Study on extended gate field effect transistor with tin oxide sensing membrane

Transcription

1 Materials Chemistry and Physics 63 (2000) Study on extended gate field effect transistor with tin oxide sensing membrane Li-Lun Chi a, Jung-Chuan Chou b,, Wen-Yaw Chung a, Tai-Ping Sun c, Shen-Kan Hsiung a a Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li, Taiwan 320 b Institute of Electronic and Information Engineering, National Yunlin University of Science and Technology, Touliu, Younlin, Taiwan 640 c Department of Management Information System, Chung-Yu Junior College of Business and Administration, Keelung, Taiwan 201 Received 30 April 1999; received in revised form 19 July 1999; accepted 20 July 1999 Abstract Ion-sensitive field effect transistor (ISFET) is a device composed of a conventional ion-sensitive electrode and MOSFET device, applied to the measurement of ion content in a solution. Extended gate field effect transistor (EGFET) is another structure to isolate FET from chemical environment. In this study, the ISFET was separated into two parts. Tin dioxide (SnO 2 ), obtained by sputtering, is used as a ph-sensitive membrane for electrode, which is connected with a commercial MOSFET device in CD4007UB or LF356N. The experimental data show that this structure has a linear ph response of about mv/ph in a concentration range between ph 2 and 12. In addition, it is easier to fabricate and package the sensitive membrane structure and the measurement is simple for the application of disposable biosensor Elsevier Science S.A. All rights reserved. Keywords: Ion-sensitive field effect transistor (ISFET); Extended gate field effect transistor (EGFET); Tin dioxide (SnO 2 ); ph response 1. Introduction The first ion-sensitive field effect transistor (ISFET) was fabricated by Bergveld [1]. The difference between IS- FET and MOSFET is that there is no metal gate electrode in the former. Silicon dioxide (SiO 2 ) was first used as a ph-sensitive membrane for the ISFET. After this, Al 2 O 3, Si 3 N 4,Ta 2 O 5 and SnO 2 were used as ph-sensitive membranes because of the higher ph response [2 4]. An extended gate field effect transistor (EGFET) is another structure to isolate FET from the chemical environment, in which a chemically-sensitive membrane is deposited on the end of the signal line extended from the FET gate electrode [5 8]. This structure has a lot of advantages, such as light insensitivity, simple to passivate and package, flexibility of shape of the extended gate area, etc. In this paper, our laboratory improved upon the EGFET structure. We constructed an EGFET by connecting the sensing structure to a commercial MOSFET. The commercial MOSFET CD4007UB has good characteristics, and we are not concerned about its fabrication processes. In addition, the commercial MOSFET is reusable for bio-application and the cost is cheaper for disposable biosensor. Corresponding author. 2. Experimental 2.1. Fabrication of sensing structure EGFET was separated into two parts. One was the sensing structure containing the sensitive membrane and the other was the MOSFET structure. The fabrication processes of sensing structure were summarized as follows and its cross-section is shown in Fig. 1: 1. Clean the silicon substrate. 2. Al was deposited on silicon by thermal evaporation method. 3. SnO 2 was deposited on Si/Al about 3000 Å by sputtering method. 4. Bounding and packaging by epoxy Measurement processes The HP4145B Semiconductor Parameter Analyzer was used to measure the threshold voltage (V T ) of the ISFET in ph 2, 4, 6, 8, 10, and 12 buffer solutions. The measurement system in this study is shown as Fig. 2. The sensing structure and the reference electrode were dipped into the buffer solution and connected to the gate of MOSFET. Our MOS- FET is a commercial device CD4007UB. In this study, the reference electrode was calomel electrode, and the distance was fixed about 1 cm between the reference electrode and tin oxide electrode /00/$ see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S (99)

2 20 L.-L. Chi et al. / Materials Chemistry and Physics 63 (2000) Fig. 1. Cross-section of sensing structure. Fig. 2. EGFET measurement system. We also investigated the optical characteristics of the EGFET in dark and constant light exposure. In order to consider practical application, a conventional lamp was used as a source of white light. In our experiment, we used a constant light exposure of about 2000 lx and the sensing structure was in ph 7 buffer solution. 3. Results and discussion The operation of the EGFET is very similar to that of a conventional MOSFET, except that an additional sensing structure is dipped in the buffer solution. The transconductance gives the same peak value in a concentration range between ph 2 and 12, shown in Fig. 3. The same I D V Ref slope can be obtained around the maximum transconductance; the threshold voltage, in turn, can be obtained; and ph sensitivity of the EGFET can be obtained through the measurement of threshold voltage shift. The result shows that the EGFET has a linear ph sensitivity of approximately 58 mv/ph, with the ion concentration ranging between ph 2 and 12. Fig. 4(a) shows the I D V DS characteristics of the EGFET in a concentration range between ph 2 and 12. The saturation region current is expressed as I D = µ 0C ox W 2 L (V Ref V T ) 2 (1 + λv DS ) (1) where µ 0 is the electron mobility in the channel, λ is the channel-length modulation factor, C ox is the oxide capacitance per unit area, W/L is the channel width-to-length Fig. 3. I D V Ref characteristics of EGFET.

