Investigation of High-Sensitivity pH Sensor Based on Au-Gated AlGaN/GaN Heterostructure

Abstract

A high-sensitivity pH sensor based on an AlGaN/GaN high-electron mobility transistor (HEMT) with a 10 nm thick Au-gated sensing membrane was investigated. The Au nanolayer as a sensing membrane was deposited by electron-beam evaporation and patterned onto the GaN cap layer, which provides more surface-active sites and a more robust adsorption capacity for hydrogen ions (H⁺) and hydroxide ions (OH⁻) and thus the sensitivity of the sensor can be significantly enhanced. A quasi-reference electrode was used to minimize the sensing system for the measurement of the microliter solution. The measurement and analysis results demonstrate that the fabricated sensor exhibits a high potential sensitivity of 58.59 mV/pH, which is very close to the Nernstian limit. The current sensitivity is as high as 372.37 μA/pH in the pH range from 4.0 to 9.18, under a 3.5 V drain-source voltage and a 0 V reference-source voltage. Comparison experiments show that the current sensitivity of the Au-gated sensor can reach 3.9 times that of the SiO₂-gated sensor. Dynamic titration experiments reveal the pH sensor’s ability to promptly respond to immediate pH variations. These findings indicate that this pH sensor can meet most application requirements for advanced medical and chemical analysis.

1. Introduction

The pH measurement is in strong demand in a wide variety of fields, such as chemistry and biomedicine [1]. The pH level plays a critical role in numerous biological and chemical reactions [2]. Therefore, it is crucial to develop sensitive methods for detecting pH values with high sensitivity. The usual pH measurement method is potentiometry using a glass reference electrode with metal inside [3]. However, the large size of conventional glass reference electrodes seriously hinders their application in biomedicine. By comparison, the ion-sensitive field-effect transistor (ISFET) combines the sensing and transducing elements into a single device, and its compatibility with complementary metal-oxide semiconductor (CMOS) processes [4], thereby allowing the device to be miniaturized for small-volume measurement [5]. The Si-based ISFET pH sensor has been studied for years. However, due to the inherent properties of Si-based materials, such as SiO₂, it is prone to corrosion in certain specific solutions [6], and faces challenges with repeatability and long-term stability, which restrict its application scope.

Gallium nitride (GaN) is a semiconductor material with a wide bandgap (3.4 eV) and high chemical bond strengths [7,8]. Since its superior thermal stability, excellent conductivity, and chemical stability, the GaN-based ISFET is a good candidate as a high-performance pH sensor [9]. The core architecture of GaN-based pH sensors is the AlGaN/GaN heterojunction, where the spontaneous and piezoelectric polarization at the interface induces exceptionally high interface charges and electric fields. This phenomenon results in a two-dimensional electron gas (2DEG) with extremely high electron concentration. Unlike traditional ISFETs, the high-electron mobility transistor (HEMT) exhibits greater sensitivity to surface state changes near the 2DEG. A slight change will bring a sensitive change in the 2DEG concentration, which leads to a change in the channel current between the drain and the source of the transistor. Therefore, the AlGaN/GaN pH sensor shows high sensitivity and quick response.

The site-binding model is widely adopted to describe the sensing mechanism of the AlGaN/GaN pH sensor. In this model, the surface potential is altered by the adsorption of H⁺ and OH⁻ onto the sensing membrane of the sensor.

Many researchers tried to change or improve the material of sensing membranes to achieve higher sensitivity. For example, one can deposit oxide materials such as Sc₂O₃ [10], Al₂O₃ [11], Ga₂O₃, and ZnO nanorods [12,13] on the GaN or AlGaN surface. Or, one can exploit the methods of oxidation treatment such as thermal oxidation treatment [9], H₂O₂ treatment [14], oxygen plasma treatment [15], and photoelectrochemical oxidation treatment [16] for the GaN or AlGaN surface.

Moreover, several studies indicated that Au modification could improve the sensitivity of the non-GaN-based pH sensor [17]. Medhat et al. reported that the sensitivity increases by 26.7% when using the graphene nanoplatelet (GP) modified by a Au nanoparticle as the sensing membrane, compared to that of the GP [18]. The field-effect-transistor (FET) pH sensor with Au-adsorbed ZnO nanorods showed better sensing performance than the pH sensor with pure ZnO nanorods [19].

