ISFET Pixel Array With Selectable Sensitivity and Bulk-Based Offset-Drift Nullification Capability for Reduction of Non-Ideality Effects

Abstract

This article presents a monolithic, cost-effective, label free, low power, robust <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times2$ </tex-math></inline-formula> fully differential ion-sensitive field-effect transistor (ISFET) pixel array aimed at pH sensing. Each pixel is based on a differential readout architecture which eliminates need for additional non-ideality compensation circuitry. Additionally, the design offers the capability to correct for residual drift and trapped charge offset through substrate-based voltage adjustment during calibration. The design also provides a unique ability for selecting pH sensitivity according to the desired mode of operation which has not been demonstrated previously. We present experimental results obtained from the pixel array fabricated in an unmodified AMS <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.35~\mu \text{m}$ </tex-math></inline-formula> CMOS process using a 3.3 V power supply. Each pixel demonstrates a differential sensitivity of 61 mV/pH when operated in high sensitivity mode and 25 mV/pH when operated in high non-ideality rejection mode over the entire pH range, with the ability to remove the already small drift of 0.3 mV/min and any offset mismatch by adjusting the substrate voltage.