a-Si:H Electrolyte-Gate Thin Film Devices for Biological Applications
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a-Si:H Electrolyte-Gate Thin Film Devices for Biological Applications Dina Goncalves1,2, Duarte M.F. Prazeres2,3, Virginia Chu1, and Joao P. Conde1,3 1 INESC-MN, Rua Alves Redol, 9, Lisbon, 1000-029, Portugal 2 Center of Biological and Chemical Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, Lisbon, 1049-001, Portugal 3 Department of Chemical and Biological Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, Lisbon, 1049-001, Portugal
ABSTRACT This paper presents the use of a-Si:H electrolyte-gate thin film transistors (EG-TFTs) for the pH sensing and the detection of DNA and proteins (horseradish peroxidase, HRP). The sensing layer used was SiNx or SiO2. The devices show linear sensitivity to pH above the point of zero charge and respond to the adsorption of oligonuleotides and HRP with typical Langmuir adsorption behavior. DNA immobilization and hybridization detection is demonstrated. Surface control and reproducibility issues are addressed by the measurement of surface contact angles. INTRODUCTION The development of new devices for the sensitive detection of chemical and biological molecules is currently of great interest for a number of applications. Ion-sensitive field-effect transistors (ISFETs) [1] allow the electronic detection of chemical and biological species in aqueous solutions. The physical or chemical adsorption of analytes on the device surface results in a change of the transverse electric field in the active semiconductor layer and causes a threshold voltage shift. ISFETs are currently mainly fabricated in crystalline silicon (c-Si). However, hydrogenated amorphous silicon (a-Si:H) offers some advantages over c-Si, such as its low deposition temperatures and the possibility to use a variety of substrates such as flexible polymers or glass. a-Si:H thin film transistors are commonly used as the switching element in active matrix liquid-crystal displays and imagers [2], but they can also be used as ISFETs as was first demonstrated by Gotoh et al. [3]. The control of the sensitive surface of ISFETs is of great importance. Changes in the processing conditions can lead to a great variability in the performance of the devices. To overcome this problem, the use of controlled processes, suitable cleaning or treatment procedures or the use of organic monolayers must be explored. In this work, the use of a-Si:H-based electrolyte-gate (EG) TFTs for biochemical sensing applications is demonstrated. The devices were fabricated on glass. However, the processing conditions used can be adapted and applied for use on flexible substrates [4, 5]. Sensitivity and surface control issues will be discussed.
EXPERIMENTAL DETAILS Device fabrication Electrolyte- and metal-gate a-Si:H TFTs were fabricated on glass substrates in a top-gate configuration. They have a width-to-length ratio of 5 (W=100 µm, L=20 µm). Source and drain contacts are comprised of 150 nm Al with a contact layer of 50 nm n+-a-Si:H. The active island is formed by 250 nm a-Si:H layer and a 100 nm SiNx dielectric. Both l
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