Nanowires are interesting due to their exceptional electrical and mechanical properties and large surface area to volume ratio. Semiconductor nanowires, especially silicon nanowires (Si NWs), can be used as channels for field effect transistors (FETs), Fig. 1a. NW FETs have promising application not only in digital logic but also as sensing devices. Instead of a physical top gate, however, such devices are gated by the attached electrically charged molecules, e.g. streptavidin as shown in Fig. 1b. The NWs can be functionalised by attaching to their surface thin layers of certain receptor molecules (biotin in Fig. 1b) that are selectively sensitive only to the target molecules (streptavidin in Fig. 1b). In this way, depending on their functionalisation, NW FETs may be used as gas sensors, chemical sensors, and biosensors. They have a potential for fast, low-cost, low-power, label-free detection, real-time response, high throughput analysis, providing insight into biological processes while not requiring large sampling quantities. Nanometre-scale cross-sections lead to depletion or accumulation of carriers in the bulk of the device channel when a charged biomolecule binds to the surface, versus surface-only modulation with the traditional planar ion sensitive field effect transistor (ISFET) sensors.
Fig. 1 Schematics of a p-channel nanowire FET (a) and the p-type JNT architecture used in this study (b). The negatively charged streptavidin molecules will bind to the biotin receptor molecules and act as a chemical gate.
Research into the use of Si NWs as FET-type sensors began by using chemically grown NWs and this is still the main route for fabrication of such devices. Although the bottom-up approach is capable of a massive production of sub-10 nm NWs, their integration into functional devices remains challenging. The main issues here are the proper contacting as well as precise positioning and alignment of grown NWs to other nanostructures. These issues can be naturally addressed by the top-down approach used in semiconductor device fabrication. In addition, this approach is CMOS compatible and improves the control on device parameters and, hence, on reproducibility.
Therefore, we chose the top-down approach. We designed and fabricated a range of Si NW sensing devices having various nanowire densities, lengths and widths (see Fig. 2). The devices are fabricated mainly by electron beam lithography (EBL) and reactive ion etching (RIE) on highly p-doped silicon-on-insulator (SOI) wafers. The operation of these devices relies on the principle of a field effect transistor. In contrast to most sensors of this type, however, our devices have a junctionless architecture, i.e. the source, channel (nanowires) and drain have the same dopant polarity (p-type in this case) without any junctions between them. The junctionless nanowire transistors (JNTs) have been recently invented and demonstrated for the first time by researchers at the Tyndall National Institute. Such devices are easier to fabricate than the traditional FETs since they do not require separate doping of the source and drain regions. They also possess a number of other advantages over the conventional inversion-mode FETs.
Currently, there are two main concepts for achieving the highest possible sensitivity with Si NW FET-type sensors: operating them (i) in the subthreshold mode or (ii) in the above-threshold mode. Usually the operation mode can be set up, e.g. by applying an appropriate backgate potential at the expense of increased power consumption. In the case of JNTs, however, their performance depends strongly on the geometry (height, width, and length) of the channel (NW) as well as on its doping level. Therefore, these parameters can be used to finely tune the operation point of devices into the appropriate mode without additional power supply, which makes them very energy efficient.
Fig. 2 Overview of the sensor design layout
Electrical characterisation of our fabricated devices revealed their excellent performance as back-gated JNTs (see Fig. 3).
To test the devices as biosensors, a microfluidic delivery system has to be arranged. Polydimethylsiloxane (PDMS) stamps with 150 mm wide microfluidic channels are attached to the devices (see Fig. 4a). The solutions are delivered from gastight syringes propelled by a syringe pump (Fig. 4b) through polytetrafluoroethylene (PTFE) tubing with 250 µm inner diameter connected to the PDMS stamps.
Experiments for sensing the ionic strength of commercially available pH 7 buffer of different concentrations demonstrated the high sensitivity of the sensors (Fig. 5).
Fig. 3 Sample output (a) and transfer (b) characteristics (Id-Vd and Id-Vbg curves, respectively)
Fig. 4 Images of microfluidic PDMS stamps attached to devices (a) and of the syringe pump with the PTFE tubing (b)
Fig. 5 Time dependence of the drain current Id demonstrating the ionic strength sensing results
To be able to sense the protein streptavidin, biotin has to be covalently tethered to the Si nanowire surface. The process involves first covalently attaching (3-Aminopropyl) triethoxysilane (APTES), which is then used as the linker molecule between the native oxide of the Si nanowires and the biotin molecules (see Fig. 6).
Fig. 6 Schematic of the functionalisation route of the silicon surface of the JNTs from the native oxide termination to the biotin terminated surface
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