ISFET

An ion-sensitive field-effect transistor (ISFET) is a <a href="/facts/Field-effect_transistor/TerYC6Nz">field-effect transistor</a> used for measuring ion concentrations in solution; when the ion concentration (such as <a href="/facts/Hydrogen_ion/6UbSyoz4">H+</a>, see <a href="/facts/PH/RnSv8D7y">pH</a> scale) changes, the current through the <a href="/facts/Transistor/Bmr8zwL0">transistor</a> will change accordingly. Here, the solution is used as the gate electrode. A voltage between substrate and <a href="/facts/Oxide/DXtoVe1C">oxide</a> surfaces arises due to an <a href="/facts/Ion/3bePpDna">ion</a> sheath. It is a special type of <a href="/facts/MOSFET/eTDJT2Km">MOSFET</a> (metal–oxide–semiconductor field-effect transistor), and shares the same basic structure, but with the <a href="/facts/Metal_gate/vFzyEhb4">metal gate</a> replaced by an ion-sensitive <a href="/facts/Membrane/FDlaJe23">membrane</a>, <a href="/facts/Electrolyte/4YE8bIVa">electrolyte</a> solution and <a href="/facts/Reference_electrode/GctgppDA">reference electrode</a>. Invented in 1970, the ISFET was the first <a href="/facts/BioFET/GgMPKWNk">biosensor FET</a> (BioFET).

The surface <a href="/facts/Hydrolysis/JTvwIHPs">hydrolysis</a> of Si–OH groups of the gate materials varies in aqueous solutions due to pH value. Typical gate materials are <a href="/facts/Silicon_dioxide/A3WILp6J">SiO2</a>, <a href="/facts/Silicon_nitride/58X9QPQJ">Si3N4</a>, <a href="/facts/Al2O3/wXM9P6pI">Al2O3</a> and <a href="/facts/Ta2O5/td1xNb2s">Ta2O5</a>.
The mechanism responsible for the oxide surface charge can be described by the site binding model, which describes the equilibrium between the Si–OH 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 reactions occurring at the oxide-electrolyte interface:

—Si–OH + H2O     ↔   —Si–O−     + H3O+
—Si–OH + H3O+   ↔   —Si–OH2+ + H2O
An ISFET's source and drain are constructed as for a <a href="/facts/MOSFET/eTDJT2Km">MOSFET</a>. The gate electrode is separated from the channel by a barrier which is sensitive to <a href="/facts/Hydrogen_ion/6UbSyoz4">hydrogen ions</a> and a gap to allow the substance under test to come in contact with the sensitive barrier. An ISFET's <a href="/facts/Threshold_voltage/IparzT9A">threshold voltage</a> depends on the pH of the substance in contact with its ion-sensitive barrier.

ISFET open-in-new

ISFET