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. 2024 Mar 20;9(13):15304–15310. doi: 10.1021/acsomega.3c09965

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

Chia-Hsun Chen †, Shu-Bai Liu ‡, Sheng-Po Chang §,*
PMCID: PMC10993268  PMID: 38585084

Abstract

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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.

Introduction

In recent years, pH sensors have garnered significant interest across various fields, such as chemical and biological applications,1 clinical detection,2 and environmental analysis.3 Continuous monitoring of health conditions is of paramount importance, with applications ranging from early diagnosis, including noninvasive glucose monitoring with contact lenses,4 to pH value detection on tooth surfaces for caries diagnosis.5,6 In pursuit of these diverse sensing applications, various materials, including silicon nitride (Si3N4),7 titanium oxide (TiO2),8 and zinc oxide (ZnO),9 have been employed and fabricated as sensing films for detecting H+ and OH– ions. However, among these materials, the wide-band-gap material ZnGa2O4, introduced in this work, offers several distinct advantages. Notably, it exhibits high chemical stability and functions as a transparent conducting oxide in the ultraviolet (UV) region, effectively mitigating the effects of light and temperature.10,11 In particular, ZnGa2O4 displays excellent faradaic efficiency properties, a quality that has been verified and documented in the previous literature.12,13 Consequently, ZnGa2O4 has attracted significant attention as a promising material for constructing sensing films for EGFET pH sensors. Earlier research explored various methods for pH value measurement. Typically, glass sensing electrodes were widely used to measure hydrogen ion concentrations and displayed favorable measurement characteristics. However, glass electrodes have certain drawbacks, such as fragility, the need for constant wet storage, and limitations in miniaturization, greatly limiting their practical applications.14 To address these shortcomings, Piet Berveld introduced the concept of an ion-sensitive field-effect transistor (ISFET) based on the MOSFET in 1970.15 This adaptation involved replacing the traditional metal component of the MOSFET with a layer of metal oxide sensing films. When the sensing film of the ISFET is immersed in a solution, it generates an interface potential that varies with changes in the ion concentration within the solution. These ion concentration variations affect the current–voltage characteristics of the field-effect transistor (FET). The ISFET offers numerous advantages, including a fast response time for real-time measurements, potential for miniaturization, and compatibility with the MOSFET process, which significantly reduces production costs. As a result, pH sensing using the ISFET structure has become a more favorable approach compared to traditional glass electrodes.16 However, ISFET technology is not without its challenges, including issues of instability, susceptibility to electrostatic discharge (ESD) damage, and the presence of a leakage current. In order to enhance the properties of ISFET and mitigate associated risks, Spiegel et al. introduced the EGFET in 1983.17 The EGFET structure effectively separates the sensing film from the FET while retaining the metal gate of the FET. In essence, the EGFET consists of two distinct components: the sensing film and the MOSFET, connected by wires. Furthermore, when compared with ISFET and other methods such as biofluid and interstitial fluid (ISF), EGFETs offer several key advantages. These include a separated structure that shields the MOSFET components from the solution, ensuring long-term stability, insensitivity to light and temperature, ease of packaging, cost-effectiveness, simplicity, and the absence of microneedles (MN).18−21 For these reasons, the EGFET structure was chosen for this study and connected to ZnGa2O4 sensing films to detect pH values. It not only has the advantage of the above reason but could also be quickly analyzed by the sensing platforms of the EGFET structure and could quickly obtain the performance of the ZnGa2O4 sensing films. The material characteristics of the ZnGa2O4 sensing films were thoroughly examined by using atomic force microscopy (AFM) and X-ray diffraction (XRD). Additionally, the electrical properties were measured and analyzed in both the constant current and constant voltage modes to determine pH voltage sensitivity and pH current sensitivity, respectively.

Results and Discussion

When a sensing film is immersed in an electrolyte solution, the reaction between the film and the solution is primarily determined by the surface properties. The film’s surface is characterized by numerous metal–OH bonds formed by dangling bonds. According to the site binding model, the metal–OH groups can accept or donate a proton and make the surface charge accrete. When the sensing film is immersed in an acidic solution (lower pH value), the high accretion of H+ ions on the sensing film provides a positive charge (OH2+). On the contrary, when the sensing film is immersed in an alkali solution (higher pH value), the high accretion of OH– ions on the sensing film provides a negative charge (O–). The mechanism is known as “protonation” and “deprotonation”, which results in a change in surface potential as the sensing film is immersed in solutions with different pH values. Therefore, the surface characteristics of the sensing film are very important for the sensing performance. Next, the surface characteristics of the ZnGa2O4 sensing film and the I–V transfer characteristics of the ZnGa2O4 EGFET pH sensor with various oxygen ratios were discussed.

