ISFET Arrays in CMOS: A Head-to-Head Comparison Between Voltage and Current Mode

This paper demonstrates and compares three configurations for ISFET arrays fabricated in unmodified CMOS that facilitate chemical imaging with linear pH-to-output conversion. Specifically, the typical voltage-mode source-follower configuration is compared against our recently proposed current-mode approach using an ISFET in velocity saturation. These two topologies are also compared against an extension of the current-mode topology to include a programmable-gate capacitor inside the pixel stack for offset compensation. Various performance metrics, such as linearity, speed, power consumption, size, attenuation, and noise have been experimentally measured to quantify the performance of each approach and a figure-of-merit (FoM) has been defined to capture the overall performance. Using this FoM, the current-mode approach results in the best overall performance. Through this paper, however, the relative merits and limitations of each approach are identified to be used as a guide for future designs which are usually application-specific. Furthermore, we illustrate methods used to obtain standard ISFET performance metrics in order to standardize the comparison of ISFET arrays using the figure-of-merit.