In this paper, a 10 nm thick Au nanolayer was used as the sensing membrane of an AlGaN/GaN HEMT, which differs from the conventional oxidation method to enhance sensitivity. Au was selected as the sensing membrane for the AlGaN/GaN pH sensor as it can be fabricated in many cleanrooms and is a well-known material for its chemical stability. A quasi-reference electrode made of Au was integrated into the self-designed printed circuit board (PCB), which can reduce the pH sensor’s overall size and measurement complexity for microliter solutions. Furthermore, the sensing mechanism of the Au-gated AlGaN/GaN pH sensor is analyzed in detail.

2. Materials and Methods

2.1. Epitaxy Growth and Device Fabrication

The device was fabricated using a commercial epitaxial GaN wafer based on a silicon substrate. Figure 1 shows a schematic cross-section diagram of the Au-gated AlGaN/GaN pH sensor. The epitaxial structure was grown by metal-organic chemical vapor deposition (MOCVD) on the silicon substrate. Starting from the substrate, one can find the structure consisted of a 1 µm thick GaN buffer layer, a 2 µm thick GaN channel layer, a 1 nm thick AlN interlayer, a 25 nm thick Al_0.26Ga_0.74N barrier layer, and a 3 nm thick GaN cap layer. The sheet electron density and electron mobility of the 2DEG at 25 °C were 1 × 10¹³ cm⁻² and 1500 cm²/(V·s), respectively.

The fabrication process and characterize method are similar to our previous report [20]. The fabrication process flow started with a mesa etching using a Cl₂/BCl₃ inductively coupled plasma (ICP) to define the sensor geometry. After that, Ti/Al/Ti/Au (20/110/40/50 nm) metal contacts were deposited via electron-beam evaporation (EBE) for the drain (D) and source (S) electrodes followed by annealing at 850 °C for 45 s in N₂ ambient. Then, a 200 nm thick SiO₂ was deposited via plasma-enhanced chemical vapor deposition (PECVD). The SiO₂ layer was etched in a buffered oxide etch (BOE) solution to open the contact pads and gate windows. A 10 nm thick nanolayer of Au or SiO₂ was deposited as the sensing membrane by EBE or PECVD, respectively. All the conducting areas, except the sensing membrane, were covered with a 5 µm thick photoresist to prevent them from contacting the solution.

After dicing, the discrete pH sensor samples were mounted on a self-designed PCB substrate, as shown in Figure 2. The Au quasi-reference electrode was used to apply a voltage bias to the electrolyte solution as well as control the charge density of the 2DEG beneath the sensing membrane. It can provide a stable electrochemical interface, especially when used with a suitable electrolyte solution that forms a stable layer on the Au surface, contributing to a stable potential and reducing unnecessary electrolysis.

Figure 3 shows the scanning electron microscope (SEM) images of the Au-gated device, with a detailed zoom-in view of the sensing membrane given in Figure 3b. Figure 4 shows the energy dispersive spectroscopy (EDS) spectra of the Au-gated sensing membrane. The inset of Figure 4 shows an SEM image of the sensing area and the elemental mapping of Ga, N, Au, Al, and O in the Au-gated sensing membrane. The measured elemental atomic percentages were Ga (45.83%), N (45.29%), Au (2.23%), Al (2.71%), and O (3.94%), respectively. Correspondingly, the elemental weight percentages were Ga (72.55%), N (14.40%), Au (9.96%), Al (1.66%), and O (1.43%), respectively. Thus confirming the deposition of Au on the sensing membrane.

2.2. Measurement Setup

The transfer (I_DS-V_Ref), output current-voltage (I_DS-V_DS) curves, and the real-time (I_DS-t) response curves were measured using two Keithley 2450 source meter instruments at 25 °C.

The pH values of four pH standard buffered solutions were 1.68, 4.0, 6.86, and 9.18 at 25 °C. Then, we dropped deionized (DI) water or pH solution between the sensing membrane and the quasi-reference electrode using a micro-pipette. A 30 μL solution can sufficiently cover the whole sensing membrane and the exposed quasi-reference electrode region.

