Biomedical Engineering Reference
In-Depth Information
Fig. 6.1 Schematic
illustration of IS-FET
Biological phenomena are found as molecular behaviors such as DNA, antigen-
antibody reaction, or ligand protein-receptor interaction at cell membrane at the
nanoscale. On the other hand, microelectromechanical systems (MEMS) technology
is useful to produce various micro- or nanoscale devices by top-down approach
[ 2 ]. The developed devices are composed of regular and functional structure, on
which biomolecules are arrayed and detected in a multiplex system. Functional
biomolecules are controlled on the multiarrayed devices and in the microfluidic
system, which are detected with an optical or electrochemical method.
In 1970, Bergverd et al. showed electrical detection method of pH variation
based on change of positive charges of hydrogen ions using field-effect transistor
(FET) on the basis of semiconductor principle [ 3 ]. This is called ion-sensitive
field-effect transistor (IS-FET), as shown in Fig. 6.1 . Semiconductor material is
separated with solution across gate insulator, of which the thickness is not more
than a few hundred nm. The gate insulator is usually composed of oxide such as
SiO 2 ,Ta 2 O 5 ,andAl 2 O 3 or nitride such as Si 3 N 4 [ 4 ]. The hydroxyl groups are
formed at the surface between solution and gate insulators and are so sensitive to
hydrogen ions. These positive charges at the gate surface interact electrostatically
with electrons at the channel in silicon crystal. The field effect caused by charge
density changes at the gate induces the change of drain-source current and threshold
voltage (V T ). This electrical response to hydrogen ions is applicable to Nernstian
response, 59.1 mV/pH.
Recently, FET biosensor is being studied and developed to apply for clinical
diagnosis, drug discovery, tissue engineering, and so on. Since FET biosensor can
detect molecular recognition events accompanied by charge density changes without
labeled materials and be easily arrayed by use of the conventional semiconductor
processes in order to measure multisamples, the platform based on FET chips is
suitable for a simple and cost-effective system for chip-based diagnosis. The down-
sizing of system is significant for personalized medicine at home. Moreover, the
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