S.D. Moss, C.C. Johnson, Jirijanata, Hydrogen, calcium and potassium ion sensitive FET transducers: A preliminary report, IEEE Trans. Biomed. Eng. 25 (1978) 49–54. [4] Hung-Kwei Liao, Jung-Chaun Chou, Wen-Yaw Chung, Tai-Ping, Shen-Ken Hsiung, Study on the interface trap density of the Si3 N4 /SiO2 gate ISFET, in: Proceedings of the Third East Asian Conference on Chemical Sensors, Seoul, South Korea, November 1997, pp. 394–400. [5] Li-Te Yin, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping, Shen-Ken Hsiung, Characteristics of silicon nitride after O2 plasma treatment for pH ISFET applications, in: Proceeding of 1998 International Electron Devices and Materials Symposia, B, C, National Cheng Kung University, Tainan, Taiwan, ROC, December 1998, pp. 267–270. [6] P. Gimmel, B. Gompf, D. Schmeiosser, H.D. Weimhofer, W. Gopel, M. Klein, Ta2 O5 gates of pH sensitive device comparative spectroscopic and electrical studies, Sensors and Actuators B 17 (1989) 195–202. [7] Hung-Kwei Liao, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, Study of amorphous tin oxide thin films for ISFET applications, Sensors and Actuators B 50 (1998) 104–109. [8] Hung-Kwei Liao, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, Study of pHpzc and surface potential of tin oxide gate ISFET, Mater. Chem. Phys. 2428 (1999) 1–6. [9] M. Esashi, T. Matsuo, Integrated micro multi ion sensor using field effect of semiconductor, IEEE Trans. Biomed. Eng. BME-25 (1978) 182–192. [10] B.H. van der Schoot, P. Bergveld, ISFET-based enzyme sensors, Biosensors 3 (1987) 161–186. [11] S. Nakamoto, N. Ito, T. Kuriyama, J. Kimura, A lift-off method for patterning enzyme-immobilized membranes in multi-biosensors, Sensors and Actuators 13 (1988) 165–172. [12] J. Kimara, T. Munakami, T. Kuriyama, An integrated multi-biosensor for simultaneous amperometric and potentiometric measurement, Sensors and Actuators 15 (1989) 435–443. [13] Y. Hanazato, M. Nakako, S. Shinon, M. Maeda, Integrated multi-biosensors based on an ion-sensitive field-effect transistor using photolithographic technique, IEEE Trans. Elect. Dev. 36 (1989) 1303–1310. [14] A.S. Poghossian, Method of fabrication of ISFETs and CHEMFETs on an Si–SiO2 –Si structure, Sensors and Actuators B 13/14 (1993) 653–654. [15] A.S. Poghossian, Method of fabrication of ISFET-based biosensors on an Si–SiO2 –Si structure, Sensors and Actuators B 44 (1997) 361–364. [16] J. Van der Spiegel, I. Lauks, P. Chan, D. Babic, The extended gate chemical sensitive field effect transistor as multi-species microprobe, Sensors and Actuators B 4 (1983) 291–298.
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L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
[17] T. Katsube, T. Araki, M. Hara, T. Yaji, Si Kobayashi, K. Suzuki, A multi-species biosensor with extended-gate field effect transistors, in: Proceedings of the Sixth Sensor Symposium, Tsukuba, Japan, 1986, pp. 211–214. [18] Li-Lun Chi, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, New structure of ion sensitive field effect transistor, in: Proceedings of the Biomedical Engineering Society 1998 Annual Symposium, Taiwan, 1998, pp. 332– 334.
[19] M.A. Martinez, J. Herrero, M.T. Gutierrez, Electrochemical stability of indium tin oxide thin films, Electrochim. Acta 37 (1992) 2565– 2571. [20] K.L. Chopra, S. Major, D.K. Pandya, Transparent conductors — a status review, Thin Solid Films 102 (1983) 1–46. [21] J.L. Chiang, J.C. Chou, Y.C. Chen, Study on the hysteresis effect of a-WO3 gate ISFET, in: Proceedings of the Fourth East Asian Conference on Chemical Sensors, Hsinchu, Taiwan, November, 1999, pp. 424–427.
