Two-Regime Drift in Electrolytically Gated FETs and BioFETs

Abstract

Drift is the most well-known reliability issue of electrolytically gated ISFETs and bioFET sensors. The shift of the threshold voltage with time of electrolytically gated FETs compromises the reliability of these sensors. Notwithstanding, relatively little work has been done to understand the issue. We distinguish two drift regimes which occur at room temperature and at approximately zero gate bias conditions: a strong initial transient drift (81-144mV) and a weaker eventual drift (-0.6 - 0.8mV/hr), and we elaborate on the strong initial transient drift. We compare drift in SiO2, SiON and HfO2 FETs and discuss kinetics, area and salinity dependence. An SC1 preclean shows no significant impact on transient drift precluding a role of siloxane oxidation in initial drift. Two hypotheses regarding the cause of the initial drift are tested with tailored experiments: gate oxide hydration and gate oxide contamination. We find contamination to most adequately explain our observations. For bioFETs, the addition of a Self-Assembled Monolayer (SAM) does not have a large impact on initial drift whereas a DNA graft strongly reduces the initial drift.