Extended-gate Field-effect Transistor pH Sensors Research Articles

Performance Improvement of a ZnGa2O4 Extended-Gate Field-Effect Transistor pH Sensor.

ZnGa2O4 sensing films were prepared using an RF magnetron sputtering system and connected to a commercial metal oxide semiconductor field-effect transistor (MOSFET) as the extended-gate field-effect transistor (EGFET) to detect pH values. Experimental parameters were adjusted by varying the oxygen flow rate in the process chamber to produce ZnGa2O4 sensing films with different oxygen ratios. These films were then treated in a furnace tube at an annealing temperature of 700 °C. The sensitivity and linearity of the constant current mode and the constant voltage mode were measured and analyzed in the pH range of 2-12. Under the deposition conditions with an oxygen ratio of 6%, the sensitivity reached 23.14 mV/pH and 33.49 μA/pH, with corresponding linearity values of 92.1 and 96.15%, respectively. Finally, the sensing performance of the ZnGa2O4 EGFET pH sensor with and without annealing processes was analyzed and compared.

Structural properties and sensing performance of Ni–Ti alloy films for extended-gate field-effect transistor pH sensors

This study presents a facile Ni–Ti alloyed film grown on the n+-type Si through a DC sputtering technique for electrochemical pH sensing and its comprehensive structural, morphological, compositional, profiling, microstructural, and elemental characterization. In addition to pH sensing measurements, the properties of the sensing layer were analyzed using X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy analysis. The impact of rapid thermal annealing (RTA) treatment (500–700 °C) on the alloyed Ni–Ti sensitive films was extensively studied. The structural feature of the alloyed Ni–Ti sensitive films is more closely related to their relative sensing performances. The performance of the alloyed Ni–Ti sensitive film at the RTA temperature of 600 °C showed a higher pH sensitivity of 60.73 mV/pH in a wide pH range of 2–12 in comparison with other RTA temperatures. Moreover, the stability of this 600 °C-annealed EGFET sensor exhibited a small drift rate of 0.21 mV/h in pH 7 and a low hysteresis voltage of 1 mV in a loop of pH 7 → 4→7 → 10→7. The alloyed Ni–Ti sensitive film is fully compatible with a standard CMOS process and demonstrates better sensing performance than other metal nitride sensitive films (e.g. RuN, TiN), suggesting it may be a strong candidate for future sensor and biosensor applications.

Impact of rapid thermal annealing on structural and sensing properties of Ni–Ti sensitive films for extended-gate field-effect transistor pH sensors

In the present work, a facile Ni–Ti film deposited on n+-type Si substrate is introduced as a sensitive film for the development of a solid-state extended-gate field-effect transistor (EGFET) pH sensor. Influence of rapid thermal annealing (RTA) treatment (500–700 °C) on structural properties of the Ni–Ti sensitive films was extensively investigated. The film structures, surface morphologies, element profiles, film microstructures, and chemical compositions of the Ni–Ti sensitive films were characterized by a combination of X-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy, respectively. The structural features of the Ni–Ti sensitive films are most closely related to their sensing characteristics. The experimental results indicated that the Ni–Ti sensitive film annealed at the RTA temperature of 600 °C exhibited better sensing performances (a higher pH sensitivity of 71.01 mV/pH, a lower hysteresis voltage of 1 mV and a smaller drift rate of 0.15 mV/h) than other RTA temperatures. The proposed Ni–Ti sensitive film is expected to be effectively useful for various long-term pH sensor and biosensor applications, since it satisfies the typically required sensing performances such as sensitivity and stability.

Structural properties and sensing performance of TaOx/Ta stacked sensing films for extended-gate field-effect transistor pH sensors

In this article, the TaOx/Ta stacked sensing films as sensing membranes were deposited onto n+-type Si substrate by a radio-frequency (RF) magnetron sputtering technique for solid-state extended-gate field-effect transistor (EGFET) pH sensors. The effect of rapid thermal annealing (RTA) treatment (300–700 °C) on structural properties of the TaOx/Ta stacked sensing films was investigated in detail. The chemical compositions, element profiles, crystallographic structures, surface morphologies, and film microstructures of the TaOx/Ta stacked sensing films were characterized by X-ray photoelectron spectroscopy, Auger electron spectroscopy, X-ray diffraction, atomic force microscopy, and transmission electron microscopy, respectively. The sensing performance of the TaOx/Ta stacked sensing films is most closely correlated with their relative structural features. Among these RTA temperatures, the best sensing performance including pH sensitivity, hysteresis and drift was achieved at the RTA temperature of 500 °C. The obtained experimental results showed that the TaOx/Ta stacked sensing film is considered a feasible alternative for pH sensor and biosensor applications.

