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Ion-sensitive field effect transistor (ISFET) has been widely used as a pH sensor. Its sensitivity can be expressed in terms of current or voltage which were defined by us as SA and SV in this article. Using current to represent sensitivity is more convenient for it can simplify the subsequent circuits. What’s more, SA contains more information including the geometry of the device, the parameter of the epitaxial material and the property of the sensitive membrane while SV only contains the information of the sensitive membrane, which helps us to analyze the device performance. In this paper, the sensitivity SA was deduced theoretically based on the previous research and was verified experimentally. On this basis, we put forward a method that can separate the effect of the sensitive membrane on the sensitivity from the material completely by designing a series of AlGaN/GaN based heterostructure ISFET pH sensors with different sizes of the channel area (width/length parameter). Experimental result shows the importance of this method in the future research.