An adaptive CMOS-based PG-ISFET for pH sensing

This paper presents a novel CMOS based PG-ISFET (programmable gate-ion sensitive field effect transistor) and readout for compensation of large threshold voltages observed with ISFETs fabricated in a standard CMOS process. The proposed device uses a capacitively coupled floating gate to allow tunability of its operating point to counteract the presence of trapped charge, thus allowing operation within a tolerable gate voltage range. By using feedback, an adaptive readout has been designed, which allows integration of the device as well as cancellation of reduced sensitivity due to extra capacitance of the programmable gate. Fabricated in a 0.35 mum CMOS process, the device can compensate for a variation of up to 14.2 V for 1 muA using a 3.3 V supply.