Titanium nitride membrane application to extended gate field effect transistor pH sensor using VLSI technology

A new process for the fabrication of the extended gate field effect transis tor (EGFET) together with complementary metal oxide semiconductor (CMOS) ci rcuits on the same chip is reported, The sensing membrane of the EGFET is t itanium nitride (TiN) conducting material and it is fabricated using the r. f. sputtering method. The chips are fabricated using the standard submicron 0.5 mum double poly double metal (DPDM) N-well CMOS IC process. No extra m ask is used in the post-process. An instrument amplifier circuit is describ ed that provides an output voltage dependent on the threshold-voltage varia tions in the sensing membrane. According to the experimental results, the h igh linear sensitivity approaches 57 mV/pH. The hysteresis voltage is 0.5 m V per cycle of buffer solutions of pH7 --> pH4 --> pH7 --> pH 10 --> pH7. T his structure is also insensitive to light. This EGFET is fabricated using the standard technology and no difficulty is experienced in realizing this multi species device. The EGFET and readout circuits are produced using VLS I technology, achieving reduced area and low cost. This device has the adva ntages of mass production.