The surface morphology of the ZnGa2O4 thin film with various oxygen flow ratios was characterized using atomic force microscopy (AFM) with a scanning area of 5 μm × 5 μm. Figure 1a–d presents the AFM images of the ZnGa2O4 sensing films with varying oxygen flow ratios. Specifically, the ZnGa2O4 sensing films with oxygen flow ratios of 0, 2, 4, and 6% exhibited root-mean-square (RMS) roughness values of 1.31, 1.55, 2.19, and 2.58 nm, respectively. It is evident that the root-mean-square (RMS) roughness increased as the oxygen flow ratio was raised. To further realize the effect of the annealing process, the ZnGa2O4 sensing films with oxygen flow ratios of 6% were annealed at a temperature of 700 °C, and a root-mean-square (RMS) roughness of 1.49 was measured, as shown in Figure 7e.

Figure 1.

Figure 1

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AFM images of the surface morphology of ZnGa2O4 sensing films under different oxygen flow ratios of (a) 0%, (b) 2%, (c) 4%, (d) 6%, and (e) 6% with an annealing temperature of 700 °C, respectively.

Figure 7.

Figure 7

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Switch test of the ZnGa2O4 extended-gate field-effect transistor pH biosensor unannealed and annealed at 700 °C.

To assess the performance of the ZnGa2O4 EGFET pH sensor, we discuss the transfer characteristics and transconductance curves obtained under the constant current mode. The ZnGa2O4 EGFET pH sensor operated in the linear region with a fixed drain voltage (VDS) of 0.2 V, while the measured reference voltage (VREF) was varied from 0 to 3 V. Figure 2 shows the transfer characteristics and transconductance curves of the ZnGa2O4 EGFET pH sensor with an oxygen flow ratio of 6%. As shown in Figure 2, the peak values of the transconductance curves within the pH range of 2–12 remain consistent, indicating that temperature effects can be considered negligible.22 As a result in Figure 2, the curves of the transconductance shifted to a positive reference voltage with increasing pH values from 2 to 12 owing to the surface reaction of protonation and deprotonation. Based on the MOSFET theory, the relationship between drain current (IDS) and reference voltage (VREF), while operating in the linear region of the ZnGa2O4 EGFET pH sensor, can be described by the following equation23,24

graphic file with name ao3c09965_m001.jpg 1

where μn is the carrier mobility of the channel, Cox is the gate oxide capacitance per unit area, W is the channel width, L is the channel length, VREF is the voltage of the applied voltage of the reference electrode, VT,EGFET is the threshold voltage of the EGFET, and VDS is the drain to source voltage. In the expression formula of IDS, VT,EGFET was affected by different pH values when the ZnGa2O4 sensing film was immersed in the buffer solution. VT,EGFET could be defined and expressed by the following equation25,26

graphic file with name ao3c09965_m002.jpg 2

where VT,MOSFET is the threshold voltage of the MOSFET, φM is the metal gate work function of the reference electrode, q is the electron charge, EREF is the potential of the reference, χSol is the surface dipole potential of the electrolytic solution, and ϕ is the surface potential between the interface of the pH buffer solution and the ZnGa2O4 sensing films. The surface potential (ϕ) that depended on the pH value of the electrolyte could be expressed by the following equation27,28

graphic file with name ao3c09965_m003.jpg 3

where pHpzc is the pH value of the buffer solution at the point of zero charge, K is Boltzmann’s constant, T is the absolute temperature, and β is the parameter of the sensitivity. Based on the site binding model of the electrical double layer,29 the parameter of the sensitivity (β) could be evaluated by using the following equation27,28

graphic file with name ao3c09965_m004.jpg 4

where Ns is the surface sites per unit area, CDL is the capacitance of the electrical double layer, Ka is the equilibrium constant of acid, and Kb is the equilibrium constant of the base. According to the theory described above, VT,EGFET is influenced by different pH values and is further affected by the surface potential (ϕ), resulting in varying drain currents (IDS) obtained in different pH buffer solutions. Therefore, the transfer characteristic curve (IDS–VREF) in Figure 2 exhibits a right shift in the threshold voltage as the pH value increases, which is a result of the decreasing concentration of hydrogen ions. The drain current output characteristics in the saturation region of the ZnGa2O4 extended-gate field-effect transistor pH sensor could be expressed as follows.28,30

graphic file with name ao3c09965_m005.jpg 5

Figure 2.

