# Investigation of High-Sensitivity pH Sensor Based on Au-Gated AlGaN/GaN Heterostructure
## 3 Results and Discussion
### Sensing Mechanism
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 2 DEG (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\({}_{\text{ads}}\)). The E\({}_{\text{ads}}\) is defined as:  
\[E_{ads}=E_{total}-E_{surface}-E_{adsorbate} \tag{4}\]  
Figure 12: Sensing schematic diagram of the Au-gated AlGaN/GaN pH sensor: (**a**) alkaline solution; (**b**) acidic solution. where E\({}_{\text{total}}\) is the total energy of the entire system after adsorption, E\({}_{\text{surface}}\) is the energy of the sensing membrane surface before adsorption, and E\({}_{\text{adsorbate}}\) is the energy of the adsorbate before adsorption. As defined, the more negative the E\({}_{\text{ads}}\), the more robust the adsorption and the more stable the structure. Table 3 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 2 DEG electrons. High electron density and high electron mobility coexist in the 2 DEG 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 2 DEG is sensitive to the change in surface charge density. For example, the positive surface charge will attract additional electrons to the 2 DEG layer, thus increasing the electron density in the 2 DEG 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 2 DEG 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.