Biomedical Engineering Reference
In-Depth Information
The inherent negative charges of anti-AI increase the V
T
and lead to a positive
V
T
shift, as shown in Fig.
5.18
b. When a V
G2
value of -2.0 V (<V
T;
DG
/ is applied,
there is no remarkable change in V
T
. In contrast, V
T
changes significantly when a
V
G2
value of 0.5 V (>V
T;
DG
/ is used. It is noteworthy that this bias condition shows
enhanced sensitivity compared to the condition of a floating V
G2
, which is the most
similar to a conventional single-gate nanowire FET.
Even for the same nanowire dimensions, the double-gate nanowire FET with a
supportive gate (G2) can deliver enhanced sensitivity compared to the single-gate
nanowire FET. Thus, the difficulties in scaling down nanowire biosensors can be
overcome by implementing a double gate using matured CMOS technology.
5.5
Sensing Metrics in a DMFET
Currently, the detected response from FET-based biosensors, that is, the sensing
parameter, is always a V
T
shift. However, this restricted sensing scheme leads
to several problems: (1) The scope of possible analyses is limited because only
the detection of biomolecules themselves is possible. An attempt to calculate the
charge-trapping properties of DNA using the V
T
shift was made [
45
]; however, this
did not provide any physical or electrical meaning about the DNA regarding the
amount of the trapped charge. (2) In addition, as only a small amount of electrostatic
force under a low concentration of target molecules participates in the biosensing
procedure, the net response is generally less than an order of magnitude; thus,
this type of sensing scheme is associated with a low signal-to-noise ratio [
46
]. To
improve the sensitivity relative to noise, the dimensions of the sensor device must
be scaled down, which complicates the fabrication of the device [
47
].
Actually, a FET has many useful and sensitive device parameters aside from
V
T
. Every device parameter can be utilized as a sensing parameter for detecting
biomolecules. In this section, new sensing parameters for use in a DMFET are
discussed.
5.5.1
Interface Trap Charge Method
One device parameter that can be utilized as a sensing parameter is the number of
interface states at the surface of the channel, or the channel/gate dielectric interface
(normally Si=SiO
2
/, which is also referred to as the interface trap density (D
it
/.The
interface trap is a very sensitive parameter that can affect the device characteristics
severely. Therefore, many techniques to investigate and extract D
it
electrically have
been developed; among them, charge-pumping [
48
]and1=f noise measurements
[
49
] have been studied since the 1960s. A recent and related development is the
advent of a biosensing technique that detects the interface state modulation resulting
from biomolecular interactions.
