Dielectric Detection Using Biochemical Assays - Point-of-Care Diagnostics on a Chip - page 101

Tabl e 5. 1 The direction of V T according to the properties of

biomolecules and the types of FETs
n-Channel FETs

p-Channel FETs

Dielectric effect V T <0 V T >0

Charge effect Negative V T >0

Positive V T <0

Thus, it is possible to detect the specific bindings of biomolecules by monitoring
V T . As shown by Eq. 5.5 , the amount of signal change increases as the dielectric
constant of the analyte (k anlt / increases. Moreover, the signal change can be
enhanced via the response coefficient, which is determined by t anlt and Q dep that,
in turn, depend on the substrate doping concentration. Although we previously
assumed that the analyte is weakly charged or neutral, strong charges exist on
some of the most analyzed biomolecules: proteins and nucleic acids. When the
analyte is negatively/positively charged, it leads to a positive/negative V T shift
(V T > 0=V T <0) in both n-channel FETs and p-channel FETs.
It is important to note that the direction of the V T shift depends on the dielectric
and charge effects of the biomolecules, as well as the types of FETs, as shown in
Tab le 5.1 . Thus, the properties of the biomolecules and the device type should be
considered to maximize the signal change.
It is well known that the charge effect is inversely proportional to the distance
from the sensor surface (here, the channel). As the charged analytes move far from
the silicon channel, the charge effect tends to be weaker, resulting in a smaller V T
shift. Thus, one should consider a binding site where receptors are immobilized
and subsequent analytes are bound. When the binding site is close to the silicon
channel, the charge effect is the dominant factor, exceeding the dielectric effect.
However, when the binding site is far from the silicon channel, the charge effect
is weaker, and the dielectric effect is relatively more influential in the detection of
the analytes. Additional details and experimental data are described in the following
section.

5.3.2 Proof of Concept and DNA Detection with DMFET

The first result involving a DMFET was reported in 2007 [ 30 ]. In that work,
researchers concentrated on the proof of concept of a DMFET with weakly charged
biomolecules: specifically biotin and a streptavidin biomolecules, which are the
most widely used biomolecules in verifications of the operation of a biosensor.
The fabricated DMFET had a thick gate oxide (10 nm) and a gold gate with
nanogaps at the edges of the gate dielectric, as depicted in Fig. 5.9 . The thick gate
dielectric reduced the V T shift caused by trapped charges or intrinsic charges from
biomolecules; hence, only a V T shift due to a change in the dielectric constant was,
in fact, observed (Fig. 5.10 ).