# Investigation of High-Sensitivity pH Sensor Based on Au-Gated AlGaN/GaN Heterostructure
## 3 Results and Discussion
### 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. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Current Sensitivity**, & & & & & & & \\
**S\({}_{\text{A}}\) (\(\upmu\)A/pH)** & **pH Range** & **Sensing Membrane** & **Gate Width**, & **Gate Length**, & **V\({}_{\text{DS}}\)** & **V\({}_{\text{Ref}}\)** & **Ref.** \\ \hline
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 & SiO\({}_{2}\) & 40 & 40 & 3.5 & 0 & This work \\
37 & 3-10 & SiO\({}_{2}\) & 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 & Al\({}_{2}\)O\({}_{3}\) & 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] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of current sensitivity with previously reported AlGaN/GaN pH sensors. Figure 11: Real-time dynamic pH responses of the Au-gated AlGaN/GaN pH sensor. 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 (2 DEG), and the threshold voltage (V\({}_{\text{th}}\)) is crucial to its sensing mechanism. The 2 DEG, 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 2 DEG. 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 2 DEG 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 2 DEG (see Figure 12a).