Nanowire Field-effect Transistor Sensors Research Articles

New Approach for Enhancing Sensitivity in Liquid‐Gated Nanowire FET Biosensors Under Optical Excitation

AbstractThe results obtained for liquid‐gated (LG) nanowire (NW) field‐effect transistor (FET) sensors analyzed in divalent ion (MgCl2) solutions, which play a crucial role in the human body, are reported. Liquid‐gated NW FET sensors fabricated in‐house are investigated using I–V characteristics and noise spectroscopy in a wide range of voltages and different intensities of optical infrared irradiation. A new effect is revealed under the influence of external optical excitation. The effect allows for the control of characteristic times of single‐trap phenomena: capture time to a single trap, emission time from the trap, and its ratio represented by the dimensionless ratio parameter (R‐factor). These parameters enabled an improvement in sensor sensitivity. The findings open prospects for controlling charge states in LG NW FET structures to optimize new parameters for achieving ultrahigh sensitivity in biosensors.

Detection of DNA nucleotides via silicon nanowire field-effect transistor sensors with a nanogap: Semi-empirical modeling

Silicon Nanowire field-effect transistors (SiNW FETs) have become very popular as they enable enhanced sensing capabilities, such as high-sensitivity, selectivity, and real-time detection. This research aimed at studying Silicon Nanowire Field effect transistor with a nanogap that is capable enough to create unique electronic signatures representing individual DNA nucleotides (Adenosine, Guanosine, Thymidine & Cytidine). In this work, two different transistor systems- back-gated SiNW FET and Gate All Around SiNW FET - were studied for the purpose of DNA nucleotide detection. Electronic transport properties such as current, transmission spectrum, and density of states were studied for both types of transistors. In the presence of Adenosine, the Gate All Around sensor reading was significantly enhanced from 0.07 nanoamperes to 3 nanoamperes, greatly improving the sensor's ability to differentiate between nucleotides.The study's findings indicate that the GAA SiNW FET outperforms the back-gated SiNW FET in terms of sensitivity when detecting DNA nucleotides. To examine the mechanics behind this process, non-equilibrium Green's function integrated with a semi-empirical model was utilized. The results showed that each type of DNA nucleotide could be detected by the designed SiNW FET sensor.

Silicon Nanowires Length and Numbers Dependence on Sensitivity of the Field-Effect Transistor Sensor for Hepatitis B Virus Surface Antigen Detection.

Silicon nanowire field effect transistor (NWFET) sensors have been demonstrated to have high sensitivity, are label free, and offer specific detection. This study explored the effect of nanowire dimensions on sensors’ sensitivity. We used sidewall spacer etching to fabricate polycrystalline silicon NWFET sensors. This method does not require expensive nanoscale exposure systems and reduces fabrication costs. We designed transistor sensors with nanowires of various lengths and numbers. Hepatitis B surface antigen (HBsAg) was used as the sensing target to explore the relationships of nanowire length and number with biomolecule detection. The experimental results revealed that the sensor with a 3 µm nanowire exhibited high sensitivity in detecting low concentrations of HBsAg. However, the sensor reached saturation when the biomolecule concentration exceeded 800 fg/mL. Sensors with 1.6 and 5 µm nanowires exhibited favorable linear sensing ranges at concentrations from 800 ag/mL to 800 pg/mL. The results regarding the number of nanowires revealed that the use of few nanowires in transistor sensors increases sensitivity. The results demonstrate the effects of nanowire dimensions on the silicon NWFET biosensors.

Nanowire-Enabled Bioelectronics.

Bioelectronics explores the use of electronic devices for applications in signal transduction at their interfaces with biological systems. The miniaturization of the bioelectronic systems has enabled seamless integration at these interfaces and is providing new scientific and technological opportunities. In particular, nanowire-based devices can yield smaller sized and unique geometry detectors that are difficult to access with standard techniques, and thereby can provide advantages in sensitivity with reduced invasiveness. In this review, we focus on nanowire-enabled bioelectronics. First, we provide an overview of synthetic studies for designed growth of semiconductor nanowires of which structure and composition are controlled to enable key elements for bioelectronic devices. Second, we review nanowire field-effect transistor sensors for highly sensitive detection of biomolecules, their applications in diagnosis and drug discovery, and methods for sensitivity enhancement. We then turn to recent progress in nanowire-enabled studies of electrogenic cells, including cardiomyocytes and neurons. Representative advances in electrical recording using nanowire electronic devices for single cell measurements, cell network mapping, and three-dimensional recordings of synthetic and natural tissues, and in vivo brain mapping are highlighted. Finally, we overview the key challenges and opportunities of nanowires for fundamental research and translational applications.

