Extended-gate Field-effect Transistor Research Articles

Bioanalytical Method Based on Extended-Gate Field-Effect Transistor Modified by Self-Assembled Monolayer

In this paper, we introduce semiconductor biosensors for detecting or monitoring various biological substances and for surface chemical technologies tailored to target molecules. To fabricate the semiconductor biosensor best suited to the target biomolecules, the gate electrodes for extended-gate type field-effect transistors (EGFETs), which are separated from semiconductor part, must be constructed by interfacial chemical modification. First, ion-sensitive EGFET was developed by self-assembled monolayer (SAM) modification on gold gate electrode. Polar functional-group-terminated alkanethiol SAM-coated-gate FET showed pH dependency. In particular, carboxy-terminated alkanethiol SAM-coated gate FET showed higher sensitivities from 42 to 56 mV/pH, which was close to the Nernstian response, in a wide range of biological environments. By using the ion-sensitive EGFET, the hydroxyapatite biomineralization process was successfully monitored by increasing the gate surface potential. Furthermore, saccharides were quantified using EGFET by changing the functional group of SAM, with phenylboronic acid as a functional molecule. In conclusion, target-specific surface modification on gate electrodes makes it possible for semiconductor devices to be applied as biosensors.

Optimization of Nanobelt Field Effect Transistor with a Capacitive Extended Gate for Use as a Biosensor

In this study, various types of extended gate field effect transistor (EGFET) biosensors were compared and then a model based on potential coupling between a disposable extended gate (EG) capacitor and the gate–source/drain capacitor of a nanobelt field effect transistor (NBFET) was developed and optimized for biosensor applications. Several parameters, including the dielectric thicknesses and the EG gate area coverage, the ionic strength of buffer solution, and the charge density of specific binding biomolecules, were included in the model. The potential coupling efficiency between the potential induced by surface charge on the EG and the gate voltage of the FET was analyzed and verified through pH detection. Biotin–streptavidin/avidin sensing was demonstrated with the optimized EG and NBFET. In addition, real-time measurements of the detection of botulinum neural toxin (BoNT) type-A were also performed using the EG NBFET, which could detect an extremely low concentration (20 fM) of BoNT type-A. Because the nanoelectronic field effect transistor does not need to be sacrificed after detection, the optimized EG NBFET biosensor has great potential for use in bio/chemical and point-of-care applications.

Selective and sensitive onsite detection of phthalates in common solvents

Phthalates, which are proven to have adverse health effects, are globally restricted for use in all kinds of plastics through various regulations. Although there are laboratory based techniques for phthalate detection, there is a pressing need for a field based technique so samples can be pre-screened. Here, we report a molecularly imprinted polymer (MIP) functionalized extended gate field effect transistor (EGFET) as a field sensor to identify di-2-ethlyhexyl phthalate (DEHP), which is the one of the most commonly used phthalate. In DI water, DEHP is detected at the extremely low concentration of 25 μg/L while exhibiting excellent selectivity. We are able to tune the linear dynamic range of the sensor by synthesizing the MIP with a different monomer-to-template ratio and by choice of the functional monomer. Finally, the sensor is calibrated for DEHP in artificial saliva at sub 50 μg/L, showing applicability in phthalate migration tests, which are used in assessing the safety of plastic toys. Furthermore, our sensor platform can be further extended to identify other phthalates as fast pre-screening tool.

Effects of UV-Ozone Treatment on Sensing Behaviours of EGFETs with Al₂O₃ Sensing Film.

The effects of UV-ozone (UVO) treatment on the sensing behaviours of extended-gate field-effect transistors (EGFETs) that use Al2O3 as the sensing film have been investigated. The Al2O3 sensing films are UVO-treated with various duration times and the corresponding EGFET sensing behaviours, such as sensitivity, hysteresis, and long-term stability, are electrically evaluated under various measurement conditions. Physical analysis is also performed to characterize the surface conditions of the UVO-treated sensing films using X-ray photoelectron spectroscopy and atomic force microscopy. It is found that UVO treatment effectively reduces the buried sites in the Al2O3 sensing film and subsequently results in reduced hysteresis and improved long-term stability of EGFET. Meanwhile, the observed slightly smoother Al2O3 film surface post UVO treatment corresponds to decreased surface sites and slightly reduced pH sensitivity of the Al2O3 film. The sensitivity degradation is found to be monotonically correlated with the UVO treatment time. A treatment time of 10 min is found to yield an excellent performance trade-off: clearly improved long-term stability and reduced hysteresis at the cost of negligible sensitivity reduction. These results suggest that UVO treatment is a simple and facile method to improve the overall sensing performance of the EGFETs with an Al2O3 sensing film.

