Amorphous Silicon Thin Film Transistor Biosensing System
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Amorphous Silicon Thin Film Transistor Biosensing System Hanbin Ma1, Ben Miller1, Sungsik Lee2, Arman Ahnood1, Marius Bauza2, William I. Milne1 and Arokia Nathan1 1 Electrical Division, Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, U.K. 2 London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH, U.K. ABSTRACT Electronic systems are a very good platform for sensing biological signals for fast pointof-care diagnostics or threat detection. One of the solutions is the lab-on-a-chip integrated circuit (IC), which is low cost and high reliability, offering the possibility for label-free detection. In recent years, similar integrated biosensors based on the conventional complementary metal oxide semiconductor (CMOS) technology have been reported. However, post-fabrication processes are essential for all classes of CMOS biochips, requiring biocompatible electrode deposition and circuit encapsulation. In this work, we present an amorphous silicon (a-Si) thin film transistor (TFT) array based sensing approach, which greatly simplifies the fabrication procedures and even decreases the cost of the biosensor. The device contains several identical sensor pixels with amplifiers to boost the sensitivity. Ring oscillator and logic circuits are also integrated to achieve different measurement methodologies, including electro-analytical methods such as amperometric and cyclic voltammetric modes. The system also supports different operational modes. For example, depending on the required detection arrangement, a sample droplet could be placed on the sensing pads or the device could be immersed into the sample solution for real time in-situ measurement. The entire system is designed and fabricated using a low temperature TFT process that is compatible to plastic substrates. No additional processing is required prior to biological measurement. A Cr/Au double layer is used for the biological-electronic interface. The success of the TFT-based system used in this work will open new avenues for flexible label-free or lowcost disposable biosensors. INTRODUCTION Traditional biological detection techniques often use dyes [1] and optical read out [2], which can be expensive and time consuming. Electronic systems for biological sensing constitute cheap, fast and sensitive alternatives [3]. Electrochemical techniques measure the electrical properties of a solution, so provide a good label-free means of characterizing biological materials. CMOS based devices using electrochemical techniques have shown suitability for biological sensing, such as cyclic voltammetry for detecting DNA probe surface density [4], and DNA sequencing using an ion-sensitive FET based sensor [5]. However, there are post fabrication processing problems associated with CMOS devices [6], as well as biocompatible electrode deposition, and the need for circuit encapsulation. These issues could be solved using thin-film transistor based sensors. They are very low cost devices, with low temperatur
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