Ion Sensitive Field Effect Transistors (ISFETs) are currently produced comm
ercially and promise to become the platform sensors for important biomedica
l applications. The drift in ISFETs is still an important inherent problem
that prevents its application to accurate in vivo measurements. The present
paper presents measurements of the drift and the drain current power spect
ral density (PSD) of pH ISFETs in the very low frequency range from 5 mHz t
o 10 kHz. The measurements have been performed in buffer solutions with pH
4, 7 and 10, at room temperature. Above a corner frequency, the measured sp
ectra correspond to 1/f noise introduced by fluctuations at the channel cur
rent. Below this corner frequency that depends on the magnitude of the drif
t, the measured spectra correspond to 1/f(2). The observed corner frequency
is similar to 1 Hz for a drift of 2 mV/h and shifts to frequencies below 0
.01 Hz for a drift of 0.1 mV/h. The measured drift is correlated to leakage
currents as well as temperature fluctuations and the inherent behaviour of
the ISFET. A method for quality evaluation based on frequency behaviour is
introduced. (C) 2000 Elsevier Science S.A. All rights reserved.