Transport and Noise Properties of Nanostructure Transistors for Biosensor Applications

Biosensors based on nano-scale electronic devices have the potential to achieve exquisite sensitivity for the direct detection of biomolecular interactions. Silicon nanowire field effect transistors (Si NW FET) are the most promising candidates for these purposes because of their biocompatibility, very high surface-to-volume ratio, fast response, and good reliability of the signal. In the last decade, several promising results based on Si NW sensors, which were either fabricated by “top-down” or “bottom-up” approaches, have been reported. However, a fundamental understanding of the principle determining the signal-to-noise ratio(SNR) of the Si NW FET biosensors is still not well understood. The aim of this PhD thesis was to fabricate Si NW FETs with optimized techniques based on the “top-down” approach and to study the electrical transport characteristics of Si NW FETs with different channel dimensions in order to disclose the intrinsic low frequency noise properties and hence give the inspiration for biosensor fabrication and application. Nanoimprint lithography (NIL) and wet anisotropic etching were employed in the fabrication of our devices. In order to increase the size resolution, reproducibility and stable electrical operation properties, KOH chemical etching was used to fabricate imprint mold. Four kinds of Si NW FETs were fabricated using the optimized CMOS compatible technologies in the cleanroom of the Helmholtz Nanoelectronic Facility (HNF), Forschungszentrum Juelich, Germany. The electrical transport properties of Si NW FETs were characterized by both current-voltage characteristics and low frequency noise measurements using different configurations, including back gate control (V$_{BG}$) and front gate control (VFG) in ambient conditions and in liquid environments, respectively. It was demonstrated that the magnitude of the flicker noise depends on the channel dimensions and that the signal to noise ratio increases with theshrinking of the channel dimensions. Furthermore, random telegraph signals (RTSs) were registered in devices with short channels and Coulomb Blockade energy was evaluated from the real time and low frequency noise measurements at different temperatures in a back gate configuration. RTS noise was also found in the front gate configuration at different pH values and different gate voltages. It was demonstrated that the capture time constant of the RTS can be used as a sensor analysis with a higher sensitivity of signal detection in sensor applications than conventional drain current measurements and the sensitivity can be further improved by a special design of the structures