# Investigation of High-Sensitivity pH Sensor Based on Au-Gated AlGaN/GaN Heterostructure
## 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 SiO\({}_{2}\), 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-performancepH 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 (2 DEG) with extremely high electron concentration. Unlike traditional ISFETs, the high-electron mobility transistor (HEMT) exhibits greater sensitivity to surface state changes near the 2 DEG. A slight change will bring a sensitive change in the 2 DEG 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 Sc\({}_{2}\)O\({}_{3}\)[10], Al\({}_{2}\)O\({}_{3}\)[11], Ga\({}_{2}\)O\({}_{3}\), 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], H\({}_{2}\)O\({}_{2}\) 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]. [PERSON] 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 microltier solutions. Furthermore, the sensing mechanism of the Au-gated AlGaN/GaN pH sensor is analyzed in detail.