Fabrication of Low Cost and Low Temperature Poly-Silicon Nanowire Sensor Arrays for Monolithic Three-Dimensional Integrated Circuits Applications.

In this paper, the poly-Si nanowire (NW) field-effect transistor (FET) sensor arrays were fabricated by adopting low-temperature annealing (600 °C/30 s) and feasible spacer image transfer (SIT) processes for future monolithic three-dimensional integrated circuits (3D-ICs) applications. Compared with other fabrication methods of poly-Si NW sensors, the SIT process exhibits the characteristics of highly uniform poly-Si NW arrays with well-controlled morphology (about 25 nm in width and 35 nm in length). Conventional metal silicide and implantation techniques were introduced to reduce the parasitic resistance of source and drain (SD) and improve the conductivity. Therefore, the obtained sensors exhibit >106 switching ratios and 965 mV/dec subthreshold swing (SS), which exhibits similar results compared with that of SOI Si NW sensors. However, the poly-Si NW FET sensors show the Vth shift as high as about 178 ± 1 mV/pH, which is five times larger than that of the SOI Si NW sensors. The fabricated poly-Si NW sensors with 600 °C/30 s processing temperature and good device performance provide feasibility for future monolithic three-dimensional integrated circuit (3D-IC) applications.

Silicon Nanowire Field Effect Transistor Sensors with Minimal Sensor-to-Sensor Variations and Enhanced Sensing Characteristics.

Silicon nanowire field effect transistor (FET) sensors have demonstrated their ability for rapid and label-free detection of proteins, nucleotide sequences, and viruses at ultralow concentrations with the potential to be a transformative diagnostic technology. Their nanoscale size gives them their ultralow detection ability but also makes their fabrication challenging with large sensor-to-sensor variations, thus limiting their commercial applications. In this work, a combined approach of nanofabrication, device simulation, materials, and electrical characterization is applied toward identifying and improving fabrication steps that induce sensor-to-sensor variations. An enhanced complementary metal-oxide-semiconductor-compatible process for fabricating silicon nanowire FET sensors on 8 in. silicon-on-insulator wafers is demonstrated. The fabricated nanowire (30 nm width) FETs with solution gates have a Nernst limit subthreshold swing (SS) of 60 ± 1 mV/decade with ∼1.7% variations, whereas literature values for SS are ≥80 mV/decade with larger (>10 times) variations. Also, their threshold voltage variations are significantly (∼3 times) reduced, compared to literature values. Furthermore, these improved FETs have significantly reduced drain current hysteresis (∼0.6 mV) and enhanced on-current to off-current ratios (∼106). These improvements resulted in nanowire FET sensors with the lowest (∼3%) reported sensor-to-sensor variations, compared to literature studies. Also, these improved nanowire sensors have the highest reported sensitivity and enhanced signal-to-noise ratio with the lowest reported defect density of 2.1 × 1018 eV-1 cm-3, in comparison to literature data. In summary, this work brings the nanowire sensor technology a step closer to commercial products for early diagnosis and monitoring of diseases.

Nanoelectronic Platform for Ultrasensitive Detection of Protein Biomarkers in Serum using DNA Amplification

Silicon nanowire field effect transistors (NWFETs) are low noise, low power, ultrasensitive biosensors that are highly amenable to integration. However, using NWFETs to achieve direct protein detection in physiological buffers such as blood serum remains difficult due to Debye screening, nonspecific binding, and stringent functionalization requirements. In this work, we performed an indirect sandwich immunoassay in serum combined with exponential DNA amplification and pH measurement by ultrasensitive NWFET sensors. Measurements of model cytokine interleukin-2 concentrations from <20 fM to >200 pM were demonstrated, surpassing the conventional NWFET urease-based readout. Our approach paves way for future development of universal, highly sensitive, miniaturized, and integrated nanoelectronic devices that can be applied to a wide variety of analytes.

