BIOMEDICAL APPLICATIONS VI: CARBON NANOTUBES AS BIOSENSING AND BIO-INTERFACIAL MATERIALS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 184

In brief, carbon atoms arranged in condensed benzene rings such as in
CNTs represent unique nanoscale tube structures. Because of their speciic
molecular structure, they show properties that distinguish them from other
materials.

7.2.2 CNTs as Electric Sensors

7.2.2.1 CNT-based electric devices

Because of the novel properties described above, CNTs were used to produce
many kinds of electric devices, such as transistors. Developments in these
areas have critical meaning for utilising CNTs for chemical and biological
sensing and analysis.
Semiconductive CNTs have been often used as the key component of
ield-effect transistors (FETs) at room temperature. 12,13 Transistors are
semiconductor devices used to amplify or switch electronic signals. The
electron transport in the transistors may be dominated by electrons or
by holes. FETs are composed of three parts: source, drain and gate, which
control the switch by electric ield. CNT-based FETs were irst prepared by
linking the source and drain electrodes by a semiconductive CNT (Fig. 7.2),
normally with Au 13 or Pt 12 used as the source and drain electrode materials.
The gate voltage was used to control the conductivity of the CNTs. The
source-drain current in a single-wall carbon nanotube (SWNT)-based FET
decreased strongly with increasing gate voltage. Such transfer characteristics
of current-gate voltage of a CNT-based transistor indicated that the transistor
was a p-channel FET and the conductance was dominated by holes (positive
carriers). 12 By modulating the gate voltage the conductance of the SWNT FET
could be modulated by several orders of magnitude.

Figure 7.2 Scheme of a CNT-based ield-effect transistor. The CNT bridges the drain
and source electrodes. The gate voltage controls the “on” and “off ” of the transistor.