3. Results and Discussion

3.1. Transfer Characteristics and Potential Sensitivity

Figure 5a–d shows the transfer (I_DS-V_Ref) and transconductance (G_m-V_Ref) characteristics versus the reference-source voltage of the Au-gated AlGaN/GaN heterostructure-based pH sensor in three pH standard buffered solutions at the different drain-source voltages (V_DS) of 0.1 V, 0.5 V, 1.0 V, and 2.0 V, respectively. The gate width (W) and length (L) of the sensor were 160 µm and 40 µm, respectively. The reference-source voltage (V_Ref) was swept from −4.5 V to 0.5 V. A refined parallel shift was observed in the I_DS-V_Ref, when the pH value changed from 4.0 to 9.18, indicating the corresponding potential change at the gate surface. The experimental results reveal that the drain-source current (I_DS) decreased with the increase in the pH.

The transconductance (G_m) is defined as the change of the drain-source current (ΔI_DS) against the change of the reference-source voltage (ΔV_Ref), which can be expressed as:

$$G_m=\frac{\Delta I_{DS}}{\Delta V_{Ref}}=\frac{W}{L}{\mu }_n{C}_i{V}_{DS}$$

(1)

where W and L are the width and length of the sensing membrane, μ_n is the electron mobility of the 2DEG, C_i is the capacitance per unit area between the quasi-reference electrode and the 2DEG channel, and V_DS is the drain-source voltage.

At pH = 6.86, the maximum transconductance (G_{m, max}) of the sensor at V_DS = 0.1 V, 0.5 V, 1.0 V, and 2.0 V was 0.25 mS, 1.23 mS, 2.45 mS, and 4.44 mS, respectively. The V_Ref of the G_{m, max} at V_DS = 0.1 V, 0.5 V, 1.0 V, 2.0 V was −2.60 V, −2.29 V, −1.93 V, and −1.09 V, respectively. The V_Ref corresponds to the G_{m, max} shift positively with the increasing V_DS.

The threshold voltage (V_th) is defined as the V_Ref at which a conductive channel forms between the source and drain, initiating current flow within the sensor. The role of V_th in the sensor is critical as it is influenced by the surface potential, which in turn is affected by the pH value. Therefore, by observing changes in V_th, the sensor can accurately detect and measure pH value fluctuations. At V_DS = 2.0 V, the V_th of the sensor at pH = 4.0, 6.86, and 9.18 is −3.42 V, −3.21 V, and −3.12 V, respectively. The V_th shifts positively with the increasing pH value, indicating that the sensor is sensitive to the different pH values. It demonstrates that the pH change of the electrolyte solution modulates the surface potential, leading to a change in the 2DEG density of the sensor. It needs less voltage to deplete the 2DEG, causing a positive shift of the V_th.

The sensor’s potential sensitivity (S_V) is defined as the surface potential change (Δφ) with the pH value change (ΔpH):

$$S_V=\frac{\Delta \phi }{\Delta pH}=\frac{\Delta V_{Ref}}{\Delta pH}$$

(2)

Figure 6 shows the relationship of the reference-source voltage versus the pH value (V_Ref-pH). The potential sensitivity (S_V) was obtained from the slope of the V_Ref-pH plot. The experimental results indicate that the S_V of the sensor could reach 58.59 mV/pH with a linearity of 97.92% in the pH range from 4.0 to 9.18. This S_V is very close to the Nernstian limit of 59.16 mV/pH at 25 °C.

Table 1. Comparison of potential sensitivity with previously reported AlGaN/GaN pH sensors.

SV (mV/pH)	pH Range	Sensing Membrane	Ref.
58.59	4–9.18	Au	This work
57.7	4–9	Al2O3	[6]
57.8	4–9	Al2O3	[9]
55.81	4–9	AlGaN	[11]
57.66	4–12	ZnO	[13]
55.2	2–12	Al2O3	[14]
55.7	4–9	AlGaN	[15]
53.3	4–10	GaN	[16]
50	4–8	GaN	[21]
49	2–11	AlGaN	[22]
55.5	4–9	AlGaN	[23]

shows the comparative analysis of this work with the previously reported AlGaN/GaN pH sensors. The significant difference between this work and some previous reports is that Au is used as the material of the sensing membrane. It is evident from Table 1 that the fabricated Au-gated AlGaN/GaN pH sensor of this work exhibits higher potential sensitivity.

