Page 52 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 29

The interaction between organic semiconductor and analyte mol-
ecules is still not completely understood. The adsorbed polar analyte
could influence the charge transport in the channel by trapping some
charges and consequently decrease the mobility of other mobile
charges in channel. However, the major effect of the trapped or other-
wise immobile charges at the dielectric interface is to induce a shift of
threshold voltage in the transistor characteristics. For alcohol sens-
ing, a constant threshold shift was observed at low gate voltages,
which became a gate voltage-dependent mobility change at high gate
voltages. It was also observed that a reverse gate bias can facilitate
the restoration of drain current to near its original value. In many
experiments, exposure of p-channel organic FETs to alcohols pro-
duced an initial increase in channel current (presumably the effect of
dipoles) followed by a decrease (due to trapping). The initial increase
may be due to dipoles in the polar analyte, inducing more charge in
the channel.

1.2.3 Transition of Sensing Response by Organic Transistors
from Micron-Scale to Nanoscale
There have been reports exploring the chemical sensing effects of
organic and conjugated polymer transistors. 102, 103, 107–109 It has been
shown that on large-scale devices (L > 1 μm), alkyl chain alcohol ana-
lytes interact with polythiophene thin films at grain boundaries and
at the dielectric interface rather than the bulk of the films. Recently,
110
there have been reports on the role of the side chain in chemical sens-
ing of polymer/oligomer based field-effect transistors. 109, 111 Other
articles have delved into the dependence of the sensing effects on the
channel length–grain size relationship in large-scale organic transis-
tors. 112, 113 There is also an increasing need to develop sensors with
very small active areas, not only for lower power requirement, but
114
also for the possibility of higher sensitivity. However, according to
our experimental findings and analysis, scaling down the geometry
of an OFET device is not a simple way, as expected, to enhance the
115
sensitivity. Liang Wang et al. first systematically investigated the
scaling behavior of chemical sensing in OFETs with channel lengths
ranging from microns to tens of nanometers and found that the sens-
ing mechanism of where and how analyte molecules affect the electri-
cal transport in an organic transistor becomes quite different when
devices scaled from micron-scale to nanoscale dimensions. The main
reason is that unlike the classic MOSFET with crystalline silicon
channel, the electrical transport and chemical sensing behaviors of a
polycrystalline OFET heavily depend on the morphology structure of
the channel material and the properties of interfaces. For large-scale
OFET devices, grain boundaries play the dominant role for both elec-
trical transport and chemical sensing behaviors. For smaller channel
lengths, the number of grain boundaries within a channel decreases