Figure 2

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IDS–VREF transfer characteristics and transconductance curves of the ZnGa2O4 EGFET pH sensor with an oxygen flow ratio of 6%.

It was observed that the drain current in the saturation region depended on the pH value and decreased with an increasing pH value. Figure 3 shows that higher pH values in the pH buffer solution result in a lower drain current, primarily because a larger pH value implies a lower concentration of H+ ions in the pH buffer solution. The decreased concentration of H+ ions leads to changes in the ZnGa2O4 sensing films’ surface potential, resulting in a negative voltage being applied to the ZnGa2O4 EGFET pH sensor. Consequently, the drain current decreases with increasing pH values.

Figure 3.

Figure 3

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ZnGa2O4 extended-gate field-effect transistor pH sensor analyzed by the method of constant voltage mode measurement.

The pH sensitivity refers to the voltage or current changes caused by variations in the pH levels. The pH sensitivity of the ZnGa2O4 thin-film EGFET pH sensor can be determined from the ID–VREF and ID–VDS transfer characteristics curve, covering a pH range from 2 to 12, using a first-order linear equation. Once the linear equation is identified, the sensitivity can be extracted from the slope and linearity is obtained, too. To assess the sensitivity and linearity of the ZnGa2O4 EGFET pH sensor, the performance of both the linear and saturation regions is depicted in Figure 4a,b, respectively. In the linear region, the pH voltage sensitivity and linearity were assessed and fitted based on the measured results obtained in the constant current mode, with the drain current fixed at 0.2 mA. The corresponding reference voltage was then plotted in Figure 4a and used to calculate the sensitivity. Therefore, the sensitivity of the linear region can be calculated using the following equation26

graphic file with name ao3c09965_m006.jpg

where ΔpH represents the pH difference of the electrolyte and ΔVT,EGFET represents the variation of the threshold voltage of EGFET corresponding to the pH value difference.

Figure 4.

Figure 4

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Sensitivity and linearity of the ZnGa2O4 extended-gate field-effect transistor pH sensor of (a) pH voltage sensitivity and (b) pH current sensitivity.

As shown in Figure 4a, the pH voltage sensitivity for oxygen ratios of 0 and 6% was found to be 8.71 and 23.14 mV/pH, respectively. The corresponding linearity for oxygen ratios of 0 and 6% stood at 96.17 and 92.16%, respectively. This indicates that the pH voltage sensitivity for an oxygen ratio of 6% exceeded that of the oxygen ratio of 0%. Additionally, evaluating device characteristics operating in the saturation region under a fixed VDS of 3 V was crucial for assessing the pH current sensitivity. The pH current sensitivity is shown in Figure 4b and could be calculated using the following equation31

graphic file with name ao3c09965_m007.jpg

where ΔpH represents the pH difference of the electrolyte and ΔIDS represents the variation of the drain current corresponding to the pH value difference.

As shown in Figure 4b, the pH current sensitivity for the ZnGa2O4 sensing films with an oxygen ratio of 6% was 33.49 μA/pH, with a fitted linearity of 96.15%. In comparison to the ZnGa2O4 sensing films with an oxygen ratio of 0%, it is evident that the ZnGa2O4 sensing films with an oxygen ratio of 6% exhibited a higher root-mean-square (RMS) roughness, providing more surface binding sites and a larger effective sensing area.

The pH voltage sensitivity and pH current sensitivity of ZnGa2O4 sensing films with various oxygen flow ratios are presented in Figure 5. As observed in Figure 5, the pH sensing performance of ZnGa2O4 EGFET pH sensors, in terms of pH voltage sensitivity and pH current sensitivity, both exhibited variations with different oxygen flow ratios. Specifically, when compared with the pH voltage sensitivity of 8.71 mV/pH with an oxygen ratio of 0%, the pH voltage sensitivity improved with increasing oxygen ratios. The pH voltage sensitivities for oxygen ratios of 2, 4, and 6% were measured at 12.43, 16.29, and 23.14 mV/pH, respectively. Furthermore, the pH current sensitivity for an oxygen flow ratio of 0% was 5.41 μA/pH and increased with the rise in the oxygen flow ratio. The corresponding pH current sensitivities for oxygen ratios of 2, 4, and 6% were measured at 15.46, 22.22, and 33.49 μA/pH, respectively. Both pH voltage sensitivity and pH current sensitivity improved as the oxygen ratio increased. According to the site binding model theory,29 with an increase in the oxygen flow rate, the root-mean-square (RMS) surface roughness became higher. This higher surface roughness suggests a greater surface contact area available for the reaction with the pH buffer solution. Consequently, this leads to improved reaction efficiency at the interface between the pH buffer solution and ZnGa2O4 sensing films. Ultimately, this results in more significant changes in surface potential as the pH value varies, particularly under the conditions of a 6% oxygen ratio. To further assess the impact of crystallinity on sensing performance, a ZnGa2O4 extended-gate field-effect transistor pH sensor was manufactured and subjected to annealing at a temperature of 700 °C.

