E-Zine September 2013
Ion-sensitive coatings on field effect transistors (ISFET) such as shown in the figure below can also be used to create measurement half-cells for pH sensors.
The field effect transistors are designed such that charge must build up on the gate to allow current to flow from the drain to the source. The gate is coated with silicon nitride or aluminum oxide that selectively adsorbs hydrogen ions from solution thus developing a positive charge on the gate. As the charge on the gate increases, the current through the transistor increases. This device can be operated with a constant gate potential where the drain current is measured and correlated with pH. It can also be operated with a constant drain current allowing the gate potential to vary. In this approach, the gate potential is measured and correlated with pH.
Excerpted from The Consumer Guide to Industrial pH and ORP Instrumentation