ISFET Practical Limitations and Reference Electrodes:
– ISFET electrodes sensitive to H+ concentration for pH measurement.
– Requires a reference electrode for operation.
– Conventional reference electrodes like AgCl or Hg can have limitations.
– Bulky and fragile classical reference electrodes hinder miniaturization.
– Research focused on embedded tiny REFETs for solutions.
– Bergveld’s impact on MOSFET-based sensors.
– Recent advances in biologically sensitive field-effect transistors (BioFETs).
– Investigation of the dominant 1/f noise source in silicon nanowire sensors.
– Ion-gated bipolar amplifier for ion sensing with enhanced signal and noise performance.
– Device noise reduction for silicon nanowire field-effect-transistor based sensors using a Schottky junction gate.
ISFET Low-Frequency Noise and Noise Suppression:
– Low-frequency noise affects signal-to-noise ratio.
– Sources of noise include external, intrinsic, and extrinsic factors.
– Methods to suppress noise include integrating a bipolar junction transistor.
– Replacement of noisy oxide/Si interface with a Schottky junction gate.
– Noise suppression crucial for accurate biomedical signal detection.
History and Applications of ISFET:
– ISFET based on MOSFET, adapted by Dutch engineer Piet Bergveld.
– Invention of ISFET in 1970 for electrochemical and biological applications.
– ISFET widely used in biomedical applications for various detections.
– Basis for later BioFETs like DNA field-effect transistor in genetic technology.
– ISFET sensors can be integrated into CMOS technology for diverse applications.
– Bergveld’s impact of MOSFET-based sensors.
– The development and application of FET-based biosensors.
– A critical evaluation of direct electrical protein detection methods.
Related Concepts and Instruments:
– Chemical field-effect transistor, ion-selective electrodes, and MOSFET.
– pH meter, potentiometry, and quinhydrone electrode.
– Saturated calomel electrode, silver chloride electrode, and standard hydrogen electrode.
– An integrated semiconductor device enabling non-optical genome sequencing.
– Practical limits for solid-state reference electrodes.
– Towards REFET – a focus on electrical and chemical requirements for REFETs.
Further Readings and Studies on ISFET:
– Rothberg’s work on non-optical genome sequencing.
– A simple REFET for pH detection in differential mode.
– Solid-state reference electrodes for potentiometric sensors.
– Design and modeling of ISFET for pH sensing.
– Plastic reference electrodes and potentiometric cells with dispersion cast membranes.
– Development of an FET-type reference electrode for pH detection.
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).
The surface hydrolysis of Si–OH 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 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 MOSFET. The gate electrode is separated from the channel by a barrier which is sensitive to hydrogen ions and a gap to allow the substance 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.