Study of indium tin oxide thin film for separative extended gate ISFET Li-Te Yin a , Jung-Chuan Chou b,∗ , Wen-Yaw Chung c , Tai-Ping Sun d , Shen-Kan Hsiung c a
b
Department of Bio-Medical Engineering, Chung Yuan Christian University, Chung-Li 320, Taiwan, ROC Institute of Electronic and Information Engineering, National Yunlin University of Science and Technology, Touliu 640, Taiwan, ROC c Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li 320, Taiwan, ROC d Department of Electrical Engineering, National Chi Nan University, Nantou 545, Taiwan, ROC Received 2 March 2000; received in revised form 13 April 2000; accepted 30 May 2000
Abstract In this study, the indium tin oxide (ITO) was used as a sensitive film for H+ ion sensitive field effect transistor (ISFET). The sensitive characteristics of ITO glass structure for separative extended gate ion sensitive field effect transistors (EGFET) were studied. ITO thin film is used for the first time as a H+ ion sensitive film which has a linear pH sensitivity of Nerstern response, about 58 mV/pH, between pH 2 and pH 12. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Indium tin oxide; ISFET; EGFET; pH sensitivity
1. Introduction The ion sensitive field effect transistor (ISFET) is an integrated device composed of a conventional ion selective electrode and a metal oxide semiconductor field effect transistor (MOSFET). The device is similar to the conventional MOSFET except that the metal gate electrode is removed in order to expose the underlying insulator layer to the solution. The gate insulator plays the role of an ion selective electrode [1]. Silicon dioxide (SiO2 ) was first used as a pH-sensitive dielectric for the ISFET [2]. Subsequently, Al2 O2 , Si3 N4 , Ta2 O5 , and SnO2 were used as pH-sensitive dielectrics because of the higher pH response [3–8]. However, an ISFET is a kind of transistor, which works in saline water. Thus, a problem arises from the poor isolation between the device and solution. It is thus very important to develop an ISFET encapsulation process that is compatible with integrated circuit technology. Until now, several fabrication methods for ISFET-based biosensors have been reported. Esashi and Matsuo [9] applied the anisotropic etching technique to make a needle-like ISFET device, which was completely isolated from the water. However, the anisotropic etching is too complex to use in
∗ Corresponding author. Tel.: +886-55342601, ext. 2500; fax: +886-55312029. E-mail address: [email protected] (J.-C. Chou).
this process. Another most frequently used method is based on a silicon-on-sapphire (SOS) structure [10–13]. However, SOS ISFETs have disadvantages, such as an instability of characteristics because of penetration of impurities (for example, Al) from the sapphire substrate, low current sensitivity and high cost of a sapphire wafer. In addition, Poghossian [14,15] solved the ISFET encapsulation problem by Si–SiO2 –Si (SIS) structure. But the silicon bonding process of SIS structure ISFETs is still too complex. An extended gate field effect transistor (EGFET) is another structure to get isolation of FET from the chemical environment, in which a chemically sensitive membrane is deposited on the end of signal line extended from the FET gate electrode [16,17]. This structure has a lot of advantages, such as light insensitivity, simple to passivate and package, the flexibility of shape of the extended gate area, etc. Recently, Chi et al. [18] introduced an improved structure of EGFET which is separated into two parts. One is sensing part with the structure of SnO2 /Al/Si and another is commercial MOSFET, CD4007UB. This structure is suitable for application for a disposable biosensor, because the separative MOSFET is reusable when the sensing part has to interchange. However, the charge could be leakaged from the silicon layer to the conductor layer, Al, in the sensing part, so the critical encapsulation still need for this structure. In addition the SnO2 sensitive film, we have applied Si3 N4 , Al2 O3 and SiO2 to the separative structure, respectively, but all this kind of insulating materials cannot show
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 7 3 - 4
L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
a pH response. By the way, the insulating material may not be a proper sensitive material for separative EGFET. So we were looking for pH-sensitive materials which have lower resistivity. Indium tin oxide (ITO) is popular for their unique electrical and optical properties, i.e. high electrical conductivity and high optical transmittance in the visible region [19]. These qualities have caused them to be widely used as transparent conducting elements for addressing various kinds of alphanumeric displays [20]. In this study, the ITO glass electrode was first used as a pH-sensitivity material for separative EGFET. In this study, we have improved the separative EGFET structure in which the sensing part is ITO glass.
2. Experimental
13
Fig. 2. Measurement circuit of instrumentation with amplifiers AD620.
drift and hysteresis characteristics of the extended gate of the separative EGFET.
2.1. Materials The ITO glasses (the sheet resistance 50–100 /䊐, ITO coating thickness: 230 Å) were granted from Wintek corporation. 2.2. Measurement processes The HP4145B Semiconductor Parameter Analyzer was used to measure the threshold voltage (VT ) of the EGFET in the pH buffer solutions. The measurement system in this study is shown in Fig. 1. The sensing structure and calomel reference electrode were dipped into buffer solution and connected with the gate of commercial MOSFET device, CD4007UB. The VDS is fixed at 0.2 V, where VDS is the voltage between drain and source of MOSFET. The distance between the reference electrode and sensing electrode is about 0.5∼2 cm which will not affect the results during the measurement. A readout circuit based on an instrumentation amplifier AD620, which is shown in Fig. 2, was used to study the
Fig. 1. EGFET measurement system.
