Bio-FET

 ( Biosensors ) 

A field-effect transistor-based biosensor, also known as a biosensor field-effect transistor ( Bio-FET or BioFET), field-effect biosensor ( FEB), or biosensor MOSFET, is a field-effect transistor (based on the MOSFET structure) 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.

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Mechanism of operation Bio-FETs couple a transistor device with a bio-sensitive layer that can specifically detect bio-molecules such as nucleic acids and proteins. A Bio-FET system consists of a semiconducting field-effect transistor that acts as a transducer separated by an insulator layer (e.g. Silicon dioxide) from the biological recognition element (e.g. receptors or probe molecules) which are selective to the target molecule called analyte.Alena Bulyha, Clemens Heitzinger and Norbert J Mauser: Bio-Sensors: Modelling and Simulation of Biologically Sensitive Field-Effect-Transistors, ERCIM News, 04,2011. Once the analyte binds to the recognition element, the charge distribution at the surface changes with a corresponding change in the electrostatic surface potential of the semiconductor. This change in the surface potential of the semiconductor acts like a gate voltage would in a traditional MOSFET, i.e. changing the amount of current that can flow between the source and drain electrodes. This change in current (or conductance) can be measured, thus the binding of the analyte can be detected. The precise relationship between the current and analyte concentration depends upon the region of transistor operation.

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Fabrication of Bio-FET The fabrication of Bio-FET system consists of several steps as follows:

1. Finding a substrate suitable for serving as a FET site, and forming a FET on the substrate,
2. Exposing an active site of the FET from the substrate,
3. Providing a sensing film layer on active site of FET,
4. Providing a receptor on the sensing film layer in order to be used for ion detection,
5. Removing a semiconductor layer, and thinning a dielectric layer,
6. Etching the remaining portion of the dielectric layer to expose an active site of the FET,
7. Removing the photoresist, and depositing a sensing film layer followed by formation of a photoresist pattern on the sensing film,
8. Etching the unprotected portion of the sensing film layer, and removing the photoresistYuji Miyahara, Toshiya Sakata, Akira Matsumoto: Microbio genetic analysis based on Field Effect Transistors, Principles of Bacterial Detection: Biosensors, Recognition Receptors and Microsystems.

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Advantages The principle of operation of Bio-FET devices based on detecting changes in electrostatic potential due to binding of analyte. This the same mechanism of operation as glass electrode sensors which also detect changes in surface potential but were developed as early as the 1920s. Due to the small magnitude of the changes in surface potential upon binding of biomolecules or changing pH, glass electrodes require a high impedance amplifier which increases the size and cost of the device. In contrast, the advantage of Bio-FET devices is that they operate as an intrinsic amplifier, converting small changes in surface potential to large changes in current (through the transistor component) without the need for additional circuitry. This means BioFETs have the capability to be much smaller and more affordable than glass electrode-based . If the transistor is operated in the subthreshold region, then an exponential increase in current is expected for a unit change in surface potential.

Bio-FETs can be used for detection in fields such as medical diagnostics, biological research, environmental protection and food analysis. Conventional measurements like optical, spectrometric, electrochemical, and SPR measurements can also be used to analyze biological molecules. Nevertheless, these conventional methods are relatively time-consuming and expensive, involving multi-stage processes and also not compatible to real-time monitoring,K.Y.Park, M.S.Kim, K.M.Park, and S.Y.Choi: Fabrication of BioFET sensor for simultaneous detection of protein and DNA, Electrochem.org. in contrast to Bio-FETs. Bio-FETs are low weight, low cost of mass production, small size and compatible with commercial planar processes for large-scale circuitry. They can be easily integrated into digital microfluidic devices for Lab-on-a-chip. For example, a microfluidic device can control sample droplet transport whilst enabling detection of bio-molecules, signal processing, and the data transmission, using an microcontroller.Choi K, Kim JY, Ahn JH, Choi JM, Im M, Choi YK: Integration of field-effect transistor-based biosensors with a digital microfluidic device for a lab-on-a-chip application, Lab Chip., 2012 Apr Bio-FET also does not require any labeling step, and simply utilise a specific molecular (e.g. antibody, ssDNA) on the sensor surface to provide selectivity. Some Bio-FETs display fascinating electronic and optical properties. An example FET would is a glucose-sensitive based on the modification of the gate surface of ISFET with SiO₂ nanoparticles and the enzyme glucose oxidase (GOD); this device showed obviously enhanced sensitivity and extended lifetime compared with that without SiO₂ nanoparticles.Jing-Juan Xu, Xi-Liang Luo and Hong-Yuan Chen: ANALYTICAL ASPECTS OF FET-BASED BIOSENSORS, Frontiers in Bioscience, 10, 420--430, January 1, 2005

