Biomedical Engineering Reference
In-Depth Information
lateral electric field from the drain voltage trigger impact ionization, thus generating
electron-hole pairs. The resulting electron-hole pairs are driven by the electric
force produced by the applied bias on the substrate, and therefore, they create the
substrate current, which is comprised of holes in an n-channel FET. Equations 5.6
and 5.7 express the substrate current and saturated drain voltage (V Dsat )which
predominantly affect the substrate current:
V Dsat /I D exp
I s u b /
.V D
V Dsat
V Dsat D
Here, B is the impact ionization coefficient [ 56 ]. The substrate current increases
monotonically as V G increases at a low or intermediated level of V G because the
first linear term on the right side in Eq. 5.6 is dominant over the substrate current.
However, when V G is very high, the substrate current is reduced by the second
exponent term in Eq. 5.6 . Thus, I s u b appears bell-shaped, with a crucial voltage
(V G @I s u b;max / that shows the maximum I s u b value upon the first increment of I s u b
ledbyanincreaseintheV G value, with the next decrement of I s u b driven by the
exponent decrease in Eq. 5.6 .TheV G @I s u b;max value is significantly affected by
the maximum electric field in which impact ionization occurs. Thus, the changed
electric field near the gate edges, that is, near the drain junctions due to air and
biomolecules in the nanogap, results in different V G @I s u b;max values. The results as
they pertain to the substrate current have yet to be reported, but I s u b could be one of
the sensing parameters used in a DMFET.
Environmental Effect
Most previously reported biosensors were characterized under aqueous condi-
tions [ 29 , 47 ]. However, a few biomolecular detection experiments were also
performed in ambient air environments [ 18 , 30 ] (depending on the exposed con-
dition, these are referred to as a “watery environment” or a “dry environment,”
respectively). Although measurements in a watery environment are common in
biosensor characterizations because aqueous conditions maintain the functionality
of biomolecules, measurements in a dry environment facilitate various device
structures without consideration of the isolation between the aqueous solution and
the device. Hence, characterizations of a biosensor in a bionic solution and in air
ambient have been performed [ 57 ]. With the same device structure as an underlap
FET, the researchers validated the biosensor functionality in a dry environment by
comparing the result in watery environment.
To maintain biosensor operation in a bionic solution, an additional passivation
layer that prevents leakage current through the bionic solution was implemented
in the underlap FET, as shown in Fig. 5.23 . The device was submerged in a bionic
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