Extended-gate Field-effect Transistor Research Articles

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.

Molecularly imprinted polymer based extended-gate field-effect transistor chemosensors for phenylalanine enantioselective sensing

Chemosensing systems were devised for the enantioselective determination ofd- andl-phenylalanine (d- andl-Phe).

Comparative analysis of static and non-static assays for biochemical sensing using on-chip integrated field effect transistors and solidly mounted resonators

The advancement in micro/nanotechnologies has been continuously providing possibilities for inventing novel biochemical sensors. However, variations in the transducer type can cause different sensing results due to the differences in their mechanisms of analyzing biomolecular interactions. In this work, we focused on the comparative analysis of static and non-static assays for molecular interactions using on-chip integrated extended-gate field effect transistor (EGFET) as a static sensing interface and solidly mounted resonator (SMR) as a non-static sensing interface. Analysis of polyelectrolytes (PETs) surface assembly and antigen-antibody interaction using the two types of biochemical sensors presented consistent and complementary sets of information. Meanwhile, due to the difference in their operating mechanisms, variations on the detection efficiency, kinetics and thermodynamics were observed. Our results highlighted the critical dependence of signal detection on biochemical sensors’ operating mechanisms and provided a valuable guidance for static and non-static assays for biomolecular detections.

Hybrid Synthetic Receptors on MOSFET Devices for Detection of Prostate Specific Antigen in Human Plasma.

The study reports the use of extended gate field-effect transistors (FET) for the label-free and sensitive detection of prostate cancer (PCa) biomarkers in human plasma. The approach integrates for the first time hybrid synthetic receptors comprising of highly selective aptamer-lined pockets (apta-MIP) with FETs for sensitive detection of prostate specific antigen (PSA) at clinically relevant concentrations. The hybrid synthetic receptors were constructed by immobilizing an aptamer-PSA complex on gold and subjecting it to 13 cycles of dopamine electropolymerization. The polymerization resulted in the creation of highly selective polymeric cavities that retained the ability to recognize PSA post removal of the protein. The hybrid synthetic receptors were subsequently used in an extended gate FET setup for electrochemical detection of PSA. The sensor was reported to have a limit of detection of 0.1 pg/mL with a linear detection range from 0.1 pg/mL to 1 ng/mL PSA. Detection of 1-10 pg/mL PSA was also achieved in diluted human plasma. The present apta-MIP sensor developed in conjunction with FET devices demonstrates the potential for clinical application of synthetic hybrid receptors for the detection of clinically relevant biomarkers in complex samples.

Interdigitated Extended Gate Field Effect Transistor Without Reference Electrode

An interdigitated extended gate field effect transistor (IEGFET) has been proposed as a modified pH sensor structure of an extended gate field effect transistor (EGFET). The reference electrode and the extended gate in the conventional device have been replaced by a single interdigitated extended gate. A metal–semiconductor-metal interdigitated extended gate containing two multi-finger Ni electrodes based on zinc oxide (ZnO) thin film as a pH-sensitive membrane. ZnO thin film was grown on a p-type Si (100) substrate by the sol–gel technique. The fabricated extended gate is connected to a commercial metal-oxide–semiconductor field-effect transistor device in CD4007UB. The experimental data show that this structure has real time and linear pH voltage and current sensitivities in a concentration range between pH 4 and 11. The voltage and current sensitivities are found to be about 22.4 mV/pH and 45 μA/pH, respectively. Reference electrode elimination makes the IEGFET device simple to fabricate, easy to carry out the measurements, needing a small volume of solution to test and suitable for disposable biosensor applications. Furthermore, this uncomplicated structure could be extended to fabricate multiple ions microsensors and lab-on-chip devices.