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Optimization The choice of reference electrode (liquid gate) or back-gate voltage determines the carrier concentration within the field effect transistor, and therefore its region of operation, therefore the response of the device can be optimised by tuning the gate voltage. If the transistor is operated in the subthreshold region then an exponential increase in current is expected for a unit change in surface potential. The response is often reported as the change in current on analyte binding divided by the initial current ($\backslash Delta\; I/I\_\{0\}$), and this value is always maximal in the subthreshold region of operation due to this exponential amplification. For most devices, optimum signal-to-noise, defined as change in current divided by the baseline noise, ($\backslash Delta\; I/\backslash delta\; i\_\backslash text\{noise\}$) is also obtained when operating in the subthreshold region, however as the noise sources vary between devices, this is device dependent.

One optimization of Bio-FET may be to put a hydrophobic passivation surface on the source and the drain to reduce non-specific biomolecular binding to regions which are not the sensing-surface.Kim JY, Choi K, Moon DI, Ahn JH, Park TJ, Lee SY, Choi YK: Surface engineering for enhancement of sensitivity in an underlap-FET biosensor by control of wettability, Biosens Bioelectron., 2013A. Finn, J.Alderman, J. Schweizer : TOWARDS AN OPTIMIZATION OF FET-BASED BIO-SENSORS, European Cells and Materials, Vol. 4. Suppl. 2, 2002 (pages 21-23) Many other optimisation strategies have been reviewed in the literature.

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History

The MOSFET was invented at Bell Labs between 1959 and 1960.

D. KAHNG (1961). 9789810202095 . ISBN 9789810202095

Bo Lojek (2025). 9783540342588, Springer-Verlag Berlin Heidelberg. ISBN 9783540342588

(2025). 9783540342588, Springer Science & Business Media. ISBN 9783540342588

In 1962, Leland C. Clark and Champ Lyons invented the first biosensor. Biosensor MOSFETs (BioFETs) were later developed, and they have since been widely used to measure physics, chemistry, biological and environmental parameters.

The first BioFET was the ion-sensitive field-effect transistor (ISFET), invented by Piet Bergveld for electrochemical and biological applications in 1970. Other early BioFETs include the adsorption FET (ADFET) patented by P.F. Cox in 1974, and a hydrogen-sensitive MOSFET demonstrated by I. Lundstrom, M.S. Shivaraman, C.S. Svenson and L. Lundkvist in 1975. The ISFET is a special type of MOSFET with a gate at a certain distance, and where the metal gate is replaced by an ion-sensitive membrane, electrolyte solution and reference electrode. The ISFET is widely used in biomedical applications, such as the detection of DNA hybridization, biomarker detection from blood, antibody detection, glucose measurement, pH sensing, and genetic technology.

By the mid-1980s, other BioFETs had been developed, including the gas sensor FET (GASFET), pressure sensor FET (PRESSFET), chemical field-effect transistor (ChemFET), ISFET (REFET), enzyme-modified FET (ENFET) and immunologically modified FET (IMFET). By the early 2000s, BioFETs such as the DNA field-effect transistor (DNAFET), gene-modified FET (GenFET), and cell-potential BioFET (CPFET) had been developed. Current research in this area has produced new formations of the BioFET such as the Organic Electrolyte Gated FET (OEGFET).

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See also

• ChemFET: chemically sensitive field-effect transistor
• ISFET: ion-sensitive field-effect transistor

Categories: Biosensors, Field-effect Transistors, Mosfets, Transistor Types

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