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

Abstract

An extended gate ion sensitive field effect transistor with signal interface was presented for continuous monitoring of H + -ion concentrations. The SnO 2 /ITO glass, fabricated by sputtering SnO 2 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 1.8V supply voltage and normal sampling rate of 6.25MHz. The circuit (without pad) occupied an area of 0.66mm × 0.43mm. 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 ±0.25pH.