Double-gate nanowire field effect transistor for a biosensor

Abstract

In this thesis, a silicon nanowire field effect transistor (FET) straddled by the double-gate is investigated for biosensor application. The separated double-gates, G1 (primary) and G2 (secondary), allow independent voltage control to modulate channel potential. Therefore, the detection sensitivity is enhanced by the use of G2. The sensing window is significantly broadened compared to the case of employing G1 only, which is nominally used in conventional nanowire FET-based biosensors. The double-gate nanowire FET is fabricated on a silicon-on-insulator (SOI) substrate. The structure of the device is very similar to the FinFET. The double-gate structure is formed by means of chemical mechanical polishing (CMP) process that separates a single-gate into double gates. Nanowire surface exposed after the CMP process is used as a sensing region to detect charged biomolecules. In addition, one of the two gates also can be served as a sensing antenna where biomolecules are immobilized. Threshold voltage and drain current are modulated due to the charge of the biomolecules. The double-gate nanowire FETs are also fabricated on a silicon bulk substrate for low-cost applications. Wet-based measurement system is constructed for real-time measurement and detection of biomolecules in the solution medium. In order to detect biomolecules on the sensor surface, surface functionalization methods are discussed. DNA is immobilized on the sensor surface with covalent and electrostatic methods. Antigen and antibody are functionalized with several methods using a silica-binding protein (SBP) and protein G. Charged polymers with positive and negative polarities are bound on the device surface with self-assembled monolayer techniques. Sensing results are presented to detect pH solution, charged polymers, DNA, and antigen-antibody interactions. By monitoring the change in threshold voltage or drain current, we can detect chemical species and biomolecules. The experiments are carried out ...