ISFET-based pH sensor-control successfully miniaturised and optimised for easy use

The Fraunhofer Institute for Photonic Microsystems IPMS has achieved another milestone in chemical liquid analysis. The electronics needed to control the ISFET have been successfully miniaturised. At the same time, manufacturing costs and power consumption have been reduced greatly. The new electronics can be made available for direct use or integration into in-house measuring systems.

ISFETs allow the continuous and precise measurement of pH values by determining the concentration of certain ions in water or other aqueous media in real-time.

Following the outstanding development of niobium pentoxide-based ISFET pH-sensors, Fraunhofer IPMS is again reporting a major success: the new measuring systems operate with even lower power consumption than before. "After almost one year of development, we have succeeded in controlling our Nb2O5-ISFETs to measure continuously with a power consumption of less than 1.3mW, including electronics," says Dr Olaf R. Hild, head of the business unit Chemical Sensors at Fraunhofer IPMS. The power consumption of the sensor system now only amounts to 190µW. Power consumption and size are essential parameters for mobile measuring systems. Applications arise in continuous water monitoring and environmental analysis. However, long-term applications in medical technology, such as the analysis of assorted body fluids, also need small, high-performance measuring systems.

The new control electronics, which will be presented at the 'Sensor and Test' trade fair in Nuremberg in May, are particularly low-powered and, therefore, more energy-efficient, very easy to handle, and ready for immediate use. They consist of analog electronics (<1.3mW) and digital electronics that can be connected via USB-C (approx. 100mW), which allows fast on-site calibration: "As the Fraunhofer IPMS ISFETs are of extremely low-drift and exhibit almost perfect Nernst dependence, a single-point calibration is adequate for the vast majority of applications," explains electronics developer Hans-Georg Dallmann. This ensures a high level of accuracy, even over longer periods.

But Hild's team is still not satisfied with what they have achieved: "The next goal is even smaller ISFET chips (<1mm2) in order to be able to address size-limited applications. Our cleanroom is perfectly well equipped for this challenge," says technologist Falah Al-Falahi confidently.

The novel ISFET from Fraunhofer IPMS is based on a MOS field-effect transistor technology, whereby the sensor area in contact with the medium is an amphoteric metal oxide layer. Hydronium or hydroxide ions from the measuring medium are reversibly deposited on this layer according to the pH value (pH-sensitive layer). The operating voltage (UDS) of the ISFET, which is applied between the source and the drain, leads to a current (IDS). This current is always kept constant during the measurement (constant charge mode). The voltage (UGS) between the source and the gate or the reference electrode (Ag/AgCl in 3M KCl) is then employed as the measurement signal.

Meet scientists at Sensor + Test at Booth 1-317, from 6-8 May 2025, where the latest developments and possible applications will be presented.