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TEST & MEASUREMENT in the characteristics of light. For this method to work, one of the reactants or products of the biorecognition reaction has to be linked to colorimetric, fluorescent or luminescent indicators. An optical fiber is sometimes used for guiding light signals from the source to the detector. Biosensor validation starts with sensor characterization Achieving a stable, reproducible interface between the biological affinity elements and an inorganic transducer element is a major challenge in biosensor design. Fast, accurate electrical characterization is essential for qualifying the sensor/transducer interface, as well as the ultimate operation of a biodetection system. Developing or verifying performance metrics for the biosensor is an important part of biosensor development. Because of the complexity of extracting cell and tissue signatures of agent activity and response, it is often desirable to conduct direct current-voltage (I-V) characterization on key components of the biosensor. Although I-V characterization requires only a small fraction of the time needed for most types of functional testing, it is a powerful predictor of the device’s actual performance in operation. For example, I-V data can be used to study anomalies, locate maximum or minimum curve slopes, and perform reliability analyses, often on sensors based on amperometric, potentiometric, conductive, resistive, and thermal principles. I-V testing typically involves applying a voltage or current to the device under test (DUT) and measuring its response to that stimulus. Temperature measurements may also be taken. The test procedures may involve probing integrated circuits to apply the stimulus to certain connection pads and measure the DUT response on others. The signal levels involved are often extremely low, which demands the use of sensitive instrumentation and techniques that minimize external sources of error. Biosensors are often designed for use in portable, batterypowered equipment, which can restrict their operational power requirements and may dictate the level of voltage or current Fig. 4: The Model 2450’s touchscreen GUI and contextsensitive help function allows even novice SMU instrument users to acquire the data they need quickly and with confidence. output that can be provided to the measurement circuitry. In battery-operated systems, sensor output current can range from nanoamps to milliamps and voltage from nanovolts to volts. Different measurement techniques and tools are required for characterizing signals over these wide ranges. Source measure unit (SMU) instruments can simplify the time-sensitive triggering involved in the I-V characterization process. Essentially, SMU instruments integrate the capabilities of a precision power supply (PPS) with those of a high-performance digital multimeter (DMM) in a single instrument. They can simultaneously source or sink voltage while measuring current, and source or sink current while measuring voltage. Figure 2 illustrates an SMU instrument configured as a constant current source and voltmeter to measure the response from a DUT. SMU instruments allow storing many different test sequences in onboard program memory, which can then be executed with a simple trigger signal. Test data can be stored in a buffer memory until an I-V sweep is completed and then downloaded to a PC for processing and analysis. Testing BioFET sensors As mentioned previously, a FET can be fabricated to work with bio-materials to become a biosensor. The FET is a transistor that uses an electric field to control the shape and, therefore, the conductivity of a channel of one type of charge carrier in a semiconductor material. Determining the I-V parameters of a bioFET helps ensure that it functions properly in its intended applications and that it meets specifications. Figure 3 illustrates how to set up an I-V characterization 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 (figure 4) 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 makes 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. Fig. 3: Two Model 2450 SMU instruments configured to test a three-terminal bioFET. 32 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
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