Fabrication and Characterization of In0.9Ga0.1O EGFET pH Sensors

In this study, the In0.9Ga0.1O sensing membrane were deposited by using the RF magnetron sputtering at room temperature and combined with commercial MOSFETs as the extended gate field effect transistor (EGFET) pH sensors. The sensing performance of the In0.9Ga0.1O EGFET pH sensors were measured and analyzed in the pH value of range between 2 to 12. In the saturation region, the pH current sensitivity calculated from the linear relationship between the IDS and pH value was approximately 56.64 μA/pH corresponding to the linearity of 97.8%. In the linear region, the pH voltage sensitivity exhibited high sensitivity and linearity of 43.7 mV/pH and 96.3%, respectively. The In0.9Ga0.1O EGFET pH sensors were successfully fabricated and exhibited great linearity. The analyzed results indicated that the In0.9Ga0.1O was a robust material as a promising sensing membrane and effectively used for pH sensing detection application.

Structural properties and sensing performances of CoNxOy ceramic films for EGFET pH sensors

This paper describes the impact of both Ar/N2 flow rate and rapid thermal annealing (RTA) on the structural properties and sensing characteristics of CoNxOy ceramic films formed through reactive sputtering and then used in extended-gate field-effect transistor (EGFET) pH sensors. The CoNxOy ceramic films were prepared under three different Ar/N2 flow rates (20/5, 20/10, and 20/15) and then annealed at three different RTA temperatures (200, 300, and 400 °C). X-ray diffraction revealed that all of the CoNxOy ceramic films featured amorphous structures. X-ray photoelectron spectroscopy demonstrated that the Co3+ content of the CoNxOy film prepared at the 20/10 flow rate was higher than those of the films prepared at the other flow rates. Atomic force microscopy indicated that the surface roughnesses of the CoNxOy films obtained at flow rates of 20/10 and 20/15 were higher than that of the film prepared at 20/5. High pH-sensitivity (64.36 mV/pH), a small drift rate (0.27 mV/h), and a low hysteresis voltage (1.4 mV) were achieved for the CoNxOy EGFET sensor that had been fabricated at a flow rate of 20/10 with subsequent RTA at 300 °C. This high performance was due to its CoNxOy film having a rough surface, a high Co3+ ion content, and a low number of oxygen vacancies or defects.

Enhancing pH Sensors Performance of ZnO Nanorods With Au Nanoparticles Adsorption

In this work, the sensing membrane of extended-gate field-effect-transistor (EG-FET) sensor with zinc oxide (ZnO) nanorods (NRs) and Au adsorbed on ZnO NRs were fabricated by hydrothermal method and DC magnetron sputtering. The Pt sheet as a reference electrode, put in sensing membrane and reference electrode to defferent pH valuse buffer solution for linearity and sensitivity analysis. Results showed the both of the ZnO based EG-FET pH sensors exhibited high linearity and sensitivity, and Au-adsorbed exhibited significantly improved sensing performance, the average voltage sensitivity and average current sensitivity of the ZnO NRs based EG-FET sensor was 24.67 mV/pH and <inline-formula> <tex-math notation="LaTeX">$21.4~\mu \text{A}$ </tex-math></inline-formula>/pH, and linearity was 0.986 and 0.994, whereas that of the Au-adsorbed ZnO NRs based EG-FET pH sensor was 76.75 mV/pH and <inline-formula> <tex-math notation="LaTeX">$45.66~\mu \text{A}$ </tex-math></inline-formula>/pH with a linearity of 0.990 and 0.980.

High Performance NiOx Extended-Gate Field-Effect Transistor Biosensor for Detection of Uric Acid

In this paper, we developed a high performance NiOx extended-gate field-effect transistor (EGFET) biosensor for detection of uric acid. The structural and sensing properties of the NiOx sensing film deposited on a n+-type Si substrate was examined for an EGFET pH sensor. X-ray diffraction, atomic force microscope and X-ray photoelectron spectroscopy were used to analyze the film features of the NiOx sensing film. The NiOx sensing film based on EGFET exhibited a high pH sensitivity of 58.53 mV pH−1, a small hysteresis voltage of 1.4 mV and a low drift rate of 0.30 mV h−1. Moreover, the NiOx EGFET biosensor showed a high linearity in the uric acid range between 1 and 30 mg dl−1. In addition, this NiOx EGFET biosensor demonstrated a very good selectivity to uric acid over other interfering substrates (ascorbic acid, glucose, urea).

