50 years of ISFET - Unbreakable pH sensors
The journey of ion-selective field-effect transistors (ISFETs) as biochemical and biological sensors
In brief
- ISFET technology is based on an MOS transistor arrangement.
- ISFET pH sensors address the critical needs of the food and life sciences industries.
- Modern ISFET sensors provide good CIP stability.
ISFET evolution
The first big milestone was the publication by P. Bergveld in 1970 which described how ion activities in electrochemical or biological systems could be measured by combining the principles of MOS transistors and glass electrodes.
In the following years ISFET technology evolved in many directions, one of which was pH measurement. The reason was clear: to create an unbreakable, glass-free pH sensor to address the critical needs of food and life sciences industries. Glass breakage in traditional pH sensors was causing costly production losses.
ISFET sensors use an MOS transistor arrangement where the metal gate is replaced by an amphoteric metal oxide like Al₂O₃ or Si₃N₄. The medium’s hydronium or hydroxide ions interact with this amphoteric layer, creating a surface charge proportional to the pH value. This creates a conductivity between source and drain which is then measured by the transmitter electronics.
Although they had a small size, flat surface and mechanical stability, the first ISFET sensors were not suitable for industrial process control because of the difficulties in encapsulating the sensor chip.
Breakthroughs and industry applications
The first process ready ISFET sensors appeared in the mid 1990s. By 1996 Endress+Hauser started to cooperate with the Fraunhofer IPMS Institute to develop an ISFET pH sensor for the food industry. Key innovations were the introduction of a new gate material, Ta₂O₅, which improved long term stability, steam sterilizability and fast response time even at low temperatures.
In 2002 Endress+Hauser launched the first ISFET sensors, starting with the CPS471 hygienic sensor. High demand led to further releases, including liquid KCl reference models and open aperture sensors. The first products were analog but with the introduction of Memosens digital technology in 2004 a big step forward was made.
Industry challenges
Although ISFET sensors were widely used in food, beverage, life sciences and chemical industry they had limitations in cleaning-in-place (CIP) processes. Standard CIP methods using hot sodium hydroxide at high temperatures damaged the gate material and required additional maintenance steps. This challenge led to the development of alkali-stable gate materials. A dual layer of Ta₂O₅ was the optimal solution, improving CIP stability and reducing contamination risk.