3 L.-L. Chi et al. / Materials Chemistry and Physics 63 (2000) Fig. 4. I D V DS characteristics of EGFET. (a) I D V DS characteristics, and (b) square root of I D for measurements between ph 2 and 12. ratio, V Ref and V DS are the applied reference electrode and drain-source voltages, respectively. In Eq. (1), V T is dependent on the ph value. The square root of Eq. (1) is shown as follows: µ0 C ox ID = W 2 L (1 + λv DS) (V Ref V T ) (2) Based on Eq. (2), Fig. 4(b) shows that the EGFET has a linear ph response in the saturation region. In this study, the constant voltage constant current circuit was also applied to study ph response. By the procedures discussed above, the drain voltage was set at 0.2 V and drain current was set at 120 ua. Fig. 5 shows that the EGFET still has good linearity. In Fig. 6, the optical characteristics were investigated in the dark and in constant light exposure of about 2000 lx. The data show that the EGFET has an effective decrease in light sensitivity. The reference electrode voltage shifted only Fig. 5. Reference electrode voltage measured under constant voltage constant current and exposed to various ph values.

4 22 L.-L. Chi et al. / Materials Chemistry and Physics 63 (2000) Fig. 6. Optical characteristics of EGFET. about 3 mv in the constant light exposure of about 2000 lx. Our MOSFET is a commercial device that has a good package, and the light cannot penetrate the oxide into the channel of MOSFET. But the strong light will increase the temperature between the sensitive film and electrolyte interface, and result in threshold voltage shift. The influence of light exposure for the conventional MOSFET is negligible, but for open-gate FET-based sensors, such as the traditional ISFET, optical radiation can cause a considerable threshold voltage shift [9,10], and shift the reference electrode voltage under constant voltage constant current. Based on the concept of Van der Spiegel [5], the amplifier LF356N was used as unity gain buffer and the sensing membrane structure was connected to the input pin. The measurement structure and experimental data are shown in Fig. 7(a, b), respectively. Experimental data of Fig. 7(b) show that this measurement setup can measure ion concentration range between ph 2 and 12, as the input stage of the amplifier LF356N is a high-impedance FET device. Comparing with the results reported in [4], the ph sensitivity of the extended gate field effect transistor is mv/ph, which is similar to the conventional SnO 2 -ISFET, whose ph response is about 58 mv/ph. It is easy to fabricate the sensing-membrane structure, and there is no need to fabricate the MOSFET. We can construct an EGFET by connecting the sensing structure to a commercial MOSFET. The EGFET used a commercial MOSFET device that is reusable, so that the measurement is simple for the application of biosensor. The extended gate field effect transistor is more stable because we do not have to dip the MOSFET into buffer solution. 4. Conclusions Fig. 7. The measurement configuration and result of the sensing structure connected to the amplifier LF356N. (a) Measurement configuration, and (b) experiment result of output voltage versus ph value. In summary, from the above results and discussion, the important conclusions that can be obtained are as follows:

5 L.-L. Chi et al. / Materials Chemistry and Physics 63 (2000) The fabrication processes of sensing membrane structure are easier. We can fabricate EGFET by connecting the sensing structure to a commercial MOSFET. 2. The measurement is simple and our MOSFET is reusable for bio-application. 3. The extended gate field effect transistor does not degrade the ph sensitivity. The ph sensitivity is mv/ph. 4. The extended gate field effect transistor has reduced the effect from the ambient light. The reference electrode voltage shifted only about 3 mv when exposed to about 2000 lx light illumination. Acknowledgements This work is supported by National Science Council, the Republic of China, under the contract NSC E References [1] P. Bergveld, IEEE Trans. Biomed. Eng. BME-17 (1970) [2] D.L. Harame, J. Bousse, J.D. Shott, J.D. Meindel, IEEE Trans. Electron Devices ED-34(8) (1987) [3] N. Mengnian, D. Xinfang, T. Qinyi, Chin. J. Semiconduct. 17(7) (1996) [4] H.K. Liao, J.C. Chou, W.Y. Chung, T.P. Sun, S.K. Hsiung, Sensors and Actuators B 50 (1998) [5] J. Van der Spiegel, I. Lauks, P. Chan, D. Babic, Sensors and Actuators B 4 (1983) [6] I. Lauks, J. Van der Spiegel, W. Sansen, M. Steyaert, in: Proc. Int. Conf. Solid-State Sensors and Actuators transducers 85, 1985, pp [7] T. Katsube, T. Araki, M. Hara, T. Yaji, Si Kobayashi, K. Suzuki, in: Proc. 6th Sensor Symp., Tsukuba, Japan, 1986, pp [8] T. Katsube, M. Katoh, H. Maekawa, M. Hara, Sensors and Actuators B 1 (1990) [9] J.V. Voorthuyzen, P. Bergveld, Sensors and Actuators B 1 (1990) [10] Y.A. Tarantov, A.S. Kartashev, Sensors and Actuators A 28 (1991)