Nano Science and Technology Institute

R&D Profile: Double Gated Silicon Nanowire Field Effect Transistors as Charge Detection Based Bio and Chemical Sensors

The current research of the NRC Nano Sensing group is targeted to develop ubiquitous embedded sensors that combine detection and information processing and use components with functional structures tailored at the nanoscale.

Overview Courtesy of Jani Kivioja, who is the Research Leader of the Nano Sensing group at Nokia Research Center (NRC) in Cambridge, UK. The research team includes J. Kivioja and A. Colli, M. Bailey, and T. Ryhänen.

Atomic force micrograph

Atomic force micrograph of the bottom gated silicon nanowire field effect transistor.

NANOSENSING IN FUTURE MOBILE DEVICES
The mobile phone will be a trusted personal device with new fundamental capabilities that will enable it to interact with the local environment via various different techniques, to carry the digital identity of a person, and to enable easy-to-use secure communication and controlled privacy in future smart spaces, and to sense local context and user’s behaviour.

The current research of the NRC Nano Sensing group is targeted to develop ubiquitous embedded sensors that combine detection and information processing and use components with functional structures tailored at the nanoscale. The research addresses the question, “will it be possible to fabricate autonomous nanoscale sensors with the intrinsic capability to process their signals into meaningful forms and even to classify that information?” Such sensors should be autonomous in terms of power management, information processing and communication capabilities.

SILICON NANOWIRE SENSORS
Semiconducting nanowire field effect transistors are promising candidates for ultrasensitive charge detection based bio- or chemical sensors due to several outstanding properties [1-4]. The critical dimensions of the nanowire sensor can be of the same order of size of biological molecules or chemical species yielding exceptional sensing possibilities [2]. In addition, the large surface/volume ratio will give high sensitivities simply because surface effects dominate over bulk properties.

Silicon nanowire based sensors are especially attractive, because of the easy CMOS compatibility and because the sensors are also likely to be easily integrated with the present CMOS read-out circuitry and transducers. In addition, nanowires are also proposed to be used as building blocks in new type of information processing devices, e.g., in neural networks and hence sensing in wider concept could probably be made by using solely nanowires [5-6].

In this work, we have studied double gated silicon nanowire structures (DG-SiNW), where the position and/or type of the charge could be tuned within the nanowire by electric field [8]. DG-NW structures have several beneficial properties, e.g., the possibility to change the distribution of the charge could be used to increase the sensitivity of the sensor by pushing the charge towards the functionalized surface. In addition, the dynamic range of the sensor could possibly be enhanced by tuning the sensitivity in a similar manner. Furthermore, the DG structures has also been proposed to be used as a velocity modulation transistor [9], which could act as a basic building block for ultrafast data processing purposes.

SUMMARY
In summary, DG-SiNW are promising candidates for sensing purposes. In this work, DG-SiNWs were studied both experimentally and by simulations. Our results confirm that the top-gate could indeed be used to tune charge distribution along the wire even in relatively narrow ~10 nm diameter nanowires. This property could potentially be used to enhance the sensitivity and the dynamic range of nanowire based bio and chemical sensors.

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