Ag metal mid layer based on new sensing multilayers structure extended gate field effect transistor (EG-FET) for pH sensor

This study focus on the effectiveness of ZnO/Ag/ZnO (ZAZ) multilayers films thickness on electrical properties of pH sensor and structural properties such as XRD, AFM, the multilayers structure ZnO/Ag/ZnO deposit on glass substrate by using RF and DC magnetron sputtering system at different thickness (200/100/200) nm, (100/50/100) nm, XRD results indicated that the multilayers have polycrystalline phase for Ag metal. ZnO/Ag/ZnO was tested as EGFET for the first time the effect of multilayers thickness on chemical sensitivity, linearity and hysteresis voltage were demonstrated and found 0.62μA1/2/pH, 99.92%, 5.4mV for multilayer thickness (200/100/200) nm while 0.25 μA1/2/pH, 97%, 3.2mV at multilayers thickness (100/50/100) nm respectively.

Sensing properties of separative paper-based extended-gate ion-sensitive field-effect transistor for cost effective pH sensor applications

In this study, we developed a cost-effective ion-sensing field-effect transistor (FET) with an extended gate (EG) fabricated on a separative paper substrate. The pH sensing characteristics of the paper EG was compared with those of other EGs fabricated on silicon, glass, or polyimide substrates. The fabricated paper-based EGFET exhibited excellent sensitivity close to the Nernst response limit as well as to that of the other substrate-based EGFETs. In addition, we found that all EGFETs, regardless of the substrate, have similar non-ideal behavior, i.e., drift phenomenon and hysteresis width. To investigate the degradation and durability of the paper EG after prolonged use, aging-effect tests were carried out in terms of the hysteresis width and sensitivity over a course of 30 days. As a result, the paper EG maintained stable pH sensing characteristics after 30 days. Therefore, we expect that paper EGFETs can provide a cost-effective sensor platform.

Extended Gate Field Effect Transistor Using GaN/Si Hybrids Nanostructures for pH Sensor

The pH sensor of an extended gate field effect transistor (EGFET) with gallium nitride/silicon hybrid nanostructure is fabricated and analyzed. Si nanowires (NWs) are fabricated via the Ag-assisted electroless etching technique and are then covered by GaN NWs through plasma-enhanced chemical vapor deposition (PECVD). The GaN nanostructure is synthesized by introducing gallium oxide (Ga2O3) and nitrogen (N2) for the growth of NWs. The GaN nanowires supply a larger surface area than that of the pristine Si NWs, where there is a better sensitivity for pH sensor. The GaN/Silicon hybrid sensors exhibit a sensitivity higher (50.4[Formula: see text]mV/pH) than that of pristine Si NWs sensors (41.2[Formula: see text]mV/pH). This GaN/Si hybrid pH sensor prepared by simple and low-cost method may be potentially applied for cheap biosensor devices.

Direct correlation between potentiometric and impedance biosensing of antibody-antigen interactions using an integrated system

A fully integrated system that combines extended gate field-effect transistor (EGFET)-based potentiometric biosensors and electrochemical impedance spectroscopy (EIS)-based biosensors has been demonstrated. This integrated configuration enables the sequential measurement of the same immunological binding event on the same sensing surface and consequently sheds light on the fundamental origins of sensing signals produced by FET and EIS biosensors, as well as the correlation between the two. Detection of both the bovine serum albumin (BSA)/anti-BSA model system in buffer solution and bovine parainfluenza antibodies in complex blood plasma samples was demonstrated using the integrated biosensors. Comparison of the EGFET and EIS sensor responses reveals similar dynamic ranges, while equivalent circuit modeling of the EIS response shows that the commonly reported total impedance change (ΔZtotal) is dominated by the change in charge transfer resistance (Rct) rather than surface capacitance (Csurface). Using electrochemical kinetics and the Butler-Volmer equation, we unveil that the surface potential and charge transfer resistance, measured by potentiometric and impedance biosensors, respectively, are, in fact, intrinsically linked. This observation suggests that there is no significant gain in using the FET/EIS integrated system and leads to the demonstration that low-cost EGFET biosensors are sufficient as a detection tool to resolve the charge information of biomolecules for practical sensing applications.

