Silicon nanowires improve transistor-based biosensors

Korean researchers have improved the fabrication of transistor-based biosensors by adding silicon nanowires to their surface.

When molecules bind on a field-effect transistor (FET), a change happens in the surface’s electric charge. This makes FETs good candidates for detecting biological and chemical elements. Dual-gate field-effect transistors (DG FETs) are particularly good candidates because they amplify this signal several times — but they could still be improved.

FETs using silicon nanowires have already been drawing attention as promising biosensors because of their high sensitivity and selectivity, but they are difficult to manufacture. The size and position of silicon nanowires fabricated using a bottom-up approach, such as chemical vapour deposition, cannot always be perfectly controlled. Top-down approaches, such as using an electron or ion beam to draw nanorods onto a surface, allow better control of size and shape but are expensive and limited by low throughput.

Seeking a better solution, researchers led by Won-Ju Cho of Kwangwoon University fabricated their own silicon nanowires using nanoimprint lithography. A thin layer of silicon was placed on top of a substrate and was then pressed using a nanoimprinter, which imprints nanosized wire-shaped lines into the surface. The areas between separate lines were then removed using a method called dry etching, which involves bombarding the material with chlorine ions. The resultant size-controlled silicon nanowires (SiNWs) were then added to a DG FET.

“Compared to previous DG FETs with a planar-type silicon channel layer, the constructed SiNW DG FETs exhibited superior electrical properties including a higher capacitive-coupling ratio of 18.0 and a lower off-state leakage current under high-temperature stress,” the researchers wrote in the journal Science and Technology of Advanced Materials. “In addition, while the conventional planar single-gate (SG) FET- and planar DG FET-based pH sensors showed the sensitivities of 56.7 mV/pH and 439.3 mV/pH, respectively, the SiNW DG FET-based pH sensors showed not only a higher sensitivity of 984.1 mV/pH, but also a lower drift rate of 0.8% for pH-sensitivity.”

The researchers stated that their DG FET-based sensor could be used for various biological events, such as enzyme-substrate reactions, antigen-antibody bindings and nucleic acid hybridisations.