Optimizing the performance of extended-gate field-effect transistor (EGFET) pH sensor by regulating the structural properties of the nanostructured porous silicon layer

We report the performance of extended-gate field-effect transistor (EGFET) pH sensors based on the properties of nanostructured porous silicon layer (nano PSi). Four different forms of nano PSi layers with different characteristic features are used as a sensor for determining the pH level. The synthesized nano PSi layer was prepared by laser-assisted etching of n-type (100) by using different illumination intensities 15, 30, 45 and 60 mW/cm2 of 530 nm laser wavelength. The structural and morphological properties of nano PSi were investigated through exploring scanning electron microscopy, X-ray diffraction (XRD) and gravimetrical measurements. Nano PSi with pore-like structures of various pore shapes and sizes are as follows: fine circular pores, star full pore, circular pores and rectangular pores; star full pore shape was prepared through varying the laser illumination intensity. The Fourier transform infrared spectroscopy, current–voltage measurements and specific surface area of were employed to describe the density of the hydrogenated dangling bonds (Si–Hx) of the nano PSi layer. The synthesized nano PSi pH sensors were tested and dry out in the pH range from 3 to 11 at room temperature. The performance of nano PSi pH sensors was comparatively analyzed based on the density of (Si–Hx), specific surface area, porosity and layer thickness of nano PSi. Concerning EGFET pH sensor showed noticeably enhanced pH sensitivity and linearity with increasing the density of (Si–Hx) and specific surface area. Higher sensitivity with excellent linearity was obtained for pore-like structures of circular pore shapes when the laser illumination intensity was about 45 mW/cm2.

Thermal evaporation based V2O5 thin film for extended gate field effect transistor pH sensor

In this work, vanadium pentoxide (V2O5) thin film was synthesized on the corning glass substrate by using thermal evaporation method. For this purpose, 99.9% pure V2O5 powder was used that was evaporated from a tungsten boat. Subsequently, the prepared film was used as pH sensor in the extended gate field-effect transistor (EGFET) system. Structural properties, morphological features, sensing performance and linearity measurements of synthesized film were studied. The structural properties indicated that the V2O5 thin film has the preferred orientation along (001) plane. The pH-EGFET sensor based on V2O5 thin film was fabricated. The sensitivity and linearity of the device were determined in the pH range of 2–12 of the buffer solution. The sensitivity and linearity values were found to be 41.7 mV pH−1 and 83% respectively in the linear region. The results of present study show that the V2O5 can act as promising material for the EGFET PH sensor.

Hydrogen ion sensing characteristics of Na3BiO4–Bi2O3 mixed oxide nanostructures based EGFET pH sensor

In the present paper, Na3BiO4–Bi2O3 films have been tested for hydrogen ion sensing. Na3BiO4–Bi2O3 mixed oxide nanostructures were deposited on the Indium–Tin-Oxide (ITO) coated glass substrate using a low-cost electrodeposition technique at room temperature. The nanostructures have been characterized using FESEM and XPS to study their morphology and composition, respectively. Vertically aligned nanostructures (thickness~90 nm) with well-defined edges were seen in FESEM studies. XPS analysis indicates the presence of Na3BiO4 and Bi2O3. The crystallinity and the mixed phase of the film were further confirmed by X-ray diffraction study. These nanostructures were explored as a potential candidate for pH sensing using them as a sensing film for Extended-Gate Field-Effect Transistor (EGFET). A sensitivity of 49.63 mV/pH has been observed with good linearity. To the best of our knowledge, for the first time vertically aligned Na3BiO4–Bi2O3 mixed oxide nanostructures are demonstrated as an EGFET-based (Hydrogen ion) pH sensor.

An Extended-Gate FET-Based pH Sensor With an InZnxOyMembrane Fabricated on a Flexible Polyimide Substrate at Room Temperature

In this letter, a room-temperature synthesized amorphous indium-zinc oxide (InZn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> ) sensing membrane on flexible polyimide is employed as an extended-gate field-effect transistor (EGFET) pH sensor for the first time. The InZn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> -based EGFET sensor exhibited a high pH sensitivity of 56.29 mV/pH with an excellent linearity of 0.999. Furthermore, it also demonstrated good stability in terms of a low hysteresis voltage of 4.6 mV and a small drift rate of 2.08 mV/h, presumably suggesting the formation of uniform, dense, small grains, and stoichiometric InZn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> thin film, which facilitates high conductivity and provides a large number of surface binding sites. The InZn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> -based EGFET pH sensor showed an excellent mechanical bendability after more than 500 bending cycles.

