Sensing with silicon nanowire field-effect transistors

All experiments presented in this thesis are focusing on the development of a reliable and stable sensing platform, which allows converting a (bio-) chemical signal into an electrical, processable one.
A sensing platform requires a proper design meeting many demands, e.g., stability and differential readout capability with in situ references to prevent misreadings due to non-specific interactions and/or thermal drifts. Nanoscale electronic detection systems, e.g. nanowires, based on an ion-sensitive field-effect transistor (ISFET) implementation (Sensors & Actuators B, 1:1, 2003) do have the potential to meet these boundary conditions.
However there is a main difficulty: controlling and understanding the interface between the transducer and the target agents is a crucial factor and needs to be carefully explored to allow reliable detection.
In this thesis we developed a fabrication protocol yielding top-down fabricated silicon nanowire field-effect transistors (SiNW FETs) with a high degree of reproducibility which we present in chapter I. The nanowire FETs are operated in the accumulation regime. The electrical characterization of a FET is performed in the linear regime by applying a low source-drain voltage and varying the gate voltage. While changing the back-gate voltage the source-drain current is recorded. With the focus on (bio-) chemical sensing, stable functionalities and operations in electrolyte solutions is mandatory.
In chapter II we discuss investigations, which were performed in order to achieve stable working conditions in liquid environments. We show that leakage currents can sufficiently be suppressed with a thin alumina layer covering the sensor. Further we discuss the dual-gate approach. A home-built liquid cell allows the integration of a platinum electrode with which a gate potential can be applied. This electrode acts as liquid gate. The liquid cell combines both types of gating, liquid (top)-gating and back-gating, in order to characterize a device.
To target sensing with our platform we focus on ion sensing experiments, which are presented in chapter III. We studied the nanowires as ISFETs and conducted pH-sensing experiments. we show that a nanowire FET can be used as a sensing device. We demonstrate that we can measure pH shifts either by sweeping the back-gate voltage or the liquid potential.
In chapter IV we expand the application of our nanowire FETs as ion-sensitive sensors. We study here the sensing capabilities at different salts and concentrations. We demonstrate that using an alumina interface between the FETÕs surface and the liquid results in insensitivity to monovalent as well as divalent ions at a large concentration range up to 10 mM. Above a concentration of 10 mM we observe a sudden transition resulting in a transfer characteristics change. The presented evaluation and the models have to be considered as work in progress.
In chapter V we discuss additional investigations, which are of importance for reliable detection of target analytes with our sensing platform.
To determine the sensing limit of our nanowires, we analyzed the signal-to-noise ratio in our fabricated nanowire FETs. To target specificity of analytes in solution the surface has to be (i) passivated against pH reactions and (i) functionalized for specific targeting. To address these issues we investigated passivation and functionalization of the surface. Finally we introduce the developed nanowire array platform. The design enables new experiments, e.g., time-resolved correlation measurements as well as differential measurements using multiple functionalization, which is beyond the scope of this thesis. As a last step we discuss first results towards (bio-) chemical sensing using streptavidin and conclude this thesis in chapter VI.
The developed sensing platform will allow conducting further experiments to explore the use of nanowire ion-sensitive field-effect transistors as (bio-) chemical sensors.