Extended Gate H+-Ion Sensitive Field Effect Transistor with Signal Interface

An extended gate ion sensitive field effect transistor with signal interface was presented for continuous monitoring of H⁺-ion concentrations. The SnO₂/ITO glass, fabricated by sputtering SnO₂ on the conductive ITO glass, was used as a pHsensitive membrane of extended gate field effect transistor (EGFET). The signal processing circuits, constructed by using differential sensing and noise shaping techniques, converted the H⁺-ion concentration into digital form. This chip, fabricated in a 0.18-um CMOS 1P6M process, operated at a 1.8 V supply voltage and normal sampling rate of 6.25 MHz. The circuit (without pad) occupied an area of 0.66 mm times 0.43 mm. The experimental data showed that this structure has a linear pH response about 97 digital counts/pH in the ion concentration range between pH2 and pH12, and the gain errors within the H⁺-ion concentrations are less than 3%. The minimum detectable pH value can reach as small as plusmn0.25 pH.