Fabrication of integrated solid state electrode for extended gate-FET pH sensor

To realize the miniaturization of an extended gate field effect transistor (EGFET) based sensor, an all solid state phase electrodes are needed. This paper presents the facile fabrication of an integrated solid state electrode (ISSE) consisting a solid state silver/silver chloride (Ag/AgCl) reference electrode (RE) and ZnO nanostructures sensing electrode (SE) on a single 2 × 2 cm2 indium tin oxide (ITO) substrate. The RE and SE was fabricated by thermal evaporation (TE) and simple immersion process respectively. The ISSE was then tested by applying it to an EGFET pH sensor setup and compared with commercialized Ag/AgCl RE; and it was found that the sensor sensitivity for both electrodes are comparable (commercial RE: 52.4 mV pH−1, ISSE: 53.4 mV pH−1).

Thermal Chemically Deposited Zno Nanostructures: Influence of Post-Deposition Annealing Temperature on Ph Sensor Performance

Zinc oxide (ZnO) nanostructures were synthesized via thermal chemical deposition (TCVD) method and applied as the sensing membrane of an extended-gate field effect transistor (EGFET) pH sensor. The ZnO nanostructures undergone a post-deposition heat treatment with the temperature ranging from 200-500 °C for 15 min in air ambient. The influence of the post-deposition heat treatment on the physical and pH sensing characteristics was investigated. The FESEM images showed that the surface morphology of the samples were dependent on post-annealing treatment. The as-deposited sample showed the biggest grain size and non-uniformed structures. Applying the post-annealing heat treatment caused the surface morphology to become denser and uniform. The most ideal temperature for the heat treatment for EGFET pH sensor application was found at 300 °C resulting in a pH sensitivity of 48.2 mV/pH with 0.9646 linearity.

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.

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.

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.

METAL OXIDE THIN FILMS AS PH SENSING MEMBRANE FOR EXTENDED GATE FIELD EFFECT TRANSISTOR

In this research, metal oxides (ZnO and TiO2) thin films were fabricated by the sol-gel spin coating method. The thin films were applied as the pH sensing membrane for the extended-gate field effect transistor (EGFET) sensor to distinguish the sensing capability between them. The surface morphology, thin film components and crystalline quality were characterized and the sensor performance of both materials were characterized and compared. The results showed that TiO2 thin film gave higher sensitivity with better linearity compared to the ZnO thin films hence was considered a more suitable material to be used as sensing membrane in EGFET pH sensor compared to zinc oxide.

Spin Speed and Duration Dependence of TiO2Thin Films pH Sensing Behavior

Titanium dioxide (TiO2) thin films were applied as the sensing membrane of an extended-gate field-effect transistor (EGFET) pH sensor. TiO2thin films were deposited by spin coating method and the influences of the spin speed and spin duration on the pH sensing behavior of TiO2thin films were investigated. The spin coated TiO2thin films were connected to commercial metal-oxide-semiconductor field-effect transistor (MOSFET) to form the extended gates and the MOSFET was integrated in a readout interfacing circuit to complete the EGFET pH sensor system. For the spin speed parameter investigation, the highest sensitivity was obtained for the sample spun at 3000 rpm at a fixed spinning time of 60 s, which was 60.3 mV/pH. The sensitivity was further improved to achieve 68 mV/pH with good linearity of 0.9943 when the spin time was 75 s at the speed of 3000 rpm.

EGFET pH Sensor Performance Dependence on Sputtered TiO2Sensing Membrane Deposition Temperature

Titanium dioxide (TiO2) thin films were sputtered by radio frequency (RF) magnetron sputtering method and have been employed as the sensing membrane of an extended gate field effect transistor (EGFET) for pH sensing detection application. The TiO2thin films were deposited onto indium tin oxide (ITO) coated glass substrates at room temperature and 200°C, respectively. The effect of deposition temperature on thin film properties and pH detection application was analyzed. The TiO2samples used as the sensing membrane for EGFET pH-sensor and the current-voltage (I-V), hysteresis, and drift characteristics were examined. The sensitivity of TiO2EGFET sensing membrane was obtained from the transfer characteristic (I-V) curves for different substrate heating temperatures. TiO2thin film sputtered at room temperature achieved higher sensitivity of 59.89 mV/pH compared to the one deposited at 200°C indicating lower sensitivity of 37.60 mV/pH. Moreover the hysteresis and the drift of TiO2thin film deposited at room temperature showed lower values compared to the one at 200°C. We have also tested the effect of operating temperature on the performance of the EGFET pH-sensing and found that the temperature effect was very minimal.