Chemical field-effect transistor

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A ChemFET is a chemically-sensitive field-effect transistor, that is a field-effect transistor used as a sensor for measuring chemical concentrations in solution. [1] When the target analyte concentration changes, the current through the transistor will change accordingly. [2] Here, the analyte solution separates the source and gate electrodes. [3] A concentration gradient between the solution and the gate electrode arises due to a semi-permeable membrane on the FET surface containing receptor moieties that preferentially bind the target analyte. [3] This concentration gradient of charged analyte ions creates a chemical potential between the source and gate, which is in turn measured by the FET. [4]

Contents

Construction

The schematic view of a ChemFET. Source, drain, and gate are the three electrodes used in a FET system. The electron flow takes place in a channel between the drain and source. The gate potential controls the current between the source and drain electrodes. ChemFET.png
The schematic view of a ChemFET. Source, drain, and gate are the three electrodes used in a FET system. The electron flow takes place in a channel between the drain and source. The gate potential controls the current between the source and drain electrodes.

A ChemFET's source and drain are constructed as for an ISFET, with the gate electrode separated from the source electrode by a solution. [4] The gate electrode's interface with the solution is a semi-permeable membrane containing the receptors, and a gap to allow the substance under test to come in contact with the sensitive receptor moieties. [5] A ChemFET's threshold voltage depends on the concentration gradient between the analyte in solution and the analyte in contact with its receptor-embedded semi-permeable barrier. [5]

Often, ionophores are used to facilitate analyte ion mobility through the substrate to the receptor. [6] For example, when targeting anions, quaternary ammonium salts (such as tetraoctylammonium bromide) are used to provide cationic nature to the membrane, facilitating anion mobility through the substrate to the receptor moieties. [7]

Applications

ChemFETs can be utilized in either liquid or gas phase to detect target analyte, requiring reversible binding of analyte with a receptor located in the gate electrode membrane. [8] [3] There is a wide range of applications of ChemFETs, including most notably anion or cation selective sensing. [5] More work has been done with cation-sensing ChemFETs than anion-sensing ChemFETs. [5] Anion-sensing is more complicated than cation-sensing in ChemFETs due to many factors, including the size, shape, geometry, polarity, and pH of the species of interest. [5]

Practical Limitations

The body of a ChemFET is generally found to be robust. [9] [4] However, the unavoidable requirement for a separate reference electrode makes the system more bulky overall and potentially more fragile.

History

The MOSFET (metal-oxide-semiconductor field-effect transistor) [10] was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng in 1959. [11] Dutch engineer Piet Bergveld later studied the MOSFET and realized it could be adapted into a sensor for chemical and biological applications. [10]

In 1970, Bergveld invented the ion-sensitive field-effect transistor (ISFET). [12] He described the ISFET as "a special type of MOSFET with a gate at a certain distance". [10] In the ISFET structure, the metal gate of a standard MOSFET is replaced by an ion-sensitive membrane, electrolyte solution and reference electrode. [13]

ChemFETs are based on a modified ISFET, a concept developed by Bergveld in the 1970s. [4] There is some confusion as to the relationship between ChemFETs and ISFETs. Whereas an ISFET only detects ions, a ChemFET detects any chemical (including ions).

See also

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MOSFET Transistor used for amplifying or switching electronic signals.

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Sensor converter that measures a physical quantity and converts it into a signal

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Organic field-effect transistor

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ISFET

An ion-sensitive field-effect transistor (ISFET) is a field-effect transistor used for measuring ion concentrations in solution; when the ion concentration (such as H+, see pH scale) changes, the current through the transistor will change accordingly. Here, the solution is used as the gate electrode. A voltage between substrate and oxide surfaces arises due to an ion sheath. It is a special type of MOSFET (metal-oxide-semiconductor field-effect transistor), and shares the same basic structure, but with the metal gate replaced by an ion-sensitive membrane, electrolyte solution and reference electrode. Invented in 1970, the ISFET was the first biosensor FET (BioFET).

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Multigate device type of MOS field-effect transistor with more than one gate

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Molecular sensor

A molecular sensor or chemosensor is a molecular structure that is used for sensing of an analyte to produce a detectable change or a signal. The action of a chemosensor, relies on an interaction occurring at the molecular level, usually involves the continuous monitoring of the activity of a chemical species in a given matrix such as solution, air, blood, tissue, waste effluents, drinking water, etc. The application of chemosensors is referred to as chemosensing, which is a form of molecular recognition. All chemosensors are designed to contain a signalling moiety and a recognition moiety, that is connected either directly to each other or through a some kind of connector or a spacer. The signalling is often optically based electromagnetic radiation, giving rise to changes in either the ultraviolet and visible absorption or the emission properties of the sensors. Chemosensors may also be electrochemically based. Small molecule sensors are related to chemosensors. These are traditionally, however, considered as being structurally simple molecules and reflect the need to form chelating molecules for complexing ions in analytical chemistry. Chemosensors are synthetic analogues of biosensors, but such sensors incorporate biological receptor such as antibodies, aptamers or large biopolymers.

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Field-effect transistor transistor that uses an electric field to control its electrical behaviour

The field-effect transistor (FET) is a type of transistor which uses an electric field to control the flow of current. FETs are devices with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

Chemiresistor

A chemiresistor is a material that changes its electrical resistance in response to changes in the nearby chemical environment. Chemiresistors are a class of chemical sensors that rely on the direct chemical interaction between the sensing material and the analyte. The sensing material and the analyte can interact by covalent bonding, hydrogen bonding, or molecular recognition. Several different materials have chemiresistor properties: metal-oxide semiconductors, some conductive polymers, and nanomaterials like graphene, carbon nanotubes and nanoparticles. Typically these materials are used as partially selective sensors in devices like electronic tongues or electronic noses.

Bio-FET

A field-effect transistor-based biosensor, also known as a biosensor field-effect transistor, field-effect biosensor (FEB), or biosensor MOSFET, is a field-effect transistor that is gated by changes in the surface potential induced by the binding of molecules. When charged molecules, such as biomolecules, bind to the FET gate, which is usually a dielectric material, they can change the charge distribution of the underlying semiconductor material resulting in a change in conductance of the FET channel. A Bio-FET consists of two main compartments: one is the biological recognition element and the other is the field-effect transistor. The BioFET structure is largely based on the ion-sensitive field-effect transistor (ISFET), a type of metal-oxide-semiconductor field-effect transistor (MOSFET) where the metal gate is replaced by an ion-sensitive membrane, electrolyte solution and reference electrode.

Piet Bergveld is a Dutch electrical engineer. He is the emeritus professor of biosensors at the University of Twente. He is the inventor of the ion-sensitive field-effect transistor (ISFET) sensor. Bergveld's work has focused on electrical engineering and biomedical technology.

References

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