Bio-FET

Field effect transistor based biosensor (Bio-FET) is a field-effect transistor that is gated by biological molecules. When biological molecules bind to the FET gate they can change the gate charge distribution resulting in a change in conductance of the FET channel.[1] A Bio-FET consists of two main compartments: one is the biological recognition element and the other is the field effect transistor (e.g. ISFET).[2]

The schematic view of an ISFET. Source and drain are the two electrodes used in a FET system. The electron flow takes place on the channel in which connects the drain and source. The gate controls the flow of current between the two electrodes.

Function of Bio-FET

Bio-FETs couple a semiconductor device to a bio-sensitive layer that detects bio-molecules such as nucleic acids.[3] A Bio-FET system consists of a semiconducting transducer separated by an insulator layer (e.g. SiO2) from the biological recognition element (e.g. receptors or probe molecules). A Bio-FET also consists of a transducer immobilized to the surface which contains the target molecule called analyte. Once the analyte binds to the recognition element, the charge distribution at the surface will change, triggering a change in the electrostatic potential of the semiconductor. The transistor controls the electron flow in the channel using two electrodes, which are called source and drain.[4] As a result, the conductance of the semiconductor changes, which can be measured. This change of conductance directly yields the concentration.[3] The current flow between the source and drain is controlled by the gate.[4]

The source and drain metals act as two electrodes. A polymer is placed in between the electrodes, which acts as a channel to allow the flow of electrons from one electrode to another. The gate oxide and semiconductor gate controls this flow. The changes are measured and detected by the bio receptors depending on the analyte used for analysis.[5]

Fabrication of Bio-FET

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

  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 photoresist[6]

Advantages

Bio-FET is widely used in 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 methods can be consuming much more time, expensive, involving multi-stage processes and also not compatible to on-line monitoring comparing to Bio-FET. Bio-FET is simply made up of a sensor and a measurement circuit. Its simple system makes it has the advantage of low weight, low cost of mass production, small size and compatible with commercial planar processes for large-scale circuitry. It could do biological molecule measurement via voltage change in a single step, which consumes less time and gives fast response.[7] An integration of Bio-FET with a digital microfluidic device forming one simple chip utilized the advantage of simple system of Bio-FET. It can move sample droplets while detecting target molecules so the transport and detection of bio-molecules, the recording of signals, signal processing, and the data transmission process can be done using an all-in-one chip.[8] The bio-receptors are always chosen to be highly specific to target molecule, which gives Bio-FET the advantage of high sensitivity. Bio-FET also does not require any labeling in its process.[7] Some Bio-FETs display fascinating electronic and optical properties. A glucose-sensitive ENFET was fabricated based on the modification of the gate surface of ISFET with SiO2 nanoparticles and glucose oxidase (GOD), which shows obviously enhanced sensibility and extended lifetime compared with that without SiO2 nanoparticle.[9] Also, Bio-FET technology could be used to monitor security level rapidly in our environment.[6]

Bio-FETs are classified based on the bio recognition element used for detection: En-FET which is an enzyme-modified FET, Immuno-FET which is an immunologically modified FET, DNA-FET which is a DNA-modified FET, CPFET which is cell-potential FET, beetle/chip FET and artificial BioFET-based.

Optimizations

Bio-FET has raised people’s attention and many scholars are trying to optimize the function of Bio-FET. One optimization of Bio-FET is to put bio liquid, which is electrically charged with ions of analytes, transport in the gate space to increase the gate voltage with the same analyte concentration therefore optimize the function of receptor. When the receptor layer is uniformly distributed in the gate space, the bio liquid effect is maximized.[10] Also, putting a hydrophobic passivation surface on the source and the drain, which is enhanced comparing to covering with a hydrophilic passivation, could enhance the sensitivity of Bio-FET, because the hydrophobic passivation surface enhanced the binding probability of biomolecules.[11][12]

References

  1. ↑ Brand U, Brandes L, Koch V, Kullik T, Reinhardt B, Rüther F, Scheper T, Schügerl K, Wang S, Wu X: Monitoring and control of biotechnological production processes by Bio-FET-FIA-sensors, Appl Microbiol Biotechnol., 1991 Nov;36(2):167-72.
  2. ↑ Joonhyung Lee, Piyush Dak, Yeonsung Lee, Heekyeong Park, Woong Choi, Muhammad A. Alam, Sunkook Kim: Two-dimensional Layered MoS2 Biosensors Enable Highly Sensitive Detection of Biomolecules, Sci Rep., 2014; 4: 7352.
  3. 1 2 Alena Bulyha, Clemens Heitzinger and Norbert J Mauser: Bio-Sensors: Modelling and Simulation of Biologically Sensitive Field-Effect-Transistors, ERCIM News, 04,2011.
  4. 1 2 Matsumoto, A; Miyahara, Y (21 November 2013). "Current and emerging challenges of field effect transistor based bio-sensing". Nanoscale 5 (22): 10702–10718. doi:10.1039/c3nr02703a.
  5. ↑ Pumera, Martin (July–August 2011). "Graphene in biosensing". Materials Today 14 (7–8): 308–315. doi:10.1016/S1369-7021(11)70160-2.
  6. 1 2 Yuji Miyahara, Toshiya Sakata, Akira Matsumoto: Microbio genetic analysisbased on Field Effect Transistors, Principles of Bacterial Detection: Biosensors, Recognition Receptors and Microsystems.
  7. 1 2 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.
  8. ↑ 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
  9. ↑ 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
  10. ↑ C.RAVARIU, F.RAVARIU: Some Optimizations of Bio-FETs with Electrical Charged Bio liquid, Romanian Reports in physcs, Vol. 58, No.2, P. 189-194, 2006.
  11. ↑ 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., 2013
  12. ↑ A. 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)
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