High sensitivity extended gate effect transistor based on V2O5 nanorods

The sensing characteristics of extended gate field effect transistor (EGFET) as pH sensor based on V2O5 nanorods (NRs) was fabricated for the first time. The as-deposited V2O5 NRs onto glass substrate by using spray pyrolysis method have a diameter of 140–160 nm and a length of 300–500 nm. The V2O5 NRs based EGFET pH sensor exhibited a remarkable pH sensing performance due to the larger effective sensing surface area. The sensing sensitivity and linearity of the pH-EGFET sensor was found to be 54.9 mV/pH and 0.9859, respectively in the range of pH 2–12. V2O5 NRs sensor showed superior pH sensing stability and reliability, thereby can be considered as potential candidate for mass fabrication in disposable biosensors.

An implementation of an electronic tongue system based on a multi-sensor potentiometric readout circuit with embedded calibration and temperature compensation

We present an electronic tongue system composed of an analog front-end (AFE) with embedded calibration and temperature compensation for interfacing with an array of Ion-Sensitive FETs (ISFETs) or Enzyme – Extended Gate FETs (EEGFET). The AFE consists of a floating bridge type constant voltage constant current (CVCC) structure, and transmission gates as sensor enable. It also includes a temperature readout circuit and an instrumentation amplifier for temperature compensation. The calibration algorithm is synthesized into a Field Programmable Gate Array (FPGA) and implemented onto an Application Specific Integrated Circuit (ASIC). The system shall be used to measure the pH and the concentration of calcium ions in a solution alongside a conductometric sensor system. The ISFETs used are from ITE-Poland and shall be utilized to measure pH; meanwhile, to measure the Ca2+ concentration, EEGFETs with calcium ionophore on polyvinyl chloride (PVC) on SnO2/ITO/Glass thin film have been fabricated. The performance of the system using pH buffer solutions of pH 4–10, and CaCl2 buffer concentrations of 0.001–1M has been verified. The sensors and the AFE readout exhibited a Nernstian response (~50mV/pH) with a very stable output having a relative standard deviation of just ~−0.4%, and a Ca2+ sensitivity of 25.02mV/pCa, with a drift of −6.82mV/M-s. An on-chip version of the AFE using TSMC 0.35µm CMOS 2P4M Technology is also presented as well as the ASIC implemented on TSMC 0.18µm CMOS 1P6M technology.

Extended-Gate pH Sensors Using Self-Aligned Four-Terminal Metal Double-Gate Low-Temperature Polycrystalline-Silicon Thin-Film Transistors on Glass Substrate

