Floating-Gate Ion Sensitive Field-Effect Transistor for Chemical and Biological Sensing
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Floating-Gate Ion Sensitive Field-Effect Transistor for Chemical and Biological Sensing Ben Zhao1, Tsunehiro Sai2, Arifur Rahman3, Kalle Levon2 1 Department of Electrical and Computer Engineering, 2 Othmer Department of Chemical and Biological Science and Engineering, Polytechnic University, Brooklyn, NY 3 Xilinx Research Labs, Xilinx, Inc, San Jose, CA Abstract This paper presents the use of Floating-Gate Ion Sensitive Field-Effect Transistor (FGISFET) as a real-time chemical and biological sensor. The structure of FGISFET is similar to that of an electrically erasable programmable read-only memory (EEPROM). The floating-gate of a FGISFET is connected to an exposed metallic structure, which serves as a probe for detecting ionic activities. By applying ion-sensitive chemical and biological materials to the floating gate, its threshold voltage can be modulated in the presence of selective chemical or biological targets. As a demonstration, FGISFETs have been fabricated in 1.2 µm process technology available through MOSIS [1]. Our preliminary measurements confirmed the basic design and operation of FGISFET, and using doped aniline trimer (TANI) as a sensing material, we were able to sense 70ppm of ammonium gas. 1. Introduction Recently, we demonstrated surface imprinting of Indium Tin Oxide (ITO), by creating cavities on ion sensitive electrode for the detection of nerve gas [2] and biological agents [3]. This type of chemosensor provided the selectivity for target detection but lacked the speed and control of real-time chemical and biological sensor. Seeking alternative solutions for real-time detection led us to the investigation of Ion Sensitive Field Effect Transistor (ISFET). The first proposed work on ISFET was presented in 1968 by Bergveld [4] using FET-based sensors to measure ionic activities. The proposed design of ISFET affected the difference in work function of gate and channel materials and thus altered the conductivity of the channel between the drain and source. Other works were performed where the gate was formed by a material which was sensitive to selective gases or analytes, and the electric field within the gate oxide was determined by the electrochemical properties of the combined (gate and gas or analyte) system. This mechanism was exploited to design various types of silicon based ISFETs [5-6]. For example, in one design, sensing material was coated on top of the gate electrode to modulate the electric field to implement chemically sensitive FETs (CHEMFETs) [7]. Another design replaced the channel material with highly conductive organic material to create potentiometric sensors or insulated gate FETs (IGFETs) [8]. However, these designs lacked robustness to function in all environments, and they were difficult to combine monolithically with integrated circuits. To address some of the limitations of conventional ISFETs, we propose a new type of ISFET, floating-gate ISFET (FGISFET) that exploits the advances in state-of-the-art semiconductor process technologies. FGISFET has the potential
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