Dielectric Detection Using Biochemical Assays - Point-of-Care Diagnostics on a Chip - page 109

In this section, two newly developed sensing techniques for FET-based biosen-
sors, charge-pumping [ 50 - 53 ], and 1=f noise measurement [ 54 ] are discussed.
Their advantages and weaknesses, operation principles, and technical issues are also
described.

5.5.1.1 Charge-Pumping Technique

The operating principle of the charge-pumping technique is as follows: a DMFET
is fabricated in which the gate dielectric is partially etched to form a nanogap,
as shown in Fig. 5.19 a. Interface traps are located at the interface between the
channel and the gate dielectric, and the measured charge-pumping current (I CP /
is proportional to the interface trap density [ 55 ]. Therefore, because the nanogap
exposes the silicon channel to biomolecules directly, additional traps can be pro-
vided by biomolecules immobilized inside the nanogap; consequently, variation
of the trap density can occur with a measurable quantity (I cp / that is highly
sensitivity.
The great advantage of the charge-pumping technique is its sensitivity; when
the frequency and the level of the applied pulse were optimized by the prediction
from the derived analytical model [ 52 ], the micro-sized FET showed high sensitivity
that was comparable to a nanowire biosensor without a dimension scaled to
the nanoscale. This makes the fabrication of a highly sensitive biosensor at a
low cost feasible. Figure 5.20 shows the measured values of I cp as a function
of the charge-pumping frequency (f cp /. It was verified experimentally that the
sensitivity can be improved if a lower pulse frequency is used during the charge-
pumping measurement [ 52 ]. The sensing margin can be improved if f cp is lowered;
consequently, the sensitivity can fall below the picomolar concentration regime
without scaling the physical size of the sensor.
Another advantage is that the charge-pumping technique is able to analyze
various properties of biomolecules electrically. Hence, not only does it enable
the detection of biomolecules, but it also extracts their fundamental electrical
properties. For example, the identification of the biomolecular charge polarity
was demonstrated using a charge-pumping technique [ 53 ]. When negatively or
positively charged biomolecules are immobilized in the nanogaps, the V T of a FET
is not uniform along the channel but instead varies locally, as shown in Fig. 5.21 a.
Accordingly, if the maximum peak level of the pulse (V h / is increased gradually, a
lateral V T profile can be expected. Consequently, the biomolecular charge polarity
can be identified. The experimental results are provided in Fig. 5.21 bandc,showing
that the biomolecular charge polarity was successfully determined by the shift of the
direction of the dI cp = dV h V h curves. Therefore, the charge-pumping technique is

useful in that it enables the analysis of various electrical properties of biomolecules
and can be utilized as an investigational tool to extract their fundamental properties
and their biosensing characteristics.