High-performance polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) on glass substrate are applicable to high-performance and low-cost pH sensors. We previously reported on four-terminal (4T) self-aligned planar embedded metal double-gate (E-MeDG) low-temperature (LT) poly-Si TFTs on glass substrate. This device includes a metal top gate (TG) and a metal bottom gate (BG), which are fabricated from a sputtered tungsten film. The BG is embedded in the glass substrate through chemical mechanical polishing, and the TG is fabricated in a self-aligned manner to the BG by using back-surface exposure. The material of the gate dielectric is silicon dioxide (SiO2), which is deposited using plasma-enhanced chemical vapor deposition, and has a thickness of 75 and 150 nm against the TG and BG, respectively. The material of the channel region is poly-Si, which is fabricated using diode-pumped solid-state continuous-wave laser lateral crystallization. The TG and BG can be used as the drive and control gates, and vice versa. In this device, we were able to control the threshold voltage (Vth) by varying the voltage applied to the control gate, and we confirmed a high Vth controllability. This Vth controllability is applicable for pH sensors because voltage variation applied to the control gate due to a change in pH can be read as the Vthvariation of the TFTs. Therefore, the 4T self-aligned planar E-MeDG LT poly-Si TFT can be used as an extended-gate field-effect transistor (EGFET) to measure the voltage variation due to a change in pH. Thus, we performed an evaluation of pH sensors by using the 4T self-aligned planar E-MeDG LT poly-Si TFT as an EGFET. The evaluation of pH sensors was performed using a typical glass electrode, reference electrode, and pH buffer solution in addition to the 4T self-aligned planar E-MeDG LT poly-Si TFTs. The glass electrode was connected to the TG or BG of the TFTs. The reference electrode was connected to the electrode, which applied a constant voltage. The glass electrode and reference electrode were set in the pH buffer solution. In this situation, we evaluated the pH sensors by measuring the transfer characteristics of the TFTs by using another gate, which was not connected to the glass electrode, as the drive gate. In the first experiment, we connected the glass electrode to the BG and measured the transfer characteristics by using the TG as the drive gate against pH = 8.8 and pH = 4.2 buffer solutions under various reference electrode voltages. We confirmed that the transfer characteristics of the drive gate in the pH = 8.8 buffer solution shifted toward the positive direction against the characteristics in the pH = 4.2 buffer solution at every reference electrode voltage. The pH sensitivity calculated from the Vthvariation is 21.7 mV/pH, which is fairly consistent with the theoretical pH sensitivity of 25.5 mV/pH. In the second experiment, we connected the glass electrode to the TG and measured the transfer characteristics by using the BG as the drive gate against pH = 8.7 and pH = 4.8 buffer solutions at various reference electrode voltages. We confirmed that the transfer characteristics of the drive gate in the pH = 8.7 buffer solution shifted toward the positive direction against the characteristics in the pH = 4.8 buffer solution. This trend is the same as that of the top-gate drive TFT, but the pH sensitivity calculated from the Vthvariation is 76.9 mV/pH, which is larger than that of the top-gate drive TFT. In addition, its magnitude is found to be fairly consistent with the theoretical pH sensitivity of 88.8 mV/pH. We consider that the difference in pH sensitivity between the TG and BG drives is caused by the difference in thickness between the TG and BG SiO2 dielectric. We previously confirmed that the ratio of the Vthvariation against the control gate voltage variation is larger in the BG drive than in the TG drive. We consider that the difference in pH sensitivity reflects this phenomenon. Thus, the experimental results indicate that 4T self-aligned planar E-MeDG LT poly-Si TFTs can be successfully operated as pH sensors. In summary, we confirmed that 4T self-aligned planar E-MeDG LT poly-Si TFTs can be successfully operated as pH sensors. In addition, to improve the pH sensitivity of EGFETs, our experimental results indicate that the combination of the capacitance of the top and bottom gates is a significant parameter.

Molecularly Imprinted Polymer (MIP) Film with Improved Surface Area Developed by Using Metal–Organic Framework (MOF) for Sensitive Lipocalin (NGAL) Determination

Electropolymerizable functional and cross-linking monomers were used to prepare conducting molecularly imprinted polymer film with improved surface area with the help of a sacrificial metal-organic framework (MOF). Subsequent dissolution of the MOF layer resulted in a surface developed MIP film. This surface enlargement increased the analyte accessibility to imprinted molecular cavities. Application of the porous MIP film as a recognition unit of an extended-gate field effect transistor (EG-FET) chemosensor effectively enhanced analytical current signals of determination of recombinant human neutrophil gelatinase-associated lipocalin (NGAL).

High Sensitivity pH Sensor Based on Porous Silicon (PSi) Extended Gate Field-Effect Transistor

In this study, porous silicon (PSi) was prepared and tested as an extended gate field-effect transistor (EGFET) for pH sensing. The prepared PSi has pore sizes in the range of 500 to 750 nm with a depth of approximately 42 µm. The results of testing PSi for hydrogen ion sensing in different pH buffer solutions reveal that the PSi has a sensitivity value of 66 mV/pH that is considered a super Nernstian value. The sensor considers stability to be in the pH range of 2 to 12. The hysteresis values of the prepared PSi sensor were approximately 8.2 and 10.5 mV in the low and high pH loop, respectively. The result of this study reveals a promising application of PSi in the field for detecting hydrogen ions in different solutions.