Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 61

g m D @I D =@V G ; (2.10)

and the conductance of the channel

g D D @I D =@V D : (2.11)

The cutoff frequency of the transistor is given by

f c D g m =2C G ; (2.12)

where C G is the gate capacitance.
The three regions of the FET described above and the parameters ( 2.10 )-( 2.12 )
are encountered in any type of FET, irrespective of the channel, which can be a 2D
electron gas (2DEG), a single nanowire or carbon nanotube (CNT), or a network of
such structures, or graphene.
The FET transistors which will be used further as label-free sensors of
biomolecules often work in the ballistic regime described in Chap. 1 . The theory of
ballistic FETs, known as Natori theory of FETs, is applicable also for nanowires
and CNT transistors and is described in detail in ( Lundstrom and Guo 2006 ); it will
not be reproduced here. Further, we will describe each important transistor used as
label-free detector for biomolecules and will focus on some examples illustrating
label-free detection.
The first example is the label-free sensor for Si-based FETs described above. The
DNA hybridization was achieved using a commercially available p-type FET having
as gate a multilayer thin film consisting of SiO 2 .100 nm/=Si 3 N 4 .100 nm/=Ta 2 O 5
(40 nm) ( Ohtake et al. 2004 ). The DNA was immobilized on the Ta 2 O 5 thin film by
immersing the gate electrode in 3 vol% 3-aminopropyltrimethoxylane comprising
CH 3 OH W H 2 O (19:1) solution for 30 min in ambient conditions. In this way, the gate
surface consisting of the thin film Ta 2 O 5 was modified by silane. Then, the silanized
FET was immersed in dimethylformamide (DMF) to create a linker for DNA.
Finally, the DNA immobilized on the gate was incubated overnight. The amount
of immobilized molecules is about 3:6 10 5 cm 2 . The label-free detection consists
in the decrease of the drain current due to the negative-charged DNA, which is
deposited on the gate. This effect is visible, for example, in the I D V G dependence.
The measurements are performed in a buffer solution of Tris-HCl C MgCl 2 ,having
Ag/AgCl as reference electrode, as indicated in Fig. 2.4 .
The difference in the threshold voltages V th of the FET functionalized with
ssDNA and dsDNA was 10 mV, for a gate length L G D 30m and width W D
350m, and the sum of gate capacitances was about 60 fF. From this data, the extra
charge due to DNA hybridization can be estimated to be 0:2 nC=cm 2 .
A similar work was done by Xuan et al. ( 2005 ). However, in comparison with
the label-free detection reported above, there is no gate surface treatment and no
immobilization chemical processes. The dsDNAs containing between 5,000 and
35,000 bases were diluted with deionized water. For the measurements, a drop of
0:5L and about 2 mm in diameter was deposited on the gate region. In the linear