Nano-Bio Integration - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 241

Fig. 9.7 ( a ) Arrow-shaped
drain (D) and source (S)
electrodes in a biological
transistor and ( b ) a typical
current-voltage characteristic

a b

I

S

D

substrate

V

molecule is placed over the central part of the CNT, on which a 50-nm-wide channel
is fabricated with the focused ion-beam bombardment technique. The poly(dG)-
(dC) DNA molecule, placed using the electromagnetic trapping method, increases
the electrical conductivity between CNT electrodes, acting as a p-type channel. The
room-temperature current-voltage characteristic, which has a typical form as that
in Fig. 9.6 b, can be explained by the small polaron hopping model, the hopping
transport mechanism being different for low- and high-bias voltage ranges. More
specifically, the estimated hopping distance is 9.08 A for biases between 10 and
10 V; this parameter decreases to 3.8 A if the whole 20-20 V bias range is taken
into consideration. The latest value is comparable to the 3.4 A distance between base
pairs in the DNA molecule. Metallized DNA, consisting of arrays of Co 2C ; Zn 2C ,
or Ni 2C metal ions that substitute the imino protons of every base pair, can also be
used as the active element of a FET, the rate of the phonon-assisted electron hopping
between the localized states of metal ions being strongly influenced by a transverse
electric field ( Nokhrin et al. 2007 ).
Electrical transport between nanosized Cr/Au arrow-shaped source and drain
electrodes bridged by self-assembled guanosine ribbons and placed above a back
gate Ag electrode has been demonstrated in Maruccio et al. ( 2003 ). This FET,
shown in Fig. 9.7 , has a p-type channel consisting of ribbon-shaped supramolecular
assembles of deoxyguanosine derivatives that induce current rectification due to the
intrinsic dipole moment oriented along the ribbon axis, which creates a preferential
charge pathway. When the currents I are measured for both polarizations of
the source-drain voltage V , the rectification ratio is found to be RR.V/ D
I.V/=I. V/ Š 3. In fact, the current increases slowly, almost linearly with the
source-drain voltage up to about 3 V, and shows an exponential increase above this
threshold voltage, which depends slightly on the gate bias. In fact, although similar
to a MOSFET, this biomolecular FET differs from the latter in that the current-gate
voltage dependence does not saturate for a given source-drain voltage, indicating
the presence of two transport mechanisms: energy band alignment and resonant
transport in the molecular channel.
Biological organic FETs have been fabricated also from DNA-based polymers
such as insulating DNA-CTMA thin films, but these devices often show large
hysteresis caused by ionic charge migration, which limits their performances. In
addition, DNA-CTMA films are soft and can be easily scratched. A significant
improvement of both electrical and mechanical properties of DNA-CTMA
FETs containing DNA purified from salmon waste occurs after cross-linking
with poly(phenylisocyanate)- co -formaldehyde (PPIF) ( Yumusak et al. 2009 ).