3.2. I-V Characteristics and Current Sensitivity

Figure 7 presents the output current-voltage (I_DS-V_DS) characteristics of the Au-gated AlGaN/GaN pH sensor with W/L = 160/40 µm in the three pH standard buffered solutions at the V_Ref from −3 to 1 V (1 V/step). The drain-source voltage (V_DS) was swept from 0 to 5 V. The sensor exhibits good current saturation characteristics. The magnitude of the V_DS influences the electric field within the heterostructure, thereby affecting the carrier mobility and the saturation current. When the V_Ref becomes more negative, the difference in the I_DS across various pH levels (such as pH = 4.0 and pH = 9.18) decreases. This is because the more negative voltage changes the electric field, decreasing the sheet electron density within the 2DEG, thereby leading to weaken the sensor’s response to pH variations.

In practical applications, the transconductance (G_m) and output current are more effective parameters to integrate with subsequent circuits, where current sensitivity (S_A) is essential. The S_A of the sensor is defined as:

$$S_A=\frac{\Delta I_{DS}}{\Delta pH}=G_m{S}_V$$

(3)

where ΔI_DS is the change of drain-to-source current and ΔpH is the change of pH value. Using Equation (3) to calculate the results from Figure 7, the current sensitivities of the sensor are shown in Figure 8 with different V_Ref values.

Figure 8 shows the S_A of the Au-gated pH sensor achieves 372.37 μA/pH (V_DS = 3.5 V, V_Ref = 0 V) with a linearity of 99.99% in the pH range from 4.0 to 9.18. The findings indicate that the sensor exhibits high sensitivity to microliter solutions.

3.3. Au-Gated vs. SiO₂-Gated Sensing Membrane

Figure 9 presents the I_DS-V_DS characteristics of the Au-gated and SiO₂-gated sensing membrane pH sensors with W/L = 40/40 µm in the three pH standard buffered solutions at the different V_Ref. The V_Ref ranges from −3 to 1 V (1 V/step). The drain-source voltage (V_DS) was swept from 0 to 5 V.

The S_A of the Au-gated and SiO₂-gated pH sensors are shown in Figure 10. The experimental results show that the S_A of the Au-gated and SiO₂-gated sensors are 136.91 μA/pH and 35.00 μA/pH (V_DS = 3.5 V, V_Ref = 0 V), respectively. Notably, the S_A of the Au-gated sensor is 3.9 times that of the SiO₂-gated sensor under these conditions, which reveals the influence of sensing membranes made of different materials on the sensitivity. The density of surface states on Au exceeds that of SiO₂, indicating that Au surfaces offer more surface-active sites for the adsorption of H⁺ and OH⁻. For Au, the adsorption energy between its surface and H⁺/OH⁻ is typically more negative than that for SiO₂, indicating a more robust adsorption propensity.

Table 2. Comparison of current sensitivity with previously reported AlGaN/GaN pH sensors.

Current Sensitivity, SA (μA/pH)	pH Range	Sensing Membrane	Gate Width, W (μm)	Gate Length, L (μm)	VDS (V)	VRef (V)	Ref.
372.37	4–9.18	Au	160	40	3.5	0	This work
136.91	4–9.18	Au	40	40	3.5	0	This work
35.00	4–9.18	SiO2	40	40	3.5	0	This work
37	3–10	Sc2O3	150	2	0.25	---	[10]
84.39	4–9	AlGaN	800	800	10	0	[11]
22.231	2–10	ZnO	---	---	0.7	---	[12]
55.8	2–12	Al2O3	200	50	2	−2	[14]
14	4–10	GaN	400	40	0.5	−1.15	[16]
6.62	4–8	GaN	500	1200	0.25	---	[21]

details some previous reports on the current sensitivity of the AlGaN/GaN pH sensors. Compared with some previous reports, the newly developed Au-gated AlGaN/GaN heterostructure-based pH sensor of this work shows much higher current sensitivity (S_A) under similar W/L ratio, V_DS, and V_Ref conditions. Further, it demonstrates that Au as a sensing membrane can achieve higher sensitivity.