Influence of CuS membrane annealing time on the sensitivity of EGFET pH sensor

In this study, CuS membranes were deposited on glass substrates using spray pyrolysis, and then annealed at 150°C for durations of 5, 15 and 30min. The precursor materials for the synthesis of the CuS membranes include copper chloride and sodium thiosulfate dissolved in deionized water. 0.4M concentration of the mixed solution was subsequently sprayed on the glass substrates at 200°C. Afterwards, the as-deposit membranes were implemented as extended gate field effect transistors (EGFET) for pH sensors. The structure, morphology, resistance, sensitivity and linearity of the membranes were determined. The structural characteristics showed that the maximum average grain size of the CuS membrane was 24.2nm after 30min of annealing. The sensitivity of the membranes was measured in buffer solutions of pH 2, 4, 6, 8, 10 and 12. The sensitivity increased with annealing time until maximum values of 31.5µA/pH and 27.8mV/pH were attained at 30min, with minimum resistance of 400Ω and minimum hysteresis of 11.4mV. Thus, the ideal annealing time for optimum sensitivity, resistance and hysteresis is 30min.

Mechanisms of Ion Detection for FET-Sensors Using FTO: Role of Cleaning Process, pH Sequence and Electrical Resistivity

The use of FTO samples as an extended gate field effect transistor biosensor is presented. The FTO samples were produced by spray pyrolysis technique. The cleaning process is shown to have a fundamental importance for the final sensitivity of the samples when multiple re-usage is adopted. The role of electrical resistivity and morphology of the films are investigated. The influence of pH sequence of measurements from 2 to 12 is presented. Both increasing and decreasing the pH values sequence of measurements are compared. Electrical, morphological, time evolution and electrochemical experiments are correlated in the main discussion. A physical-chemical model is presented to explain the main mechanisms of charge adsorption and desorption. Parameters not commonly reported in the literature are proven to have fundamental importance in sensors behavior and characterization.

Core-Shell P-N Junction Si Nanowires as Rapid Response and High-Sensitivity pH Sensor

High density, vertical well-aligned p-type, n-type, and core-shell p-n homojunction Si nanowires were synthesized on p-Si substrate. The morphology, crystalline structure, composition, and doping ratio of these Si nanowires were investigated via field emission scanning electron microscope, Raman spectra, and X-ray photoelectron spectroscopy. The pH sensors were fabricated with the typical extended-gate field-effect transistor method. The core-shell p-n junction Si nanowires samples had the best performance for pH sensitivity at 68 mV/pH compared with the p-Si NWs and n-Si NWs pH sensors. Furthermore, the pH sensing current of the core-shell p-n junction Si NWs sensor featured rapid response, good repeatability, and stability. The built-in potential $V_{bi}$ of the core-shell p-n homojunction Si nanowires is ~0.78 V, which improved the pH sensitivity.

Characteristics of Extended-Gate Field-Effect Transistor (EGFET) Based on Porous n-Type (111) Silicon for Use in pH Sensors

Following the advances in pH sensors based on porous silicon (p-Si) in the late 20th century, several studies have been carried out to take advantage of the intrinsic properties of p-Si for development of chemical sensors. This study investigates the characteristics and pH sensitivity of an extended-gate field-effect transistor (EGFET) based on n-type p-Si with (111) orientation. Porous silicon was applied directly without coating. The x-ray diffractogram revealed only n-type (111) crystal orientation. p-Si was comparatively analyzed against a silicon wafer (flat and porous surface) in the pH range from 2 to 12. Regarding EGFET operation, p-Si exhibited significantly enhanced pH sensitivity of 56.13 mV/pH and linearity of 0.9857 (at drain–source current I DS of 0.1 mA, temperature of 300 K, and immersion time of 300 s) because of its high surface area, whereas the silicon wafer (flat and porous surface) exhibited comparatively poor sensitivity of 25.41 mV/pH and linearity of 0.99 under similar conditions. In addition, we demonstrate use of current as a second parameter with high linearity for pH sensing. The low hysteresis depth (9 mV) of the EGFET sensor based on p-Si indicates good stability and reversibility.

Multilayer ZnO/Pd/ZnO Structure as Sensing Membrane for Extended-Gate Field-Effect Transistor (EGFET) with High pH Sensitivity

