Separate structure extended gate H  -ion sensitive ®eld effect transistor on a glass substrate Li-Te Yin a , Jung-Chuan Chou b,* , Wen-Yaw Chung c , Tai-Ping Sun d , Shen-Kan Hsiung c a Department of Bio-Medical Engineering, Chung Yuan Christian University, 320 Chung-Li, Taiwan, ROC b Institute of Electronic and Information Engineering, National Yunlin University of Science and Technology, 640 Toulin, Taiwan, ROC c Department of Electronic Engineering, Chung Yuan Christian University, 320 Chung-Li, Taiwan, ROC d Department of Electrical Engineering, National Chi Nan University, 545 Nantou, Taiwan, ROC Received 7 February 2000; received in revised form 16 June 2000; accepted 30 June 2000 Abstract In our research, glass was used as a substrate for an H  ion sensitive ®eld effect transistor (ISFET). The sensitive characteristics of ®ve structures for separate extended gate ion sensitive ®eld effect transistors (EGFET) were studied. The components included tin oxide (SnO 2 )/ aluminum/micro slide glass, tin oxide/aluminum/corning glass, indium tin oxide (ITO) glass, tin oxide/indium tin oxide glass and tin oxide/ micro slide glass. Indium tin oxide (ITO) thin ®lm was ®rst time used as an H  ion sensitive ®lm, which has a linear pH Nerstern response sensitivity, about 58 mV/pH, between pH 2 and 12. In addition, the sensing area effect of the tin oxide/glass, tin oxide/ITO glass and ITO glass structure is discussed. The results show that the tin oxide/ITO glass structure EGFET has the best drift, hysteresis and sensing area characteristics. # 2000 Elsevier Science B.V. All rights reserved. Keywords: pH-ISFET; EGFET; Indium tin oxide (ITO) glass; Tin oxide (SnO 2 ); Contact area effect 1. Introduction Ion sensitive ®eld-effect transistors (ISFETs) have been developed on the basis of the MOSFET (metal oxide ®eld effect transistor). Since Bergveld [1] ®rst employed the ®eld-effect transistor in neurophysiological measurements in 1970, ISFETs have been developed into a new type of chemical-sensing electrode. Many theoretical and experi- mental studies have been published for describing the behavior of this chemical-sensing electronic device [2]. Silicon dioxide (SiO 2 ) was ®rst used as a pH-sensitive dielectric for the ISFET [1]. Subsequently, Al 2 O 3 , Si 3 N 4 , Ta 2 O 5 , and SnO 2 have been used as the pH-sensitive dielec- tric because of their higher pH responses [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 inte- grated circuit technology. Until now, several fabrication methods for ISFET-based biosensors have been reported. Esashi and Matsuo employed the anisotropic etching tech- nique to make a needle-like ISFET device, which was completely isolated from the water [9]. However, the aniso- tropic etching is too complex to use in this process. Another method most frequently used is based on a silicon-on- sapphire (SOS) structure [10±13]. However, SOS ISFETs have some disadvantages, such as characteristic instability because of the penetration of impu- rities (for example, Al) from the sapphire substrate, low current sensitivity and the high cost of a sapphire wafer. In addition, Poghossian solved the ISFET encapsulation pro- blem using a Si-SiO 2 -Si (SIS) structure [14,15]. But, the silicon bonding process of the SIS structure ISFETs is still too complex. An extended gate ®eld effect transistor (EGFET) is another structure to produce FET isolation from the chemical environment, in which a chemically sensitive membrane is deposited on the end of a 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, and the shape ¯exibility in the extended gate area, etc. Recently, Chi et al. have introduced an improved EGFET structure which is separated into two parts. One is a sensing part with a structure of SnO 2 /Al/Si and the other is a commercial MOSFET, CD4007UB [18]. This structure is suitable for application to a disposable biosensor, because the separate MOSFET is reusable when the sensing part is Sensors and Actuators B 71 (2000) 106±111 * Corresponding author. Tel.: 886-55342601 ext. 2500; fax: 886-55312029. E-mail address: choujc@pine.yuntech.edu.tw (J.-C. Chou). 0925-4005/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0925-4005(00)00613-4