Figure 5.

Figure 5

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pH voltage sensitivity and pH current sensitivity of ZnGa2O4 films with various oxygen flow ratios.

X-ray diffraction (XRD) was employed to measure and characterize the lattice characteristics. Figure 6 presents the X-ray diffraction (XRD) curve for the ZnGa2O4 sensing films with an oxygen ratio of 6%, both without annealing and after annealing at 700 °C. As shown in Figure 6, the as-deposited ZnGa2O4 exhibits no regular arrangement of atoms, and no discernible peaks indicative of an amorphous structure are observed. This observation is consistent with the results reported in the previous literature.32 However, following annealing at a temperature of 700 °C, the films underwent crystallization and exhibited a polycrystalline phase. Notably, the diffraction peaks were observed near 35.90, 37.35, 57.60, and 63.75°, corresponding to the (311), (222), (511), and (440) planes of ZnGa2O4, respectively (JCPDS card 381240), indicative of a spinel structure. These peaks closely align with 35.73, 37.37, 57.45, and 63.10°, respectively.33 The pH current sensitivity and pH voltage sensitivity are also shown in Figure 5. It is evident that the pH current sensitivity and pH voltage sensitivity, when using an oxygen ratio of 6% under annealing at 700 °C, were measured at 19.48 μA/pH and 18.86 mV/pH, respectively. However, a comparison with the results obtained without the annealing process reveals that the sensitivity was smaller with the annealing temperature set at 700 °C. This difference can be attributed to the significant impact of the surface passivation process resulting from the annealing temperature of 700 °C, leading to a phase transition from the amorphous phase to the crystalline phase, as demonstrated in Figure 6. Furthermore, with an annealing temperature of 700 °C, the crystallization of the material became more distinct and the full width at half-maximum (fwhm) narrowed, signifying higher film quality. While the thin film quality could be enhanced through the annealing process, it did not lead to improved sensitivity performance. This lack of improvement can be attributed to the reduction in surface site density, which hinders the interaction between the solution and the ZnGa2O4 sensing films. To further illustrate the influence of the surface roughness and vacancies, ZnGa2O4 sensing films with a 6% oxygen flow ratio were annealed at a temperature of 700 °C. First, the root-mean-square (RMS) roughness decreased from 2.58 nm (as-deposited) to 1.49 nm (after annealing at 700 °C). Second, characterization using X-ray photoelectron spectroscopy (XPS) was carried out, where the OII peak area to the total peak area of O1s was defined to compare oxygen deficiency in the various films. The higher OII ratio correlated with more vacancies in the films. The relative proportions of the OII/O1s ratio decreased from 57.64% (unannealed) to 26.93% after annealing at 700 °C.34 Therefore, both surface roughness and vacancies are crucial factors that need to be considered and analyzed.

Figure 6.

Figure 6

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XRD spectra of ZnGa2O4 films with and without an annealing process.

In order to study the reliability of the ZnGa2O4 extended-gate field-effect transistor pH biosensor, a switch test was measured and revealed the real-time drain current of the loop with pH 2–12, as shown in in Figure 7. The switch test is based on the constant voltage mode measurement of VDS and VGS operated at 3 V. When the current value changes suddenly, the drain current is recorded in the current time mode to monitor the change of the saturation current. As shown in Figure 7, it could be observed that the ZnGa2O4 sensing films have great response and corrosion resistance.

Table 1 shows the comparison of the pH sensing performance of various metal oxide materials published in the previous literature. As shown in Table 1, the pH voltage sensitivity of ZnO pH sensors ranged from 15.4 to 24.67 mV/pH, whereas InN and CuO exhibited 22.66 and 18.4 mV/pH, respectively.35−38 Compared to the above results, the ZnGa2O4 pH sensor studied in this work exhibited a pH voltage sensitivity of 23.14 mV/pH, which is larger than those of the first ZnO pH sensor and CuO pH sensor but slightly lower than those of the second ZnO pH sensor and InN pH sensor. Besides, the pH detected range in our study was from 2 to 12, wider than the previous literature studies. While the pH sensitivity of nanorod and nanowire structures is not always superior to that of ZnGa2O4 thin films based on these literature studies, the nanorod and nanowire structures offer the advantage of increased surface contact area with the electrolyte. In the future, this will remain a research direction worthy of further exploration to enhance the performance of pH sensors.

