Biology Reference
In-Depth Information
microfabrication and can provide devices at tens of nanometers
workingatGHzfrequencyranges.However,forbiosensorarraysthis
high performance is far over-specification as previously discussed.
On the other hand, considering manufacturing and convenience of
operation, both the sensor element itself and the whole chip cannot
be made too small. In the existing examples, the size of the chip is
4mm 2 for 50 sensor spots using a 0.18- μ m process and 20 mm 2
for 128 sensor spots using a 0.5 μ m process. Even with this small
number of sensors in the array, using CMOS technology leads to
substantial cost on the manufacturing of the chip, typically a few
dollars in these two cases, excluding the costs of the biomolecular
probes, post-processing, design, and installation fee for the masks.
This cost is mainly due to the expensive single crystal silicon
substrate,andhencewouldscaleupwhenalargernumberofsensor
elements or a larger area for each sensor element is needed. As
disposability is highly desired for biosensor arrays, the high cost of
CMOS process makes it impractical for large scale applications such
as diagnosticsand disease screening.
On the other hand, although TFTs cannot provide such high
performance electronic devices, it can be manufactured on much
cheaper substrates such as glass and even plastics, making the
technology an ideal candidate for biosensor arrays in the view
of cost. The main limitation of TFTs is the low mobility of the
semiconductor material. This does not only affect the performance
whenitisusedasatransducer,but,incaseofamorphousTFT,italso
prevents itsuse for theaddressing logicand measurement circuit.
To be used as the addressing matrix switches for individual
sensorelements,theon/offstatecurrentratioistheparametertobe
considered.Forabiosensorarraywiththousandsofsensorelements
theoff-stateresistancemustbeatleast3ordersofmagnitudelarger
that of the on-state to secure precise measurement of data. This can
be easily achieved by the use of a single FET based on either CMOS
[5], poly-Si TFT [15], or amorphous-Si TFT [59].
Forthelogicdrivingcircuit,theTFTneedstoworkat10 4 Hzwith
normalsequentialmeasurement,and10 7 HziftimemultiplexedEIS
is to be implemented. The highest working frequency of a FET is
mainly determined by the mobility of the semiconductor material,
as well as by its geometry size and fabrication process. It has been
 
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