Biology Reference
In-Depth Information
As previously introduced, the FET itself can be used as a
potentiometric biosensor transducer to translate biomolecular
interactions into variations of flat-band voltage or source-drain
current. The ability of miniaturization makes FETs ideal candidates
for applications on biosensing arrays, as the signal-to-noise ratio
is independent of the geometry size. This feature allows FET-
based biosensorsto be integrated on extremely high-density arrays,
with the limit of detection determined by the immobilization of
biomolecular probes and practical operations. The performance of
an FET-based potentiometric transducer depends on the internal
gain of the FET, which is measured by the transconductance, and on
the fabrication geometry, which determines the ratio of the double
layer capacitance to the gate dielectric capacitance.
Independently of the electrochemical technique employed for
DNA sensing, FETs can have an important role in the development
of electronically addressed biosensor arrays. Acting as switches for
individual cell elements, there are two basic requirements for the
addressing circuits. First, the electronic switch attached to each
sensor must have a high on/off current ratio. This is to make sure
that when the designated sensor element is measured, interference
from other sensor elements does not affect the characterization.
The second requirement is that the logic circuit, which translates
the input signal into the address information and selects the sensor
element,mustworkatahighenoughfrequency.Asthebiomolecular
reaction is often a dynamic process when the measurement is
carried out, all the sensor elements need to be characterized in a
relatively short time window, typically a few seconds. The driver
logic circuit needs to switch on all the sensor elements sequentially
within thistime window to allow the measurements.
Two possible architectures for TFT-addressed biosensor arrays
are illustrated in Figs. 6.10 and 6.11. For potentiometric sensing,
the biosensing pad is connected to the gate of the TFT (see
Fig. 6.10), which acts as the transducer. A dummy transistor, where
no biomolecular interaction occurs, can be used for differential
measurements [45]. For current detection, the sensing pad needs
to be connected to the source or the drain of the TFT as shown in
Fig. 6.11.
As the biomolecular interaction delivers a very weak electronic
signal, integrated amplification and noise canceling are often
