Low frequency noise and drift in Ion Sensitive Field Effect Transistors

Ion Sensitive Field Effect Transistors (ISFETs) are currently produced commercially and promise to become the platform sensors for important biomedical 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 spectral density (PSD) of pH ISFETs in the very low frequency range from 5 mHz to 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 spectra correspond to 1/f noise introduced by fluctuations at the channel current. Below this corner frequency that depends on the magnitude of the drift, the measured spectra correspond to 1/f². The observed corner frequency is approx. 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.