Electrolyte-Gate a-Si:H Thin Film Transistors
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A17.4.1
Electrolyte-Gate a-Si:H Thin Film Transistors Dina I. Gonçalves1,2, Duarte M. Prazeres2,3, Virginia Chu1, João P. Conde1,3 1 INESC Microsistemas e Nanotecnologias (INESC-MN), Lisbon, Portugal 2 Center of Biological and Chemical Engineering, Instituto Superior Técnico, Lisbon, Portugal 3 Department of Chemical Engineering, Instituto Superior Técnico, Lisbon, Portugal ABSTRACT This paper presents the fabrication and characterization of electrolyte-gate (EG) hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). In these devices, the metal gate is replaced by a Pt electrode immersed in an electrolyte. The source-drain current of these devices is modulated by the voltage applied through the Pt electrode. Device characteristics are compared with structurally equivalent top-gate a-Si:H TFTs. The EG devices show higher mobility and smaller subthreshold slope than their counterparts with metal gate and work in a narrower voltage range. EG-TFTs show chemical sensitivity, illustrated by a voltage shift in the transfer curve as a consequence of pH variation. The sensitivity of the devices to pH is different depending on whether the top layer in contact with the electrolyte is SiO2 or SiNx. INTRODUCTION The history of ion sensitive field effect transistors (ISFETs) goes back to the 1970’s when Bergveld [1] first reported them. Recently, the study of these devices for chemical or biological sensing applications has become a very active field of research. ISFET devices with polymers [2], crystalline silicon [3], diamond [4, 5] or GaN [6] as the active semiconductor material have been reported, and their use in the discrimination of electrolyte concentration, pH or detection of biomolecules such as DNA [7] or cells has been demonstrated. These devices could potentially be important in lab-on-a-chip, biosensor and DNA- and protein-chip applications. a-Si:H is widely used as the semiconductor active layer of TFTs for large-area electronic applications such as liquid-crystal displays and imagers. This paper introduces the use of a-Si:H as the active layer in EG-TFTs. a-Si:H-based EG-TFTs are a promising general platform for chemical and biosensing on glass or plastic substrates, or as backend-fabricated arrays on a CMOS substrate, and may allow the label-free electronic detection of molecules, such as DNA and proteins, immobilized or adsorbed on the surface of an arrayed device. EXPERIMENTAL DETAILS Device fabrication: Top-gate TFTs were made with a width-to-length ratio (W/L) of 5 (W=100 µm, L=20 µm). The substrate is glass (figure 1). Source and drain contacts are made of 150 nm Al layer with a contact layer of 50 nm n+-a-Si:H (see table I) (step 1 in fig. 1). An island of 150 nm a-Si:H active layer and 100 nm SiNx dielectric is deposited by PECVD (step 2 in fig. 1) and patterned. To electrically isolate the source-drain metal contacts from the electrolyte solution, a 150 nm SiNx layer is deposited over the full device except on the contact pads (step 3 in fig. 1). The devices are subsequently divided into 3
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