Table 1. pH Sensing Performance Comparison of Other Metal Oxide Materials.

sensing film method structure pH voltage sensitivity (mV/pH) linearity pH range refs
ZnO hydrothermal process nanorod 15.4   4–12 (35)
ZnO hydrothermal process nanorod 24.67 0.986 4–10 (36)
InN MBE nanorod 22.66   4–10 (37)
CuO thermal annealing of a Cu film at 450 °C for 5 h in air nanowire 18.4   4–10 (38)
ZnGa2O4 RF sputtering film 23.14 0.921 2 –12 this work
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Conclusions

In this study, we prepared different oxygen ratio ZnGa2O4 sensing films by an RF magnetron sputtering system. The ZnGa2O4 sensing films were analyzed by AFM and XRD, and they were connected with a commercial MOSFET to form an extended-gate field-effect transistor pH sensor. From the AFM analysis, it was observed that as the oxygen flow ratio increased, the higher surface roughness increased the sensing surface-to-volume ratio and further improved the response on the interface between the pH buffer solution and ZnGa2O4 sensing films. It could be found that the pH voltage sensitivity values with oxygen ratios of 0, 2, 4, and 6% were 8.71, 12.43, 16.29, and 23.14 mV/pH, respectively. The corresponding pH current sensitivity values with oxygen ratios of 0, 2, 4, and 6% were 5.41, 15.46, 22.22, and 33.49 μA/pH, respectively. Compared to fabricating with different oxygen ratios, the annealing treatment method could not improve the properties of the ZnGa2O4 extended-gate field-effect transistor pH sensor owing to a reduction in the surface site density, resulting in a poor reaction.

Experimental Section

The schematic structure of the ZnGa2O4 sensing films is depicted in Figure 8a. The fabrication process involved several steps. First, quartz substrates were cleaned in an ultrasonic cleaning machine using a sequence of acetone, isopropyl alcohol, and deionized water, with each step lasting 5 min. Following the rinsing process, the substrates were dried using nitrogen gas. Next, ZnGa2O4 thin films were deposited using RF sputtering. The sputtering process was carried out with a fixed sputtering power of 80 W and a chamber pressure of 5 mTorr. The oxygen flow ratio within the sputtering chamber was varied to study its effect on the characteristics of the sensing films. Four oxygen flow ratios were examined, namely, 0, 2, 4, and 6%. Then, half of the ZnGa2O4 thin film was coated with Ni/Au (30/70 nm) by using an e-beam evaporator to serve as the contact electrode. Finally, a bonding wire was used to establish a connection between the bonding pad on the ZnGa2O4 thin film and the terminal of the MOSFET. The bonding pad was then encapsulated with epoxy resin to protect the metals from corrosion when exposed to the solution. The measurement setup, as depicted in Figure 8b, was employed to evaluate the performance of the ZnGa2O4 thin film. The setup involved the following components and steps. The ZnGa2O4 sensing films and Ag/AgCl reference electrodes were immersed in a buffer solution. The ZnGa2O4 sensing films were connected to a commercial MOSFET (CD4007UBE) to serve as the gate of the EGFET pH sensors. The EGFET pH sensor was further connected to an Agilent B1500A semiconductor parameter analyzer and controlled using LabVIEW software. The EGFET pH sensors with different oxygen ratio sensing films were measured, and the sensing sensitivity and linearity of the device were analyzed in the pH range of 2–12.

Figure 8.

Figure 8

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(a) Schematic structure of ZnGa2O4 sensing films. (b) Measurement system setup of the extended-gate field-effect transistor pH sensor with the ZnGa2O4 sensing films.

Acknowledgments

This work was supported by the Ministry of Science and Technology under contract number, 111-2222-E-273-001-MY2, 111-2221-E-006-030-MY3, NSTC 111-2222-E-150-003-MY2, and 111-2515-S-273-001.

Author Contributions

∥ C.H.C., S.B.L., and S.P.C. contributed equally to this work. C.H.C. wrote the paper.

The authors declare no competing financial interest.

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