Dielectric Detection Using Biochemical Assays - Point-of-Care Diagnostics on a Chip - page 104

5.4 Structural Modification of a DMFET

5.4.1 Underlap FET

The DMFET has the advantages of high compatibility with the conventional CMOS
process and adaptability to readout circuits for on-chip integration; however, the
bio-reaction probability is lower than that of other biosensors because the sensing
part in a DMFET is retracted and covered by the gate material. To overcome this
shortcoming while maintaining the advantages of the DMFET, an underlap FET
was proposed (Fig. 5.12 )[ 39 ]. With a novel structure derived from a conventional
FET, the underlap FET has all of the merits of the DMFET, as well as additional
advantages originating from the underlap structure [ 39 ]. The underlap structure
shows higher bio-reaction probability and structural stability compared to the carved
nanogap architecture due to its opened sensing area. In addition, small changes in
the current can be accurately measured because the channel potential in the underlap
region is highly sensitive to external charges [ 39 ].
Figure 5.12 shows a schematic of the underlap FET in which an offset (underlap
region) is introduced between the gate and the drain. The underlap region serves
as the sensing area; target molecules on the underlap region will affect the channel
potential of the underlap region, which results in a drain current change.
For more details, see Fig. 5.13 , which shows an expanded view of an underlap
FET to explain its operational principle. After the immobilization of charged
biomolecules on the underlap region, the number of inverted electrons in the
underlap region is modulated, thus causing a conductance change to occur. As a
result, the drain current at a particular gate voltage will change.
The operation of an underlap FET was demonstrated using an avian-influenza
(AI) antigen/antibody. As shown in Fig. 5.14 , after the binding of the AI antibody
(anti-AI), the drain current was significantly decreased. The abrupt drop in drain
current was attributed to the negative charges of the anti-AI: negatively charged
molecules increase the channel potential, resulting in a decrease in the drain current.
With its simple fabrication process, CMOS process compatibility, and enhanced
sensitivity, the underlap FET is a promising candidate for use in chip-based
biosensors.

5.4.2 Double-Gate FET

As mentioned previously, there have been two main approaches used in preparing
nanowire biosensors: bottom-up and top-down [ 40 ]. In the bottom-up process,
integration issues and incompatibilities with the conventional CMOS process cannot
be avoided, even though the size of the nanowire can be reduced beyond the limit
of lithographic resolution. In contrast, the top-down approach is restricted by the
lithographic resolution limit, but it enables more precise control of the position