Biomedical Engineering Reference
In-Depth Information
Fig. 5.22 ( a ) Time domain conductance measurement of a p-type Si nanowire FET sensor
modified with PSA monoclonal antibodies. Different concentrations of PSA solutions and pure
buffer were sequentially delivered to the sensor. Dashed circles indicate the time windows of PSA
binding on the nanowire surface [ 54 ]. ( b ) The power spectrum of the same Si nanowire FET sensor
in a buffer and in solutions with 0.15-pM PSA concentrations shows 1=f frequency dependence
[ 54 ] (Copyright 2010 American Chemical Society)
The techniques described above, charge-pumping and 1=f noise measurement,
were recently developed. Therefore, only a few preliminary studies of the techniques
have been reported thus far. Both techniques demonstrated higher sensitivity
compared to previous FET-based biosensors; however, the stability, reliability, and
reproducibility of the biosensor operation have yet to be confirmed. In particular,
the measurement procedure of both techniques is complicated, which could be
problematic for their use in POCT systems. For the charge-pumping technique, a
continuous pulse stream must be applied to the gate electrode, which requires an
additional peripheral circuit to generate pulse. To measure the 1=f noise, additional
equipment (generally an amplifier and a spectrum analyzer) is required for high-
quality measurements, and these parts are not compatible with the miniaturization
desired in POCT systems.
Substrate Current Method
Another parameter significantly affected by the status of the nanogap site in a
DMFET is the substrate current (I sub /. The substrate current is generated when
a high drain bias is applied while the channel is inverted. The high drain bias makes
a high lateral electric field near the drain junction edge. Electrons accelerated by the
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