Metal oxide nanostructures have attracted considerable attention as pH-sensitive membranes because of their unique advantages. Specifically, the special properties of ZnO thin film, including high surface-to-volume ratio, nontoxicity, thermal stability, chemical stability, electrochemical activity, and high mechanical strength, have attracted massive interest. ZnO exhibits wide bandgap of 3.37 eV, good biocompatibility, high reactivity, robustness, and environmental stability. These unique properties explain why ZnO has the most applications among all nanostructured metal oxides based on its structure and properties. Moreover, ZnO has excellent electrical characteristics, enabling its use in accurate sensors with rapid response. ZnO nanostructures can be used in novel pH and biomedical sensing applications. However, ZnO thin film exhibits large sheet resistance and low conductivity. Increasing the conductivity or reducing the resistivity of ZnO sensing membranes is important to achieve low impedance. We propose herein a new design using a multilayer ZnO/Pd/ZnO structure as a pH-sensing membrane. Multiple layers were deposited by radio frequency (RF) sputtering for ZnO and direct current (DC) sputtering for Pd to achieve low sheet resistance. These multilayers with low sheet resistance of 15.8 Ω/sq were then successfully used to control the conductivity in extended-gate field-effect transistors (EGFETs). The resulting multilayered EGFET pH-sensor demonstrated improved sensing performance. The measured sensitivity of the pH sensor was 40 μA/pH and 52 mV/pH within the pH range from 2 to 12, rendering this structure suitable for use in various applications, including pH sensors and biosensors.

(Invited) Application of Extended-Gate Field-Effect Transistor Sensors with Molecularly Imprinted Polymer Recognition Layers for Determination of Renal Dysfunction Biomarkers

In this presentation, we will discuss development of molecularly imprinted polymer (MIP) films selective towards some prospective biomarkers of renal dysfunction, namely, inosine and lipocaline-2. This artificial recognition units can easily be integrated with extended gate-field effect transistors (EG-FETs) allowing for development of selective and robust chemosensors for the target analytes [1, 2]. For the last decade, chemically-sensitive field-effect transistors (ChemFETs) have been applied as transduction units in chemosensors. They offer numerous advantages, such as, ease of miniaturization, possibility of high-throughput sensing, high sensitivity, important detection limit, and possibility to adjust sensitivity according to specific needs. Various reports have described great variety of such devices for determination of different analytes of interest, ranging from ions, gases, small molecules, macromolecules and even whole viruses [3, 4]. Selectivity of the devised chemo- and biosensors is typically introduced by modifying gate region of the FET with suitable recognition units selective towards the analyte. In this regard, MIPs are attractive artificial recognition units offering good sensitivity and selectivity combined with robustness and resistance to harsh experimental conditions [5]. Here, we have made an attempt to combine the selectivity and robustness of MIPs with sensitivity and amplification features of EG-FET transducers to devise sensitive and selective chemosensors for selected bioanalytes. Bibliography Z. Iskierko et al., Biosens. Bioelectron., 74, 526, (2015).Z. Iskierko et al. ACS Appl. Mater. Interfaces, 8, 19860, (2016).Y. Cheng, et al., Biosens. Bioelectron., 26, 4538, (2011).F. Patolsky, et al., Proc. Natl. Acad. Sci. U.S.A., 101, 14017 (2004).P. S. Sharma, et al. Trends Ana l. Chem., 51, 146, (2013). Figure 1

Development of paper-gate transistor toward direct detection from microbiological fluids

In this study, a paper-gate transistor was developed to detect glucose using an extended-gate field-effect transistor (FET). A filter paper was used as an extended gate electrode, in which Au nanoparticles (AuNPs) modified with phenylboronic acids (PBAs) were included. PBA–AuNPs play an important role as a support to not only be entrapped in cellulose fibrils but also bind to the targeted glucose in a paper. The surface properties of PBA–AuNPs were investigated to elucidate the electrical properties of the paper-gate electrode using an absorption spectrum and a zeta potential analysis. Moreover, the paper-gate electrode enabled us to detect glucose at the micromolar level on the basis of the principle of FET devices. A platform based on the paper-gate transistor is suitable for a highly sensitive system to detect glucose in trace samples such as tears, sweat, and saliva in the future.

Hierarchical templating in deposition of semi-covalently imprinted inverse opal polythiophene film for femtomolar determination of human serum albumin

Nanostructured artificial receptor materials with unprecedented hierarchical structure for determination of human serum albumin (HSA) are designed and fabricated. For that purpose a new hierarchical template is prepared. This template allowed for simultaneous structural control of the deposited molecularly imprinted polymer (MIP) film on three length scales. A colloidal crystal templating with optimized electrochemical polymerization of 2,3′-bithiophene enables deposition of an MIP film in the form of an inverse opal. Thickness of the deposited polymer film is precisely controlled with the number of current oscillations during potentiostatic deposition of the imprinted poly(2,3′-bithiophene) film. Prior immobilization of HSA on the colloidal crystal allows formation of molecularly imprinted cavities exclusively on the internal surface of the pores. Furthermore, all binding sites are located on the surface of the imprinted cavities at locations corresponding to positions of functional groups present on the surface of HSA molecules due to prior derivatization of HSA molecules with appropriate functional monomers. This synergistic strategy results in a material with superior recognition performance. Integration of the MIP film as a recognition unit with a sensitive extended-gate field-effect transistor (EG-FET) transducer leads to highly selective HSA determination in the femtomolar concentration range.

