Page 37 - Organic Electronics in Sensors and Biotechnology

14 Chapter One

current density–longitudinal field plot. Therefore these solid lines of
calculated ohmic channel currents at varying temperatures serve as a
reference to investigate the effects of contact injection-limited trans-
port and field-dependent mobility on the charge transport through
the channel, both of which feature superlinear relation of current
density-longitudinal field. Injection-limited transport occurs at low
longitudinal field which results in lower currents and effective mobil-
ities compared to the ideal gate-induced ohmic channel current, while
field-dependent mobility takes place at high longitudinal field which
is larger than the value expected when compared to the ideal gate-
induced ohmic channel current.
Based on all the above understanding, in Fig. 1.8 the lower
portion of the measured curves below the solid line represents the
injection-limited transport, while the upper portion of the measured
curves beyond the solid line represents the transport regime of field-
dependent mobility. It can also be observed that the field dependence
of mobility is stronger at lower temperatures. All the current-field
curves in logarithmic scale measured for different channel lengths
shift downward when the temperature decreases, as represented
clearly by the downward shifts of the ohmic reference lines with low-
ering temperature. This is so because, for all channel lengths, the
threshold voltage shifts toward higher gate bias at lower tempera-
tures. Note that the data in the quadrant labeled with 4.8 K contain
some self-heating effects which make the actual device temperature
deviate from the readings. After careful analysis, it was determined
that device’s temperature was not significantly affected by the self-
heating at temperatures beyond 40 K. Figure 1.9 shows that threshold
voltages shift with temperature quasi-linearly, and this trend holds
for all channels ranging from 5 μm to 270 nm. The temperature depen-
dence of the threshold voltage in an organic field-effect transistor
50
(OFET) is significantly larger than that in a Si-MOSFET, and this is
attributed to the presence of deep trap states in an organic semicon-
51
ductor. At low temperatures the thermal energy (kT) of charge
carriers becomes very small, and thereby a large percentage of gate-
induced charge carriers are falling into trap states whose levels are
much deeper than kT and not being released and thus do not contri-
bute to the channel transport. In this case much higher gate bias is
required for the same amount of mobile charges to be present in the
channel, and therefore at lower temperature the threshold voltage of
an OFET significantly shifts toward higher gate bias.

1.1.4 Field-Dependent Mobility Model for the Scaling
Behavior of Charge Transport
To understand the scaling behavior observed in organic transistors as
shown in the previous section, a physical model is needed that explains
the mechanisms of charge transport with regard to temperature, field,