Liquid and back gate coupling effect: towards biosensing with lowest detection limit

We employ noise spectroscopy and transconductance measurements to establish the optimal regimes of operation for our fabricated silicon nanowire field-effect transistors (Si NW FETs) sensors. A strong coupling between the liquid gate and back gate (the substrate) has been revealed and used for optimisation of signal-to-noise ratio in sub-threshold as well as above-threshold regimes. Increasing the sensitivity of Si NW FET sensors above the detection limit has been predicted and proven by direct experimental measurements.Comment: 18 pages, 6 figure

Graphene on Silicon Hybrid Field-Effect Transistors

The combination of graphene with silicon in hybrid devices has attracted attention extensively over the last decade. Most of such devices were proposed for photonics and radiofrequency applications. In this work, we present a unique technology of graphene-on-silicon heterostructures and their properties as solution-gated transistors. The graphene-on-Silicon field-effect transistors (GoSFETs) were fabricated exploiting various conformations of drain-source regions doping and channel material dimensions. The fabricated devices were electrically characterized demonstrating hybrid behavior with features specific to both graphene and silicon. Although GoSFET's transconductance and carrier's mobility were found to be lower than in conventional silicon and graphene field-effect transistors (SiFETs and GFETs), it was demonstrated that the combination of both materials within the hybrid channel contribute uniquely to the charge carrier transport. A comprehensive physics-based compact modeling was specifically developed, showing excellent agreement with the experimental data. The model is employed to rationalize the observed hybrid behavior as the theoretical results from the electrostatics and the carrier transport under a drift-diffusion approach show that graphene acts as a shield for the silicon channel, giving rise to a non-uniform potential distribution along it, especially at the subthreshold region. This graphene screening effect is shown to strongly affect the device subthreshold swing when compared against a conventional SiFET due to a non-negligible diffusion current in this operation regime

Characteristic Frequencies and Times, Signal-to-Noise Ratio and Light Illumination Studies in Nanowire FET Biosensors : Invited paper

The detection of cardiac biomolecules is of paramount importance for the prospects of fast medical diagnostics of cardiovascular diseases. Silicon nanowire field-effect transistors (NW FETs) are perfect candidates for (bio)sensing studies due to their tremendous sensitivity to changes in surface charge. We present the results of an investigation of transport, fluctuation and modulation phenomena with certain characteristic times in n+-p-n+ liquid-gated field-effect transistors (FETs) and compare them with those of p+-p-p+ structures. We reveal the gate coupling effect to be a tool for influencing the channel noise mechanism. n+-p-n+ liquid-gated FETs demonstrate higher signal-to-noise ratios (SNRs) compared to p+-p-p+ structures. We show the results of the influence of light waves on the electrical properties of NW FET structures and of studying modulation phenomena in these structures. Excitation of NW samples by light waves allows the effective control of conductance in nanowire channels. Noise spectra and time-dependent modulations of the drain current show promising prospects for enhancing the sensitivity and SNR of nanowire biosensors. The direct translation of periodical signals at a frequency around 1 kHz from the biological object into surface potential changes, caused by interaction with cardiac cells, enables the highly sensitive monitoring of cell dynamic activity before and after pharmacological treatments. Electrical properties of the fabricated Si NW FETs demonstrate high sensitivity for the detection of human C-reactive protein (CRP) – a biomolecule which has recently emerged as a reliable biomarker used in clinical practice for predicting and tracking the state of cardiovascular diseases. The response of the sensor to different concentrations of target CRP molecules, which represent predictable cardio-biomarkers, is studied. Moreover, we reveal that the periodical modulation of drain current due to the single-trap effect may be used to achieve enhanced sensitivity of biosensors. The results are promising for cost-efficient lab-on-chip monitoring, which is especially necessary in the case of acute cardiovascular diseases, where every minute is critical for saving life

Transport Phenomena in Liquid-Gated Si Nanowire FETs for Biosensing Applications

We review transport and noise properties of liquid-gated Si nanowire field-effect transistor structures designed and fabricated for biosensing applications, aiming to study the response at the level of a single molecule or single neuron. It is shown that the characteristic capture time of a single trap, determined from random telegraph signal noise, contains important information about surface potential changes, with enhanced sensitivity compared to standard measurements of the drain current’s dependence on the pH concentration of the solution. The gate coupling effect determined was used to improve the signal-to-noise ratio: an important characteristic to register small signals from neuronal activity

Advanced Biosensors on the Basis of Single Trap PhenomenaInvited Contribution

Many different realizations of nanowire (NW) structures have been studied over the past decade from the point of view of biosensor development. However, scaling down does not only lead to a positive increase in sensitivity due to increased surface-to-volume ratio, but also to increased negative surface trap-assisted noise. The latter deteriorates the response signal from biological samples. In this presentation, we will review our results reflecting the novel approach of employing single trap phenomena as a useful signal for the detection of biological response. Our results show that in order to describe the single trap phenomena it is important to give consideration to both the quantum mechanical approach of carrier distribution in the channel of NW transistors and change in the charged state of single traps situated in the immediate vicinity of the channel and their influence on the effective barrier thickness of the dielectric layer. Moreover, we have discovered that signal response to changes of surface potential due to biological objects can be enhanced by one order of magnitude and even higher response can be obtained by utilizing the single trap phenomena