Zinc Oxide Nanorods Grown on Printed Circuit Board for Extended-Gate Field-Effect Transistor pH Sensor

Zinc oxide (ZnO) nanorods (NRs) were grown directly on printed circuit boards with a 35-μm-thick copper layer using a seedless galvanic-cell hydrothermal process. The hexagonal structure of the synthesized ZnO NRs was observed by scanning electron microscopy. The microstructural characteristics of the as-grown ZnO NRs were investigated by x-ray diffraction analysis, revealing preferred (002) growth direction. Raman and photoluminescence spectra confirmed the high crystalline quality of the ZnO NRs. As-grown ZnO NRs were then grown for 7 h using the galvanic effect for use as the pH membrane of an extended-gate field-effect transistor pH sensor (pH-EGFET). The current–voltage characteristics showed sensitivity of 15.4 mV/pH and 0.26 (μA)1/2/pH in the linear and saturated region, respectively. Due to their cost effectiveness, low-temperature processing, and ease of fabrication, such devices are potential candidates for use as flexible, low-cost, disposable biosensors.

Extended–gate field effect transistor (EGFET) for carbaryl pesticide detection based on enzyme inhibition assay

An extended – gate filed effect transistor (EGFET) is a device which can be applied to chemical sensing devices. In this study, EGFET was constructed by using an indium tin oxide (ITO) glass substrate. The small piece ITO with sensing area around 3x3 mm2 was used as carbaryl pesticide sensor based on enzyme inhibition assay. The prepared ITO-EGFET sensors were test with pH solutions to obtain sensitivity compared with commercial ion sensitive field effect transistor (ISFET). Preparation of ITO substrate for enzyme inhibition assay by dropped acetylcolinesterase enzyme at 0.5 Unit which were trapped in an agarose gel matrix onto substrates. This prepared ITO acts as extended gate sensing membrane for EGFET. The amounts of drop volume of enzyme were varied to obtain the highest signal for reaction with acetylcholine substrate. Cabaryl can inhibit enzyme function resulting in the decreasing signal of interaction between enzyme and acetylcholine substrate. This ITO-EGFET can be used to detect carbaryl in concentration rage 0.001 – 1 mM by using the ITO-EGFET without enzyme as control condition. This work shows possibility of ITO-EGFET for application in pesticide detection with an easy substrate preparation, low cost and can be applied in field used with others pesticides.

Extended Gate Field-Effect Transistor Biosensors for Point-Of-Care Testing of Uric Acid.

An enzyme-free redox potential sensor using off-chip extended-gate field effect transistor (EGFET) with a ferrocenyl-alkanethiol modified gold electrode has been used to quantify uric acid concentration in human serum and urine. Hexacyanoferrate (II) and (III) ions are used as redox reagent. The potentiometric sensor measures the interface potential on the ferrocene immobilized gold electrode, which is modulated by the redox reaction between uric acid and hexacyanoferrate ions. The device shows a near Nernstian response to uric acid and is highly specific to uric acid in human serum and urine. The interference that comes from glucose, bilirubin, ascorbic acid, and hemoglobin is negligible in the normal concentration range of these interferents. The sensor also exhibits excellent long term reliability and is regenerative. This extended gate field effect transistor based sensor is promising for point-of-care detection of uric acid due to the small size, low cost, and low sample volume consumption.

A 7 μW Offset- and Temperature-Compensated pH-to-Digital Converter

This paper demonstrates a micropower offset- and temperature-compensated smart pH sensor, intended for use in battery-powered RFID systems that monitor the quality of perishable products. Low operation power is essential in such systems to enable autonomous logging of environmental parameters, such as the pH level, over extended periods of time using only a small, low-cost battery. The pH-sensing element in this work is an ion-sensitive extended-gate field-effect transistor (EGFET), which is incorporated in a low-power sensor front-end. The front-end outputs a pH-dependent voltage, which is then digitized by means of a co-integrated incremental delta-sigma ADC. To compensate for the offset and temperature cross-sensitivity of the EGFET, a compensation scheme using a calibration process and a temperature sensor has been devised. A prototype chip has been realized in a 0.16 μm CMOS process. It occupies 0.35 × 3.9 mm2 of die area and draws only 4 μA from a 1.8 V supply. Two different types of custom packaging have been used for measurement purposes. The pH sensor achieves a linearity of better than ±0.1 for pH values ranging from 4 to 10. The calibration and compensation scheme reduces errors due to temperature cross-sensitivity to less than ±0.1 in the temperature range of 6°C to 25°C.