ISFET Technology
Miniature solid-state pH and ion sensing for real-time chemical monitoring
In short
The ISFET (Ion Sensitive Field Effect Transistor) is a modified version of the traditional MOSFET. Instead of a metal gate, it uses an ion-sensitive membrane that is in direct contact with a liquid. The electric field created by the ion concentration at the sensor surface (e.g., H⁺ ions in pH sensing) modulates the current flow between the source and drain terminals.
Through the interaction between the ions and the sensor surface the ISFET essentially transduces chemical (ionic) information into an electrical signal.
Advantages
- suitable for integrated circuits
- fast response time
- solid state
- can operate in small sample volumes
In detail
How ISFET sensing works
When the ISFET gate insulator is exposed to an electrolyte solution, ions in the solution interact with the insulator surface. As a result, the surface potential of the insulator changes based on the concentration of specific ions (e.g.,H⁺ ions for pH sensing). This change in surface potential modulates the channel conductivity between the source and drain, similar to how a gate voltage would in a traditional FET.
Key components of an ISFET
- Semiconductor Substrate (e.g. Si)
- Source and Drain regions
- Gate Insulator (e.g. Ta2O5) - exposed to the liquid
- Reference electrode (e.g. MSREF) - placed in the liquid to apply a stable gate voltage
- Liquid - where to ion concentration is to be measured
ISFET functionalization
When an ISFET needs to detect something beyond basic pH (H⁺ concentration), the ion-sensitive gate area can be modified to make the ISFET selective to specific ions or chemical species.
ISFET functionalization is achieved by depositing a thin polymer membrane onto the gate surface. The specific selectivity of the membrane is tailored through the inclusion of ionophores.
Application
Versatile pH and ion measurement across industries
- Nutrient monitoring in soil and hydroponic installations
- Water quality control
- Chemical process supervision
- Food safety and fermentation tracking
- Laboratory instrumentation
- Biomedical analysis