Specific detection of biomolecules in physiological solutions using graphene transistor biosensors

Nanomaterial-based field-effect transistor (FET) sensors are capable of label-free real-time chemical and biological detection with high sensitivity and spatial resolution, although direct measurements in high-ionic-strength physiological solutions remain challenging due to the Debye screening effect. Recently, we demonstrated a general strategy to overcome this challenge by incorporating a biomolecule-permeable polymer layer on the surface of silicon nanowire FET sensors. The permeable polymer layer can increase the effective screening length immediately adjacent to the device surface and thereby enable real-time detection of biomolecules in high-ionic-strength solutions. Here, we describe studies demonstrating both the generality of this concept and application to specific protein detection using graphene FET sensors. Concentration-dependent measurements made with polyethylene glycol (PEG)-modified graphene devices exhibited real-time reversible detection of prostate specific antigen (PSA) from 1 to 1,000 nM in 100 mM phosphate buffer. In addition, comodification of graphene devices with PEG and DNA aptamers yielded specific irreversible binding and detection of PSA in pH 7.4 1x PBS solutions, whereas control experiments with proteins that do not bind to the aptamer showed smaller reversible signals. In addition, the active aptamer receptor of the modified graphene devices could be regenerated to yield multiuse selective PSA sensing under physiological conditions. The current work presents an important concept toward the application of nanomaterial-based FET sensors for biochemical sensing in physiological environments and thus could lead to powerful tools for basic research and healthcare.

Double-gated Si NW FET sensors: Low-frequency noise and photoelectric properties

The transport, noise, and photosensitivity properties of an array of silicon nanowire (NW) p+-p-p+ field-effect transistors (FETs) are investigated. The peculiarities of photosensitivity and detectivity are analyzed over a wide spectrum range. The absorbance of p-Si NW shifts to the short wavelength region compared with bulk Si. The photocurrent and photosensitivity reach increased values in the UV range of the spectrum at 300 K. It is shown that sensitivity values can be tuned by the drain-source voltage and may reach record values of up to 2–4 A/W at a wavelength of 300 nm at room temperature. Low-frequency noise studies allow calculating the photodetectivity values, which increase with decreasing wavelength down to 300 nm. We show that the drain current of Si NW biochemical sensors substantially depends on pH value and the signal-to-noise ratio reaches the high value of 105. Increasing pH sensitivity with gate voltage is revealed for certain source-drain currents of pH-sensors based on Si NW FETs. The noise characteristic index decreases from 1.1 to 0.7 with the growth of the liquid gate voltage. Noise behavior is successfully explained in the framework of the correlated number-mobility unified fluctuation model. pH sensitivity increases as a result of the increase in liquid gate voltage, thus giving the opportunity to measure very low proton concentrations in the electrolyte medium at certain values of the liquid gate voltage.

Drift-Free pH Detection With Silicon Nanowire Field-Effect Transistors

The drift of drain current ( $I_{D})$ in silicon nanowire field-effect transistor sensors is analyzed under various conditions of pHs and liquid gate voltages ( $V_{LG})$ . It is found that H+ penetration into Helmholtz layer or sensing insulator is the cause of the current drift. To suppress the drift, a novel and fast measurement method with a two-step $V_{LG}$ is proposed and demonstrated. The drift could be completely suppressed by controlling the duration of the first step pulse. The time required to remove the $I_{D}$ drift is significantly reduced by the proposed method, from $\sim 1200$ s to below 100 s on average.

Application of Single-Electron Transistor to Biomolecule and Ion Sensors

The detection and quantification of chemical and biological species are the key technology in many areas of healthcare and life sciences. Field-effect transistors (FETs) are sophisticated devices used for the label-free and real-time detection of charged species. Nanowire channels were used for highly sensitive detections of target ion or biomolecule in FET sensors, however, even significantly higher detection sensitivity is required in FET sensors, especially when the target species are dilute in concentration. Since the high detection sensitivity of nanowire FET sensors is due to the suppression of the carrier percolation effect through the channel, the channel width has to be decreased, leading to the decrease in the transconductance (gm). Therefore, gm should be increased while keeping channel width narrow to obtain higher sensitivity. Single-electron transistors (SETs) are a promising candidate for achieving higher detection sensitivity due to the Coulomb oscillations. However, no reports of an SET-based ion sensor or biosensor existed, probably because of the difficulty of the room-temperature operation of SETs. Recently, room-temperature SET operations were carried out using a Si multiple-island channel structure. This review introduces the mechanism of ultra-sensitive detection of ions and biomolecules based on an SET sensor and the experimental results.

