We report on an electrostatically formed nanowire (EFN)-based sensor with tunable diameters in the range of 16 nm to 46 nm and demonstrate an EFNbased field-effect transistor as a highly sensitive and robust room temperature gas sensor. The device was carefully designed and fabricated using standard integrated processing to achieve the 16 nm EFN that can be used for sensing without any need for surface modification. The effective diameter for the EFN was determined using Kelvin probe force microscopy accompanied by threedimensional electrostatic simulations. We show that the EFN transistor is capable of detecting 100 parts per million of ethanol gas with bare SiO 2 .

The evolution of nanotechnology based sensors has enabled detection of ultra-low-level concentrations of target species owing to their high aspect ratio. However, these sensors have a limited dynamic range at room temperature characterized by saturation in the sensor response following certain concentration exposure. In this work, we show that the dynamic range towards a target gas can be significantly enhanced using the electrostatically formed nanowire sensor. The size and shape of the nanowire conducting channel are defined and tuned by controlling the bias applied to the surrounding gates. The nanowires thus formed vary in their response, detection limit, and dynamic range for a given target gas exposure depending on its size and shape. By electrostatically tuning to the appropriate nanowire, we can not only enhance the sensor response in the low concentration regime, but also broaden the overall dynamic range capacity using a single sensor. It is demonstrated that the sensor is capable of detecting ∼26–2030 ppm ethanol and ∼40–2800 ppm of acetone efficiently with reasonably high response (≥20%) throughout the whole range. The broad dynamic range concept is also demonstrated using scanning gate microscopy measurements of the device. This represents the first nanotechnology-inspired work towards tunable dynamic range of a sensor using a single electronic device.

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Scanning gate microscopy is used to determine the electrostatic limit of detection (LOD) of a nanowire (NW) based chemical sensor with a precision of sub-elementary charge. The presented method is validated with an electrostatically formed NW whose active area and shape are tunable by biasing a multiple gate field-effect transistor (FET). By using the tip of an atomic force microscope (AFM) as a local top gate, the field effect of adsorbed molecules is emulated. The tip induced charge is quantified with an analytical electrostatic model and it is shown that the NW sensor is sensitive to about an elementary charge and that the measurements with the AFM tip are in agreement with sensing of ethanol vapor. This method is applicable to any FET-based chemical and biological sensor, provides a means to predict the absolute sensor performance limit, and suggests a standardized way to compare LODs and sensitivities of various sensors.

The ability to control surface-analyte interaction allows tailoring chemical sensor sensitivity to specific target molecules. By adjusting the bias of the shallow p-n junctions in the electrostatically formed nanowire (EFN) chemical sensor, a multiple gate transistor with an exposed top dielectric layer allows tuning of the fringing electric field strength (from 0.5 × 10 to 2.5 × 10 V/m) above the EFN surface. Herein, we report that the magnitude and distribution of this fringing electric field correlate with the intrinsic sensor response to volatile organic compounds. The local variations of the surface electric field influence the analyte-surface interaction affecting the work function of the sensor surface, assessed by Kelvin probe force microscopy on the nanometer scale. We show that the sensitivity to fixed vapor analyte concentrations can be nullified and even reversed by varying the fringing field strength, and demonstrate selectivity between ethanol and n-butylamine at room temperature using a single transistor without any extrinsic chemical modification of the exposed SiO surface. The results imply an electric-field-controlled analyte reaction with a dielectric surface extremely compelling for sensitivity and selectivity enhancement in chemical sensors.