In this paper, we use the extended gate field effect transistor (EGFET) and the coated wire electrode (CWE) to design a differential pH-sensing device. The SnO 2 /ITO glass structure is the EGFET used as the pH sensor because of its excellent pH sensitivity of about 57.10 mV/pH. The contrast pH sensor is the polypyrrole/SnO 2 /ITO glass structure CWE, which has the lower pH sensitivity of about 27.81 mV/pH, and we use the third SnO 2 /ITO glass structure as the reference electrode to serve the base potential of the electrolyte solution. The pH sensitivity of this differential pH-sensing device is about 30.14 mV/pH and it is linear. Hence, this device is a good pH sensor. By using this technology, the differential pH-sensing device has a lot of advantages, such as simple fabrication, solid-state electrodes, easy packaging, low cost, etc.