A comparison between bipolar transistor and nanowire field effect transistor biosensors

Important performance metrics, such as sensitivity and signal to noise ratio (SNR) of bipolar transistor based biosensors, are compared to those for nanowire field effect transistor (FET) sensors. The sensor consists of a heterojunction bipolar transistor (HBT) with silicon germanium base connected to a sensing surface in contact with the solution. The measured sensitivity is ≥2 times and SNR is &amp;gt;20 times higher in comparison to those for nanowire FET sensors. More importantly, the HBT biosensor sensitivity is constant over the sensing range of ∼5 decades and depends only on the temperature. In comparison, the nanowire FET sensor sensitivity varies in a complex manner over the sensing range and exhibits significant fabrication induced sensor to sensor variations. Consequently, HBT sensors would require minimal calibration for quantitative sensing studies. Furthermore, the bipolar transistor SNR is not only significantly higher but is also constant over the sensing range. In comparison, the nanowire FET sensor SNR varies with the peak value confined over a narrow sensing range. Hence, HBT sensor has &amp;lt;20 times lower detection limit that remains constant over the sensing range. In summary, HBT sensors are demonstrated to have superior performance metrics and are better suited for quantitative studies. Lastly, these HBT sensors also provide simultaneous temperature measurement.

Optimized operation of silicon nanowire field effect transistor sensors

Ion-sensitive field effect transistors have been advanced in recent years by utilizing silicon nanowires (Si-NWs), but establishing their optimized operation regime is an area of ongoing research. We propose a modified configuration of SiNWs in the form of a honeycomb structure to obtain high signal to noise ratio and high current stability. The low-frequency noise characteristics and the electrical stress are systematically considered for the optimization and compared against conventional SiNW devices. The operation voltage of the device severely affects the sensing stability; as the gate voltage is increased, the signal-to-noise ratio is enhanced, however, the stress effect becomes severe, and vice versa. The honeycomb nanowire structure shows enhanced noise characteristics in low voltage operation, proving to be an optimum solution for achieving highly stable sensor operation.

Detecting Interleukin-1β Genes Using a N2O Plasma Modified Silicon Nanowire Biosensor

Background Developing ultrasensitive and selective biological sensors has increased in interest for applications in disease detection, drug discovery, and biomedical diagnostics. This paper reports a CMOS-compatible technique for fabricating a silicon nanowire field-effect transistor. The nanowire sensor was applied to detect a fragment of the cancer-related interleukin-1β (IL-1β) gene, which is characterized as an indicator of breast, colon, lung, oral, head, and neck cancers. Methods We used the advanced integrated circuit (IC) technique to fabricate a back-gated nanowire field-effect transistor sensor. The dimensions of the nanowire were 60 nm in width and 20 μm in length. To enhance the sensitivity of the nanowire sensor, a plasma modification treatment with various parameters was employed on the surface of the device. Results The sensitivity of the silicon nanowire field-effect transistor could be improved after a 1-minute N2O plasma treatment, because the morphology of the detection region is rougher after the N2O plasma treatment. Hence, a more functional linker could be bound to the surface, thereby increasing the probability of DNA immobilization and hybridization. We used a specific sequence of the IL-1β gene to verify the sensitivity enhancement of the nanowire sensor by using plasma treatment. Conclusion The sensitivity and detection limit of the plasma-treated sensor can be extrapolated to 0.12/decade and 2.5fM, respectively. The detection results demonstrate that real-time monitoring of the expression of IL-1β using nanowire sensors could be useful for evaluating cancer treatment.

Accumulation mode field-effect transistors for improved sensitivity in nanowire-based biosensors

In this work, nanowire field-effect transistors (NW-FETs) constructed from a top-down approach has been utilized for the detection of biomolecules. Here, we demonstrate that the sensitivity of NW-FET sensors can be greatly enhanced when the same dopant type is used for both channel region and source and drain. This type of FET, known as accumulation mode field-effect transistors (AM-FETs), functions under different operating principle compared with conventional inversion mode FETs. The improved sensitivity is attributed to the different conduction mechanism and current components of AM devices. The results have been verified through a direct comparison with a conventional FET.

