Page 28 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 5

as bandlike transport, hopping, or tunneling. This is the basic picture
for transport within a single grain (a domain of well-ordered mole-
cules or well-aligned polymer chains). The situation for transport
between grains is complicated. Generally the presence of domain or
grain boundaries (the region between grains) limits the mobility of the
polycrystalline organic semiconductors, because the disorder at grain
boundaries leads to more localized states within the energy gap which
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trap the mobile charge carriers. Based on this concept, the density of
states (DOS) in polycrystalline systems can be described by bands with
tails which extend into the bandgap. Inside the band the DOS varies
slowly with energy while its tails decays exponentially into bandgap. 26
Around the edge of HOMO or LUMO, the band tail, composed of local-
ized states, has a much wider distribution at grain boundaries than
within each grain, due to the increased disorder at grain boundaries. 27
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According to the multiple trapping and release model, the gate-
induced charges in an organic field-effect transistor are composed of
two parts: the free (mobile) charges in the channel, which contribute to
the channel conductance, and the trapped charges in trapping levels
within the energy gap, which do not contribute to the channel conduc-
tance. The channel conductivity is a product of free charge density and
free charge mobility. There is another way (which is the convention in
this area) to view the channel conductivity; namely, it could also be
considered as a product of total gate-induced charge density (includ-
ing both free charges and trapped charges) and effective mobility.
Therefore the value of the effective mobility is determined by the ratio
of free charges to the sum of free and trapped charges. 28–30

1.1.2 Characterization of Nanoscale Organic Transistors
In addition to the above-mentioned transport mechanisms within the
active organic semiconductor layer, the performance of an organic
field-effect transistor depends also on many extrinsic factors, espe-
cially at the interfaces. The dielectric/semiconductor interface prop-
erty affects the molecular ordering within one/two monolayers
(roughly equal to the channel depth) of the organic semiconductor.
Interface traps at grain boundaries also play a role in device mobility.
The contact or electrode/semiconductor interface property deter-
mines the injection barrier. In nanoscale transistors, the injection at
contacts limits the transport of charge carriers at low gate bias and
low source-drain bias, as well as leads to a sensing mechanism which
is quite different from that in large-scale devices. A bottom-gate tran-
sistor structure, as shown in Fig. 1.3, has been commonly used in
organic FETs comprised of a substrate, usually a highly doped silicon
wafer or conductor-covered plastic sheet, followed by a dielectric layer
atop the substrate, such as silicon dioxide or an insulating polymer.
The organic or polymer semiconductor and metallic source and drain
contacts are then deposited on top of the gate dielectric with different
patterning technologies. 31–33 This kind of structure reduces fabrication