Nano-Bio Integration - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 242

a b

G

D

D

I2

D

I2

I1

G

I1

P

P

S

S

I1

SiO 2

Fig. 9.8 ( a ) Side view (cross-section) and ( b ) top view of an ambipolar central-gate vertical FET
with biological channel

The n-channel FETs using 80-nm-thick C 60 semiconductor channels in contact
with 0.6/60-nm Cr/Au source and drain electrodes and separated from a bottom
60-nm Au electrode by a thin layer of DNA-CTMA-PPIF with a width of 0:5m,
displayed no hysteresis. Similarly, p-channel FETs with same dimensions but '-
sexithiophene as channel showed significantly reduced hysteresis due to ionic
charge migration inhibition by cross-linking; the length and widths of the channels
were 20 and 200m, respectively.
In electrical DNA-based devices, high-yield DNA immobilization on anchor
pads is an important issue. A technique describing the immobilization of individual
DNA molecules on silicon substrates by binding to 3-aminopropyltriethoxysilane
(APTES) anchoring pads is detailed ( Gao et al. 2010 ). These pads were fabricated
by electron-beam lithography, which defined the pad shapes by drilling holes in a
poly(methyl methacrylate) (PMMA) layer, followed by APTES deposition by self-
assembly from an aqueous solution, in the holes, and a final molecular lift-off in
which the PMMA layer is removed. DNA rafts in solution bind with an 85% yield
on the positively charged anchoring pads after a deposition time of 12-14 h.
Molecular transistors can even be mass produced. In particular, ambipolar
central-gate vertical FETs with a cylindrical geometry, the cross-section of which
is represented in Fig. 9.8 , can be fabricated in parallel using photolithography
and self-assembling ( Mentovich et al. 2009 ). This FET has as channel the self-
assembled monolayer of bovine serum albumin (BSA) protein P, with a length
of 4 nm determined by the thickness of BSA, while the source, drain, and gate
electrodes, denoted in Fig. 9.8 by S, D, and G, respectively, consist of thin layers
of Au, Pd, and Ti. The source and drain electrodes are separated by the I1 Si 3 N 4
dielectric layer, while the gate is isolated by an 8-nm-thin TiO 2 oxide layer labeled
as I2. The structure is grown on a Si wafer covered by a 100-nm-thick SiO 2 layer.
Arrays of such transistors with diameters between 800 and 1:5mandshowingn-
or p-type conduction depending on the gate voltage were fabricated. The operating
voltages are below 1 V. Transport in FET devices that include single molecules
is expected to depend also on the strength of the electronic coupling between
the molecule and the source and drain electrodes. This coupling can be weak,
intermediate, or strong, each regime being characterized by specific phenomena
( Moth-Poulsen and Bjørnholm 2009 ).
An example of a transistor based on a chemically modified graphene (CMG)/
bacterial

DNA and protein compound has been reported in Mohanty and