ISFET-REFET Ion Sensor Differential Measurement Microreservoir
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Solution Overview
Problem
Current ion measurement technologies, such as test strips and ion-selective electrodes, are imprecise, expensive, require maintenance, and are not easily miniaturized, while ISFET-REFET sensors are costly to manufacture and have a short lifespan due to reference solution contamination and evaporation issues.
Innovation Solution
A novel ion sensor using differential measurement with at least one ISFET and one REFET transistor, where the REFET has a microreservoir filled with a reference solution connected via a microchannel, allowing for precise ion concentration measurement without a separate reference electrode, and a manufacturing method that enables mass production at low cost, including encapsulation and hydrogel use to extend sensor life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If test strips are used for ion measurement, then the measurement process is simple and inexpensive, but the measurement precision is low (0.5 units resolution) and diagnostic value is insufficient
Solution Approach 1:
The patent replaces the mechanical/chemical test strip system with an electronic ISFET-REFET sensor system. The ISFET transistor detects ion concentration through electrical field effects at the gate oxide interface, while the REFET provides reference potential, enabling precise differential measurement. This substitution of mechanical chemical reaction-based measurement with electrical field-based measurement achieves both simplicity and high precision (0.01 pH unit resolution).
Solution Approach 2:
The patent creates a reference copy (REFET) of the measurement sensor (ISFET) that is identical in structure but isolated from the measured solution through a liquid membrane. The REFET copies the ISFET's electrical characteristics and response behavior, allowing differential measurement that eliminates common-mode errors and drift, thereby achieving stable, high-precision measurements over time.
2Device complexity
If ion-selective electrodes (ISEs) are used, then equipment complexity is reduced and cost is lowered, but the device requires a separate reference electrode and periodic maintenance for reference solution refilling
Solution Approach 1:
The patent merges the reference electrode function directly into the REFET transistor structure. The REFET's gate oxide and underlying semiconductor substrate form an integrated reference potential system that eliminates the need for separate reference electrodes and liquid membrane assemblies. This integration reduces device complexity and eliminates maintenance requirements for reference solution refilling, as the reference potential is generated solid-state within the transistor itself.
Solution Approach 2:
The patent replaces the liquid-based reference system (reference electrode with liquid membrane requiring periodic refilling) with a solid-state semiconductor reference system. The REFET transistor uses its semiconductor-substrate interface to generate a stable reference potential without liquid electrolytes, eliminating the mechanical refilling operation and associated maintenance.
3Measurement precision
If ISFET-REFET sensors are used, then measurement precision is improved, but manufacturing cost increases and sensor lifespan is reduced due to reference solution contamination and evaporation
Solution Approach 1:
The patent replaces the liquid membrane-based reference isolation system with a solid-state semiconductor reference system. The REFET transistor uses its semiconductor-substrate interface and gate oxide structure to provide reference potential without requiring liquid membranes that can leak or evaporate. This substitution eliminates contamination and evaporation problems while reducing manufacturing complexity and cost through standard semiconductor fabrication processes.
Solution Approach 2:
The patent employs standard semiconductor fabrication techniques to manufacture the ISFET-REFET sensor pair, making the sensors inexpensive and potentially disposable. The use of conventional CMOS-compatible processes allows mass production at low cost, and the solid-state construction eliminates long-term degradation issues, making the sensors economically viable for single-use or limited-life applications where replacement is cheaper than maintenance.
4Adaptability or versatility
If conventional ISFET sensors are used, then ion measurement capability is achieved, but the sensor requires a separate reference electrode which increases device complexity and maintenance requirements
Solution Approach 1:
The patent merges the reference electrode function into the REFET transistor, which is fabricated alongside the ISFET on the same semiconductor substrate. Both transistors share the same gate oxide and substrate infrastructure, and the REFET provides the reference potential directly through its semiconductor interface, eliminating the need for separate reference electrode assemblies and reducing overall device complexity while maintaining full ion measurement capability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The sensor provides accurate, cost-effective, and long-lasting ion concentration measurements by eliminating the need for a separate reference electrode and reducing manufacturing costs, while allowing for simultaneous measurement of multiple ions and extended sensor life through controlled rehydration and contamination prevention.
Implementation Method 1
The potential of the solution (which is the transistor gate potential) is controlled by a reference electrode such as those used for the measurement with ISE-type electrodes. The ISFET is a field effect transistor whose threshold voltage varies with the ion concentration of the solution in contact with its gate dielectric.
Implementation Method 2
A second object of the present invention is the manufacturing method of the sensor previously described which allows its mass production at low cost. In 1978, a solution was disclosed for pH measurement with ISFET-type devices without reference electrode which consisted of the differential measurement of an ISFET and a REFET. The REFET gate is maintained exposed to a constant pH by the incorporation of a microreservoir filled with reference solution (internal solution).
Implementation Method 3
The REFET gate is maintained exposed to a constant pH by the incorporation of a microreservoir filled with reference solution (internal solution). Said microreservoir is connected to the exterior via a microchannel which acts as liquid bond, so that the difference in potential between the external solution and the reference solution is small and is not greatly influence by the pH or the concentration of other ions in the external solution.
Data Source
AI summary
Ion sensor based on differential measurement comprising an ISFTET-REFET pair wherein the REFET is defined by a structure composed of an ISFET covered by a microreservoir where an internal reference solution is contained. The sensor comprises a first and a second ion-selective field effect transistor, an electrode, a substrate on the surface whereof are integrated the two transistors, connection tracks and the electrode and a structure adhered on the first ion-selective field effect transistor which creates a microreservoir on the gate of said first transistor, with the microreservoir having a microchannel which connects the microreservoir with the exterior and the microreservoir being filled with the reference solution.