3.4. Real-Time Measurement

Figure 11 shows the real-time dynamic pH responses of the Au-gated AlGaN/GaN pH sensor with W/L = 160/40 µm were recorded at V_DS = 0.5 V and V_Ref = −0.5 V. In the experiments, four aliquots of 2 μL each from standard buffered solutions, with pH values precisely at 9.18, 6.86, 4.0, and 1.68, were sequentially dropped into 30 μL of deionized (DI) water. The I_DS response of the sensor fluctuates as pH values range from 9.18 to 1.68 with a times interval of 0.5 s. The I_DS significantly increases with the decrease in pH values. The experimental results of dynamic titration show the pH sensor’s ability to quickly respond to immediate pH variations of the microliter solution.

As shown in the inset of Figure 11, the response time of the sensor is about 2 s, defined as the time required to reach 90% of the final value, which underscores the prompt adaptability of the sensor to immediate pH fluctuations. In the experiments, to minimize the disturbance caused by instantaneous fluctuations during titration, the electrolyte solutions were dropped at a considerable distance from the sensing membrane. Therefore, this response time mainly depends on the diffusion distance of the electrolyte solutions from the sensing membrane, and the actual response time will be faster.

3.5. Sensing Mechanism

The static and dynamic experimental findings demonstrate that a 10 nm thick Au-gated sensing membrane applied to an AlGaN/GaN HEMT markedly increases the pH sensor’s sensitivity. This sensitivity enhancement is derived from two primary factors. Firstly, Au can provide more surface-active sites for the adsorption of H⁺ and OH⁻. Secondly, the Au-gated sensing membrane exhibits a more robust adsorption capacity for H⁺ and OH⁻, leading to improved sensor performance.

The electronic configuration of Au, characterized by a filled 5d orbital and a partially filled 6s orbital, confers unique surface chemical activity [24,25,26]. These electronic properties facilitate effective charge transfer with H⁺ and OH⁻ ions, enhancing electron dynamics during the adsorption process. The effective charge transfer between Au and H⁺/OH⁻ ions not only promotes the stable adsorption of ions but also results in changes in the interfacial potential, forming the foundational mechanism for the detection of pH fluctuations.

The electronic characteristics and active sites on Au surfaces facilitate accelerated kinetics of the interface, allowing the sensor to quickly respond to minor pH changes in the solution [27]. This refined attribute of Au surfaces facilitates real-time monitoring and rapid response to environmental pH variations.

In the Au-gated AlGaN/GaN heterostructure-based pH sensor, the relationship between the two-dimensional electron gas (2DEG), and the threshold voltage (V_th) is crucial to its sensing mechanism. The 2DEG, inherently formed at the interface between the AlGaN and GaN layers due to the spontaneous and piezoelectric polarization effects, serves as the conductive channel for the pH sensor. Variations in surface pH alter the surface potential, thereby modifying the charge distribution on the Au surface. This modulation in surface charge induces changes in the electric field across the AlGaN/GaN interface, directly impacting the density of the 2DEG. This change in density directly impacts the sensor’s threshold voltage. The quasi-reference electrode maintains a constant potential, ensuring that variations in threshold voltage are predominantly due to pH changes rather than fluctuations in the external circuit or electrode potentials.

The sensing schematic diagram of the Au-gated AlGaN/GaN pH sensor is illustrated in Figure 12, highlighting the interactions with H⁺ and OH⁻, while other irrelevant ions are omitted for clarity. The sheet electron density of the 2DEG channel can be greatly affected by the ion absorption on the Au-gated sensing membrane surface, which finally causes the changes in drain-source current. Upon exposure to an alkaline solution (pH > 7), the Au-gated sensing membrane surface acquires a negative charge due to the adsorption of OH⁻ ions, leading to a decrease in the sheet electron density within the 2DEG (see Figure 12a). This reduction in electron density consequently lowers the sensor’s threshold voltage. In contrast, exposure to an acidic solution (pH < 7) imparts a positive charge to the Au-gated sensing membrane surface due to the adsorption of H⁺ ions, causing an increase in the sheet electron density of the 2DEG (see Figure 12b). This elevation in electron density correspondingly raises the sensor’s threshold voltage.

