Dielectric Detection Using Biochemical Assays - Point-of-Care Diagnostics on a Chip - page 99

Fig. 5.7 Schematics of ( a ) a conventional FET ( b ) a DMFET with a nanogap between the gate
and the gate dielectric ( c ) receptor functionalization in the nanogap, and ( d ) analyte binding on a
DMFET
The nanogap is typically fabricated using advanced lithographic techniques such
as electron beam lithography and dry etching [ 34 , 35 ], which make the process
expensive and complex. The novelty of the DMFET is its simple nanogap fabri-
cation method, which provides more design flexibility than conventional methods.
The nanoscale gap is defined not by lithography but by a thin film deposition and
a wet etching process, which are conventional CMOS fabrication methods. The
nanogap size corresponds to the deposited thin film, which can be controlled at
atomic resolution using an atomic layer deposition (ALD) technique. Thus, the
nanogap size is not constrained by a lithographic resolution limit.
When analyzing the effects of biomolecules in DMFETs, an analogy between a
conventional FET and the DMFET may be noted. In both cases, the gate voltage
(V G / modulates the drain current, which flows horizontally from the drain (D) to the
source (S), as illustrated in Fig. 5.7 a. The current starts to flow when the gate voltage
exceeds the threshold voltage (V T /. In the operation of conventional FETs, trapped
charges in the gate oxide induce a V T shift. This V T shift, caused by trapped charges,
can be exploited in memory applications [ 36 ]. In a similar manner, a V T shift occurs
in DMFETs due to the intrinsic charges of the biomolecules bound in the nanogap.
The electrical characteristics of FET are mostly governed by the gate field, which
is applied across the gate dielectric. For example, the vertical field from the gate is
strengthened as the dielectric constant (k/ of the gate dielectric increases. Hence,
the drain current can be further increased by the use of a “high-k” gate dielectric
material [ 37 ]. Similarly, when biomolecules are introduced onto the nanogap of
DMFETs, the dielectric constant is increased (k>1) from unity. Thus, it is evident
that the dielectric properties of the biomolecules affect the electrical characteristics
of DMFETs, especially the threshold voltage and the corresponding drain current.