Channel length dependent sensitivity of Schottky contacted silicon nanowire field-effect transistor sensors

In this paper, we examine the dependence of channel length on the sensitivity of Schottky contacted silicon nanowire field-effect transistor sensors. The fabricated experimental devices are used as photosensors as well as chemical sensors for pH sensing. The difference in light illuminated current response depends on the channel length in the linear regime. However, the current ratio (ΔILight-Dark/IDark) shows a different trend, being much improved for a longer channel in the subthreshold regime, which can be explained by the different subthreshold swings. Finally, devices of two different channel lengths are applied to detect the pH value of a solution yielding results similar to photosensing. From these results, we suggest that it is desirable to shorten the channel if the sensor is working in the linear region and increase the channel length if the sensor is used in the subthreshold region.

Optimization of pH sensing using silicon nanowire field effect transistors withHfO2 as the sensing surface

Silicon nanowire field effect transistor sensors withSiO2/HfO2 as the gate dielectric sensing surface are fabricated using a top down approach. Thesesensors are optimized for pH sensing with two key characteristics. First, the pH sensitivityis shown to be independent of buffer concentration. Second, the observed pHsensitivity is enhanced and is equal to the Nernst maximum sensitivity limit of59 mV/pH with a corresponding subthreshold drain current change of ∼ 650%/pH. These two enhanced pH sensing characteristics are attributed to the use ofHfO2 as the sensing surface and an optimized fabrication process compatible with siliconprocessing technology.

Gold-Nanoparticle-Functionalized In2O3 Nanowires as CO Gas Sensors with a Significant Enhancement in Response

We present the room-temperature sensing of gold nanoparticle (AuNP)-functionalized In(2)O(3) nanowire field-effect transistor (NW-FET) for low-concentration CO gas. AuNPs were functionalized onto In(2)O(3) nanowires via a self-assembled monolayer of p-aminophenyltrimethoxysilane (APhS-SAM). The nanowires were mounted onto the Au electrodes with both ends in Schottky contacts. High sensor response toward low concentration of CO gas (200 ppb-5 ppm) at room temperature is achieved. The presence of AuNPs on the surface of In(2)O(3) nanowire serves to enhance the CO oxidation due to a higher oxygen ion-chemisorption on the conductive AuNP surfaces. Detailed studies showed that the sensing capabilities were greatly enhanced in comparison to those of bare nanowires or low coverage of Au NP-decorated nanowires. When the sensor is exposed to CO, the CO molecules interact with the preadsorbed oxygen ions on the AuNP surface. The CO oxidation on the AuNPs leads to the transfer of electrons into the semiconducting In(2)O(3) nanowires and this is reflected as the change in conductance of the NW-FET sensor. This work provides a promising approach for fabricating nanowire devices with excellent sensing capabilities at room temperature.

Schottky Contacted Nanowire Field-Effect Sensing Device With Intrinsic Amplification

In this letter, we present a novel semiconductor nanowire field-effect transistor-based sensing device with an intrinsic amplification mechanism. In this novel device, a nanowire field-effect transistor is integrated with an orthogonal sensing nanowire that amplifies any detected signal. The device operating and intrinsic amplification mechanism has been proposed and experimentally validated. Fabricated exclusively using the top-down processes, the novel device prototypes have demonstrated an appreciable improvement in photodetection against the cofabricated generic nanowire field-effect transistor sensors.

Subthreshold Regime has the Optimal Sensitivity for Nanowire FET Biosensors

Nanowire field-effect transistors (NW-FETs) are emerging as powerful sensors for detection of chemical/biological species with various attractive features including high sensitivity and direct electrical readout. Yet to date there have been limited systematic studies addressing how the fundamental factors of devices affect their sensitivity. Here we demonstrate that the sensitivity of NW-FET sensors can be exponentially enhanced in the subthreshold regime where the gating effect of molecules bound on a surface is the most effective due to the reduced screening of carriers in NWs. This principle is exemplified in both pH and protein sensing experiments where the operational mode of NW-FET biosensors was tuned by electrolyte gating. The lowest charge detectable by NW-FET sensors working under different operational modes is also estimated. Our work shows that optimization of NW-FET structure and operating conditions can provide significant enhancement and fundamental understanding for the sensitivity limits of NW-FET sensors.