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In recent years, there has been demonstrated a rapid progress of the AlGaN/GaN system applications in optical and electronic device research due to its unique features. One of unique features of AlGaN/GaN system includes chemically stable where stability of surface is essentially important for liquid-phase sensor applications. Besides, AlGaN/GaN also allows highly sensitive detection of surface phenomena with the presence of a high-density two-dimensional electron gas (2DEG) in the high electron mobility transistor (HEMT) structure. Furthermore, the GaN material allows sensing operations at high temperatures due to large bandgap energies. This paper presents the design and fabrication of an open-gate structure on undoped AlGaN/GaN and n-AlGaN/GaN HEMT structure for sensing the pH level of certain electrolyte solutions. It is found that the drain-source current decreases with the pH level as expected.