3. Results and discussion The ITO thin film is a well-known material used as a conductor of high optical transmittance, but a novel material used as a pH-ISFET sensitive film. In this paper the fundamental characteristic of ITO glass electrode that applied as a separative EGFET sensing gate was studied. 3.1. pH sensitivity of ITO gate ISFET The transconductance gives the same peak value in a concentration range between pH 2 and pH 12 as shown in Fig. 3 under the same temperature (25◦ C). The slop of the same ID versus Vref can be obtained around the maximum transconductance. This slop has a concept with Vref that is the threshold voltage of ISFET. The pH sensitivity of the ISFET was investigated through a shift in the threshold voltage of an ISFET sensor. The result shows that the ITO gate separative
Fig. 3. I–V characteristics of ITO sensitive film EGFET which has a contact window of 10 × 10 mm2 .
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L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
Fig. 4. Connection diagram of AD620.
EGFET sensor has a linear pH sensitivity of approximately 58 mV/pH in a concentration range between pH 2 and pH 12. In this study, the instrumentation amplifier AD620 was used as a readout circuit which is shown in Fig. 2. The connection diagram of AD620 is shown in Fig. 4. ITO was connected to one of the input terminal that is terminal 2 or 3 and the other was connected to ground, the output of which is shown in Fig. 4. Fig. 5(a) and (b) show the results of the ITO which were connected to terminal 2 and 3 of AD520, respectively, and both the results show the pH-sensitivity about 58 mV pH−1 . In more base solution which means that more negative ions (OH− ) will be accumulated on ITO sensitive film. So in Fig. 5(a) which connect to negative input terminal of AD620 shows a positive slop of pH sensitivity, and Fig. 5(b) shows a negative slop. 3.2. Hysteresis of ITO sensing gate EGFET
Fig. 5. Output voltage versus pH value of the ITO sensing gate connected with instrumentation amplifier AD620: (a) ITO connected to terminal 2 and (b) ITO connected to terminal 3.
In the study of hysteresis of the ITO sensing gate EGFET sensor, the ITO was connected to terminal 2 of instrumentation amplifier AD620. To evaluate the hysteresis of the EGFET, we measured the output offset voltage after solution change such as pH 7 → pH 4 → pH 7 and pH 7 → pH 10 → pH 7. The results show that the hysteresis of the ITO sensing gate EGFET is about 9.8 mV as shown in Fig. 6. Table 1 shows the sensitivity and hysteresis comparison with SiO2 , SnO2 and a-WO3 thin films ISFET sensors [1,7,21]. As can be seen from the table, ITO sensing gate EGFET has a better pH-sensitivity characteristic than other thin films and normal hysteresis characteristic in comparison with others. 3.3. The contact area effect of ITO sensing gate EGFET In this study, we have determined that the structure of ITO gate EGFET has a pH sensitivity of about 55 mV pH−1 as the contact window between the sensitive layer and buffer solution is beyond 6 mm2 . However the pH sensitivity is
Fig. 6. Hysteresis characteristic of ITO sensing gate EGFET.
L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
15
Table 1 pH sensitivity and hysteresis characteristics of various ISFETs
(mV pH−1 )
pH-Sensitivity Hysteresis (mV)
SiO2 [1]
SnO2 [7]
a-WO3 [21]
ITO
25∼35, pH>7; 37∼48, pH<7 Unstable
55∼58 2.5a
50∼58, pH<7 12.5b
54∼58 9.8a
Measured the output offset voltage after solution exchanges of pH pH 7 → pH 4 → pH 7 and pH 7 → pH 10 → pH 7. Measured the output offset voltage after solution exchanges of pH 4 → pH 7 → pH 4 and pH 4 → pH 1 → pH 4 (a-WO3 can work only in acid environment). a
b
References
Fig. 7. Relation between contact area and pH-sensitivity of ITO glass structure.
decreased as the contact window is smaller than 6 mm2 , the result of which is shown in Fig. 7. This phenomenal may be caused by that the voltage sharing effect sets off a reducing of MOSFET output signal, the pH sensitivity of the small contact window on the ITO gate EGFET becomes lower.
4. Conclusions ITO can be used as a pH-sensitive film of EGFET, and it has a linear sensitivity about 58 mV pH−1 in a pH concentration ranging from pH 2 to pH 12. The instrumentation amplifier AD620 can be used as a readout circuit for separative gate EGFET. The ITO gate EGFET has the hysteresis characteristic of 9.8 mV after solution change such as pH pH 7 → pH 4 → pH 7 and pH 7 → pH 10 → pH 7. The pH sensitivity of ITO sensing gate EGFET is decreased as the contact window is smaller than 6 mm2 .
