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ChemFET, orchemical field-effect transistor, is a type of a field-effect transistoracting as a

chemical sensor. It is a structural analog of a MOSFET transistor, where the charge on the gate

electrode is applied by a chemical process. It may be used to detect atoms, molecules, and ions inliquids and gases.

ISFET, an

ion-sensitive field-effect transistor, is the best known subtype of ChemFET devices.It is used to detect ionsin electrolytes.

ENFET is a CHEMFET specialized for detection of specific biomolecules using enzymes

ISFET

An ISFET is an ion-sensitive field-effect transistorused for measuring ion concentrations in

solution; when the ion concentration (such as H+ , seepH scale) changes, the current through the

transistorwill change accordingly. Here, the solution is used as the gate electrode. A voltage

between substrate and oxide surfaces arises due to an ionssheath.

The surface hydrolysis of SiOH groups of the gate materials varies in aqueous solutions due to

pH value. Typical gate materials are SiO2, Si3N4, Al2O3 and Ta2O5.

The mechanism responsible for the oxide surface charge can be described by thesite bindingmodel, which describes the equilibrium between the SOH surface sites and the H+ ions in the

solution. The hydroxyl groups coating an oxide surface such as that of SiO2 can donate or accept

a proton and thus behave in an amphoteric way as illustrated by the following acid-base reactionsoccurring at the oxide-electrolyte interface:

SiOH + H2O SiO

+ H3O

+

SiOH + H3O+ SiOH2

+ + H2O

An ISFET's source and drain are constructed as for a MOSFET. The gate electrode is separated

from the channel by a barrier which is sensitive to hydrogen ions and a gap to allow thesubstance under test to come in contact with the sensitive barrier. An ISFET's threshold voltage

depends on the pH of the substance in contact with its ion-sensitive barrier.

Practical limitations due to the reference electrode

An ISFET electrode sensitive to H+ concentration can be used as a conventional glass electrode

to measure thepH of a solution. However, it also requires areference electrodeto operate. If the

reference electrode used in contact with the solution is of the AgCl orHgCl2 classical type, itwill suffer the same limitations as conventional pH electrodes (junction potential, KCl leak, and

glycerolleak in case of gel electrode). A conventional reference electrode can also be bulky and

fragile. A too large volume constrained by a classical reference electrode also precludes the

miniaturization of the ISFET electrode, a mandatory feature for some biological or in vivo

http://en.wikipedia.org/wiki/Field-effect_transistorhttp://en.wikipedia.org/wiki/Field-effect_transistorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/ISFEThttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Field-effect_transistorhttp://en.wikipedia.org/wiki/Hydrogen_ionhttp://en.wikipedia.org/wiki/Hydrogen_ionhttp://en.wikipedia.org/wiki/Hydrogen_ionhttp://en.wikipedia.org/wiki/PHhttp://en.wikipedia.org/wiki/Transistorhttp://en.wikipedia.org/wiki/Oxidehttp://en.wikipedia.org/wiki/Ionshttp://en.wikipedia.org/wiki/Ionshttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Al2O3http://en.wikipedia.org/wiki/Al2O3http://en.wikipedia.org/wiki/Al2O3http://en.wikipedia.org/wiki/Al2O3http://en.wikipedia.org/wiki/Ta2O5http://en.wikipedia.org/wiki/Ta2O5http://en.wikipedia.org/wiki/Ta2O5http://en.wikipedia.org/wiki/Ta2O5http://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/Hydrogen_ionhttp://en.wikipedia.org/wiki/Threshold_voltagehttp://en.wikipedia.org/wiki/Glass_electrodehttp://en.wikipedia.org/wiki/PHhttp://en.wikipedia.org/wiki/Reference_electrodehttp://en.wikipedia.org/wiki/Reference_electrodehttp://en.wikipedia.org/wiki/Reference_electrodehttp://en.wikipedia.org/wiki/Silver_chloride_electrodehttp://en.wikipedia.org/wiki/Saturated_calomel_electrodehttp://en.wikipedia.org/wiki/Saturated_calomel_electrodehttp://en.wikipedia.org/wiki/Potassium_chloridehttp://en.wikipedia.org/wiki/Glycerolhttp://en.wikipedia.org/wiki/Glycerolhttp://en.wikipedia.org/wiki/Field-effect_transistorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/ISFEThttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Field-effect_transistorhttp://en.wikipedia.org/wiki/Hydrogen_ionhttp://en.wikipedia.org/wiki/PHhttp://en.wikipedia.org/wiki/Transistorhttp://en.wikipedia.org/wiki/Oxidehttp://en.wikipedia.org/wiki/Ionshttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Silicon_nitridehttp://en.wikipedia.org/wiki/Al2O3http://en.wikipedia.org/wiki/Ta2O5http://en.wikipedia.org/wiki/MOSFEThttp://en.wikipedia.org/wiki/Hydrogen_ionhttp://en.wikipedia.org/wiki/Threshold_voltagehttp://en.wikipedia.org/wiki/Glass_electrodehttp://en.wikipedia.org/wiki/PHhttp://en.wikipedia.org/wiki/Reference_electrodehttp://en.wikipedia.org/wiki/Silver_chloride_electrodehttp://en.wikipedia.org/wiki/Saturated_calomel_electrodehttp://en.wikipedia.org/wiki/Potassium_chloridehttp://en.wikipedia.org/wiki/Glycerol

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clinical analyses (disposable mini-catheter pH probe). The breakdown of a conventional

reference electrode could also make problem in on-line measurements in the pharmaceutical or

food industry if highly valuable products are contaminated by electrode debris or toxic chemicalcompounds at a late production stage and must be discarded for the sake of safety.

For this reason, since more than 20 years many research efforts have been dedicated to on-chipembarked tiny reference field effect transistors (REFET). Their functioning principle, or

operating mode, can vary, depending on the electrode producers and are often proprietary andprotected by patents. Semi-conductor modified surfaces required for REFET are also not always

in thermodynamical equilibrium with the test solution and can be sensitive to aggressive or

interfering dissolved species or not well characterized aging phenomena. This is not a realproblem if the electrode can be frequently re-calibrated at regular time interval and is easily

maintained during its service life. However, this may be an issue if the electrode has to remain

immersed on-line for prolonged period of time, or is inaccessible for particular constrains relatedto the nature of the measurements itself (geochemical measurements under elevated water

pressure in harsh environments or under anoxic or reducing conditions easily disturbed by

atmospheric oxygen ingress or pressure changes).

A crucial factor for ISFET electrodes, as for conventional glass electrodes, remains thus thereference electrode. When troubleshooting electrode malfunctions, often, most of the problems

have to be searched for from the side of the reference electrode.