The interaction between the Au surface and H⁺/OH⁻ not only includes physical adsorption but also chemical adsorption. This chemical adsorption generates larger adsorption energy, ensuring more stable and enduring adsorption of H⁺ and OH⁻ on Au surfaces.

The adsorption capacity can be characterized by the adsorption energy (E_ads). The E_ads is defined as:

$$E_{ads}=E_{total}-E_{surface}-E_{adsorbate}$$

(4)

where E_total is the total energy of the entire system after adsorption, E_surface is the energy of the sensing membrane surface before adsorption, and E_adsorbate is the energy of the adsorbate before adsorption.

As defined, the more negative the E_ads, the more robust the adsorption and the more stable the structure.

Table 3. Comparison of adsorption energy for H adsorbed on sensing membrane surfaces of different materials.

Sensing Membrane	Adsorption Energy, Eads (eV)	Ref.
ZnO	−0.367	[28]
GaN	0.61	[29]
Ga2O3	−0.29	[30]
SiO2	−1.00	[31]
Al2O3	−1.31	[31]
Au	−2.49	[32]

compares adsorption energy values for H adsorbed on sensing membrane surfaces of different materials, as reported in the literature. The adsorption energy between Au and H is more negative than in some materials, indicating a more robust adsorption capacity. This robust adsorption effect facilitates Au to not only respond quickly to pH changes but also to exhibit higher sensitivity compared to some materials.

Figure 13 shows the energy band diagram of the Au-gated AlGaN/GaN heterostructure-based pH sensor under different pH values: alkaline (blue dotted lines), neutral (solid black lines), and acidic (red dotted lines) solutions. A 1 nm thick AlN interlayer is used to reduce the Coulomb scattering of the 2DEG electrons. High electron density and high electron mobility coexist in the 2DEG channel of the AlGaN/GaN heterostructure-based pH sensor. The strong spontaneous and piezoelectric polarization leads to a high electric field at the AlGaN/GaN interface, so the conduction band bends down below the Femi level and forms a quantum well, thus generating extremely high electron density at the AlGaN/GaN interface. Because of its electrostatic properties, the 2DEG is sensitive to the change in surface charge density. For example, the positive surface charge will attract additional electrons to the 2DEG layer, thus increasing the electron density in the 2DEG channel. As shown in Figure 13, the conduction band bends deeper below the Fermi level when introducing an acidic solution. As the pH value gradually increases, the surface of the sensing membrane becomes more negative, causing a decrease in the electron density at the 2DEG channel.

The high sensitivity of Au-gated AlGaN/GaN can be attributed to several factors. First, the high electron mobility in the AlGaN/GaN heterostructure ensures rapid response to pH changes. Second, the strong chemical stability of GaN in various environments contributes to the sensor’s reliability. Moreover, the use of Au as the sensing membrane provides more surface-active sites and a more robust adsorption capacity for hydrogen ions (H⁺) and hydroxide ions (OH⁻), thereby enhancing sensitivity.

4. Conclusions

We designed and fabricated the Au-gated AlGaN/GaN heterostructure-based pH sensor. A 10 nm thick Au nanolayer was used as the sensing membrane of the pH sensor, which differs from the conventional oxidation method to improve sensitivity. The sensor demonstrates very high potential sensitivity and current sensitivity to microliter solutions. It is noteworthy that the current sensitivity of the Au-gated sensor can reach 3.9 times that of the SiO₂-gated sensor. Dynamic experimental results show that the Au-gated pH sensor can quickly respond to the pH variations of microliter solutions. The fabricated Au-gated AlGaN/GaN pH sensor exhibits higher sensitivity than some previous reports. Furthermore, an in-depth analysis is provided, elaborating on the reasons why the Au-gated sensing membrane can achieve higher sensitivity, focusing on the aspects of surface-active sites and adsorption energy. In conclusion, our study confirms the viability of the Au-gated AlGaN/GaN pH sensors and provides a beneficial path for researching high-sensitivity pH sensors.