Acknowledgements This work was supported by National Science Council, the Republic of China under the contracts NSC88-2215-E033006.
[1] T. Matsuo, M. Esashi, Methods of ISFET fabrication, Sensors and Actuators 1 (1981) 77–96. [2] P. Bergveld, Development of an ion sensitive solid-state device for neurophysiological measurements, IEEE Trans. Biomed. Eng. 17 (1970) 70–71. [3] S.D. Moss, C.C. Johnson, Jirijanata, Hydrogen, calcium and potassium ion sensitive FET transducers: A preliminary report, IEEE Trans. Biomed. Eng. 25 (1978) 49–54. [4] Hung-Kwei Liao, Jung-Chaun Chou, Wen-Yaw Chung, Tai-Ping, Shen-Ken Hsiung, Study on the interface trap density of the Si3 N4 /SiO2 gate ISFET, in: Proceedings of the Third East Asian Conference on Chemical Sensors, Seoul, South Korea, November 1997, pp. 394–400. [5] Li-Te Yin, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping, Shen-Ken Hsiung, Characteristics of silicon nitride after O2 plasma treatment for pH ISFET applications, in: Proceeding of 1998 International Electron Devices and Materials Symposia, B, C, National Cheng Kung University, Tainan, Taiwan, ROC, December 1998, pp. 267–270. [6] P. Gimmel, B. Gompf, D. Schmeiosser, H.D. Weimhofer, W. Gopel, M. Klein, Ta2 O5 gates of pH sensitive device comparative spectroscopic and electrical studies, Sensors and Actuators B 17 (1989) 195–202. [7] Hung-Kwei Liao, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, Study of amorphous tin oxide thin films for ISFET applications, Sensors and Actuators B 50 (1998) 104–109. [8] Hung-Kwei Liao, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, Study of pHpzc and surface potential of tin oxide gate ISFET, Mater. Chem. Phys. 2428 (1999) 1–6. [9] M. Esashi, T. Matsuo, Integrated micro multi ion sensor using field effect of semiconductor, IEEE Trans. Biomed. Eng. BME-25 (1978) 182–192. [10] B.H. van der Schoot, P. Bergveld, ISFET-based enzyme sensors, Biosensors 3 (1987) 161–186. [11] S. Nakamoto, N. Ito, T. Kuriyama, J. Kimura, A lift-off method for patterning enzyme-immobilized membranes in multi-biosensors, Sensors and Actuators 13 (1988) 165–172. [12] J. Kimara, T. Munakami, T. Kuriyama, An integrated multi-biosensor for simultaneous amperometric and potentiometric measurement, Sensors and Actuators 15 (1989) 435–443. [13] Y. Hanazato, M. Nakako, S. Shinon, M. Maeda, Integrated multi-biosensors based on an ion-sensitive field-effect transistor using photolithographic technique, IEEE Trans. Elect. Dev. 36 (1989) 1303–1310. [14] A.S. Poghossian, Method of fabrication of ISFETs and CHEMFETs on an Si–SiO2 –Si structure, Sensors and Actuators B 13/14 (1993) 653–654. [15] A.S. Poghossian, Method of fabrication of ISFET-based biosensors on an Si–SiO2 –Si structure, Sensors and Actuators B 44 (1997) 361–364. [16] J. Van der Spiegel, I. Lauks, P. Chan, D. Babic, The extended gate chemical sensitive field effect transistor as multi-species microprobe, Sensors and Actuators B 4 (1983) 291–298.
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L.-T. Yin et al. / Materials Chemistry and Physics 70 (2001) 12–16
[17] T. Katsube, T. Araki, M. Hara, T. Yaji, Si Kobayashi, K. Suzuki, A multi-species biosensor with extended-gate field effect transistors, in: Proceedings of the Sixth Sensor Symposium, Tsukuba, Japan, 1986, pp. 211–214. [18] Li-Lun Chi, Jung-Chuan Chou, Wen-Yaw Chung, Tai-Ping Sun, Shen-Kan Hsiung, New structure of ion sensitive field effect transistor, in: Proceedings of the Biomedical Engineering Society 1998 Annual Symposium, Taiwan, 1998, pp. 332– 334.
[19] M.A. Martinez, J. Herrero, M.T. Gutierrez, Electrochemical stability of indium tin oxide thin films, Electrochim. Acta 37 (1992) 2565– 2571. [20] K.L. Chopra, S. Major, D.K. Pandya, Transparent conductors — a status review, Thin Solid Films 102 (1983) 1–46. [21] J.L. Chiang, J.C. Chou, Y.C. Chen, Study on the hysteresis effect of a-WO3 gate ISFET, in: Proceedings of the Fourth East Asian Conference on Chemical Sensors, Hsinchu, Taiwan, November, 1999, pp. 424–427.