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Perform electrical characterisation of biosensors

Posted: 10 Mar 2014 ?? ?Print Version ?Bookmark and Share

Keywords:biosensors? transducer? analyte <b>validation? sensor characterisation? </b>digital multi-meter?

Figure 3 is a highly simplified illustration of whats involved in fabricating a bioFET semiconductor transducer. The dielectric layer, an oxide such as silicon dioxide, has two functions: to isolate the channel of the FET from the liquid and to couple the surface layer charge electrostatically into the channel. A biofunctionalized layer that exhibits immobilized biomolecule receptors capable of binding the desired molecule lies on top of the dielectric. The analyte is a solution that contains the dissolved sample molecules. The reference electrode allows adjusting the device in order to maximise its sensitivity. If the target molecules bind to the receptors, a change in the surface charge density occurs. This change alters the potential in the semiconductor and thus the conductivity in the channel of the FET [2].

Determining the I-V parameters of a bioFET helps ensure that it functions properly in its intended applications and that it meets specifications. Figure 4 illustrates how to set up an I-V characterisation system for a three-terminal bioFET using two Keithley Model 2450 SourceMeter SMU Instruments. The number of instruments required for testing depends on the number of FET terminals that must be biased and measured. The Model 2450 is suitable for a wide range of I-V tests, including gate leakage, breakdown voltage, threshold voltage, transfer characteristics, and drain current. Just as important, its touchscreen-based graphical user interface make instrument navigation an intuitive experience by representing many functions and parameters graphically, which helps substantially reduce the learning curve associated with using a new instrument.

Although figure 5 illustrates the results of generating a drain family of curves (VDS-ID) on a three-terminal FET, the same techniques are applicable to bioFET devices.

Figure 4: Two Model 2450 SMU instruments configured to test a three-terminal bioFET.

Figure 5: Typical FET drain family of curves generated with two Model 2450 SMU instruments.

Conclusion
The right I-V instrumentation and characterisation techniques can greatly simplify qualifying sensors for bio detection systems and analytical instruments.

References
[1] Wikipedia, http://en.wikipedia.org/wiki/Biosensor.
[2] http://www.iue.tuwien.ac.at/phd/windbacher/node24.html.

About the author
Jonathan Tucker is Senior Marketing and Product Manager for Keithley Instruments, which is part of the Tektronix test and measurement portfolio. He joined Keithley in 1987. During his tenure, he has served in a variety of positions, including manufacturing test engineer, applications engineer, applications manager, product manager, and business development manager.

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