curves. For example, for almost all organic FETs, the threshold voltage extracted
from the extrapolation of the linear regression of the square root of I DS in the
saturation regime to zero is not the same as the gate voltage, at which the current
starts to increase. This point is called the onset voltage (V ON ) and is often quoted
instead of the threshold voltage. More realistic models of charge transport in
organic FETs, which take into account the dependence of mobility on charge carrier
density or contact resistance, are also available [9, 10].
The carrier mobility ultimately determines the resistance of the channel at a
given gate voltage and channel length, that is, how much current can go through the
FET. This is important for current-driven light-emitting diodes in display pixels. It
is also an important factor for the switching speed of any transistor and therefore for
possible circuits. Clearly, the higher the mobility the better. For organic electronics,
carrier mobilities between 0.1 and 1 cm
2 V
À1 s
À1 are considered to be the minimum
for useful application and substantial efforts have been made over the last decade to
increase the carrier mobility in organic semiconductors [11].
Another crucial parameter is the ratio of the on-current (at V G ) V Th ) to the
off-current (at V G < V Th ) in an FET. Even in the off-state of a transistor there is
some current flow, called the ‘off-current.’ This results from various sources such
as unintentional doping, the finite resistance of the semiconductor, and leakage to
the gate electrode. The off-current should be as low as possible, whereas the
on-current, when charges are accumulated, should be as high as possible. In
traditional metal-oxide-semiconductor FETs (MOSFETs) this problem is solved
by the pn-junction at the contacts [12]. In organic FETs, the bulk resistance of the
undoped semiconductor largely determines the off-current. Depending on the
intended application, on/off ratios of 10
6 to 10
9 are required.
The current–voltage characteristics of transistors are usually presented either as
transfer curves, that is, the drain current is plotted versus the gate voltage for a
constant source-drain voltage, or as output curves, that is, the drain current is
plotted versus the source-drain voltage for a set of constant gate voltages (see
Fig. 2). The output curves are instructive for determining the source-drain voltage
range for which the linear and saturation regimes are valid. They can also indicate
non-ohmic contact resistance, that is, the current does not increase linearly with low
V DS but exhibits a typical S-shaped curve. The transfer characteristics are most
frequently presented in the literature and allow simple extraction of the on/off
current ratio, onset voltages, and mobilities. Note that ideally both forward and
reverse voltage sweeps should be plotted to estimate any charge trapping, which
causes current hysteresis.
Charge accumulation and transport occur very close to the semiconductor–
dielectric interface; hence, the surface of the dielectric and the order of the
semiconductor directly at the interface have a large impact on device performance.
This is especially important for P3HT as we will see in section 3.4. Consequently,
the choice of device structure (e.g., bottom gate or top gate) is extremely important
for the device performance. A popular, because convenient and easy to fabricate,
structure is the bottom gate geometry with doped silicon as the gate electrode and
thermally grown silicon dioxide as the gate dielectric. The silicon dioxide can be
P3HT and Other Polythiophene Field-Effect Transistors
111
from the extrapolation of the linear regression of the square root of I DS in the
saturation regime to zero is not the same as the gate voltage, at which the current
starts to increase. This point is called the onset voltage (V ON ) and is often quoted
instead of the threshold voltage. More realistic models of charge transport in
organic FETs, which take into account the dependence of mobility on charge carrier
density or contact resistance, are also available [9, 10].
The carrier mobility ultimately determines the resistance of the channel at a
given gate voltage and channel length, that is, how much current can go through the
FET. This is important for current-driven light-emitting diodes in display pixels. It
is also an important factor for the switching speed of any transistor and therefore for
possible circuits. Clearly, the higher the mobility the better. For organic electronics,
carrier mobilities between 0.1 and 1 cm
2 V
À1 s
À1 are considered to be the minimum
for useful application and substantial efforts have been made over the last decade to
increase the carrier mobility in organic semiconductors [11].
Another crucial parameter is the ratio of the on-current (at V G ) V Th ) to the
off-current (at V G < V Th ) in an FET. Even in the off-state of a transistor there is
some current flow, called the ‘off-current.’ This results from various sources such
as unintentional doping, the finite resistance of the semiconductor, and leakage to
the gate electrode. The off-current should be as low as possible, whereas the
on-current, when charges are accumulated, should be as high as possible. In
traditional metal-oxide-semiconductor FETs (MOSFETs) this problem is solved
by the pn-junction at the contacts [12]. In organic FETs, the bulk resistance of the
undoped semiconductor largely determines the off-current. Depending on the
intended application, on/off ratios of 10
6 to 10
9 are required.
The current–voltage characteristics of transistors are usually presented either as
transfer curves, that is, the drain current is plotted versus the gate voltage for a
constant source-drain voltage, or as output curves, that is, the drain current is
plotted versus the source-drain voltage for a set of constant gate voltages (see
Fig. 2). The output curves are instructive for determining the source-drain voltage
range for which the linear and saturation regimes are valid. They can also indicate
non-ohmic contact resistance, that is, the current does not increase linearly with low
V DS but exhibits a typical S-shaped curve. The transfer characteristics are most
frequently presented in the literature and allow simple extraction of the on/off
current ratio, onset voltages, and mobilities. Note that ideally both forward and
reverse voltage sweeps should be plotted to estimate any charge trapping, which
causes current hysteresis.
Charge accumulation and transport occur very close to the semiconductor–
dielectric interface; hence, the surface of the dielectric and the order of the
semiconductor directly at the interface have a large impact on device performance.
This is especially important for P3HT as we will see in section 3.4. Consequently,
the choice of device structure (e.g., bottom gate or top gate) is extremely important
for the device performance. A popular, because convenient and easy to fabricate,
structure is the bottom gate geometry with doped silicon as the gate electrode and
thermally grown silicon dioxide as the gate dielectric. The silicon dioxide can be
P3HT and Other Polythiophene Field-Effect Transistors
111