A novel pH-to-time ISFET pixel architecture with offset compensation

A novel pixel architecture is presented to be part of a pH-based DNA microarray using ISFETs as chemical sensors. The design, based on APS architectures, performs pulse width modulation to encode the pH in time. It allows compensation for a major ISFET nonideality, offset in the threshold voltage, which is caused by trapped charge on the floating gate and the passivation layer of the ISFET. Also, the drop in sensitivity due to the passivation capacitance inherent to CMOS processes is attenuated. The system is implemented using a 0.35 fim standard CMOS technology. The pixel is shown to achieve a high and tunable accuracy of between 0.55 μs/dpH and 2.9 μs/dpH. Along with an estimation of the noise and the incidence of calibration, this leads to a resolution of approximately 20 mpH on a 1 pH-range. Trapped charge compensation proves to be effective up to an offset voltage of 1.25 V. The pixel is compact and reaches a total area of 16.5 μm × 16.25 μm.