Design guideline of biosensor based on field-effect transistor

Abstract

In this thesis, field-effect transistor (FET) is designed for highly sensitive and stable sensor for label-free detection of biological species. Sensitivity enhancement is investigated by engineering in front-end and back-end process on FET biosensors. Also, FET sensors endurable in electrolyte solution for long time were proposed. First, surface property of a passivation layer, isolating adjacent interconnection lines, is controlled. The hydrophobic passivation layer dramatically enhances the sensitivity of the underlap-embedded FET when compared with a hydrophilic passivation layer. A thin film of CYTOPTM and silicon nitride is used as the hydro-phobic and hydrophilic passivation layers, respectively. The surface antigen and its specific antibody of the avi-an influenza (AI) virus were employed as the probe and target biomolecule, respectively, to confirm the en-hanced sensitivity of the proposed biosensor. By using hydrophobic passivation layer, the limit of detection of the biosensor was improved up to 100-fold compared with that resulting from hydrophilic passivation layer. Second, nanoscale FET was investigated. The label-free electrical detection of the binding of antibodies and antigens of AI and human immunodeficiency viruses (HIV) is demonstrated through an underlap-embedded silicon nanowire field-effect transistor (SiNW FET). The proposed sensor was fabricated on a silicon bulk wafer by a top-down process. Specifically, a SiNW was fabricated by a combined isotropic and anisotropic etching, which is one route plasma etching process. The sensor was fabricated by a self-aligned process to the gate with tilted implantation, and it allowed precise control of the underlap region. This was problematic in earli-er underlap-embedded FET fabricated by a conventional gate-last process. As a sensing metric to detect the binding of a targeted antibody, the change of transfer characteristics was traced. The differences between be-fore and after antibody binding results were caused by changes in the channel potential on the underlap region due to the charge effect arising from the biomolecules

this is also supported by a simulation. Furthermore, the multiplex detection of AI and HIV is demonstrated, showing distinctive selectivity in each case. Last, a SiNW surrounded by a poly-Si gate is demonstrated for the label-free electrical detection of bio-logical and chemical species. The SiNW was fabricated by a top-down process, with the gate dielectric and gate encapsulating the SiNW. Also, separating the sensing region and transducer was suggested for long-term stability. With this design, it showed high sensitivity and enhanced compatibility with state-of-the-art CMOS technology, as well as immunity against electrolyte solutions. We demonstrated the performance of the sensor by detecting pH value, charged polymers and avian influenza (AI) antibody. The proposed sensor promised to be a cost-effective sensor with identical structure with CMOS logic devices. Thus, proposed sensors have inherent benefits for label-free, electrical, and multiplex detection of bio-molecules. Moreover, its processes are still compatible with commercialized technology presently used to fabri-cate semiconductor devices. This advantage is attractive for those involved in the construction of a point-of-care testing (POCT) system on a chip involving simple, high performance, low-cost and low-risk fabrication pro-cesses of novel structures and materials.