In this article, we fabricated a series of different geometries for pentacene based thin film transistors (TFTs), including
top contact (TC) and bottom contact (BC) configurations, to monitor variations in characteristics. The threshold voltage
(Vth) in the saturation regime shifted toward a positive voltage, i.e., from 0.5, 3.2, 9.1, 12.1, and 19.5 V for the channel
lengths 67, 47, 23, 19, and 15 μm, respectively, in BC TFTs. All of the TFTs were operated in depletion mode. However,
the Vth in the linear regime shifted from -9.3, -9.0, -3.8, -1.8, and 1.5 V for the same devices. Most of the TFTs in the
linear regime were operated in enhanced mode. The phenomenon is believed to be strongly correlated with the
longitudinal electric field (VDS/L). The high VDS induces a high carrier injection, which makes the pentacene TFTs
behave like a depletion type transistor. This assumption is evidenced by the low carrier injection with small VDS results.
The device configuration and space charge region must be discussed in more detail. Furthermore, the field effect
mobility variation and structure configuration effect is discussed in more detail within the full manuscript.
In this study, device characterization and carrier transport properties of n-type fluorinated copper phthalocyanine
(F<sub>16</sub>CuPc) organic thin-film transistors (OTFTs) were investigated using bottom gate device configuration with oxygen plasma-treatment indium-tin-oxide (ITO) bottom source and drain (S/D) contact. We fabricated F<sub>16</sub>CuPc-based OTFTs
having comb-shaped channels with a series of channel lengths of 5~100 μm at a fixed channel width. The electrical characteristics of OTFTs were measured by a Keithley 4200-SCS semiconductor parameter analyzer in a dark glove box in a nitrogen atmosphere to avoid the influence of measuring environments. The device characteristics were analyzed using the charge-sheet metal-oxide-semiconductor field-effect transistor model equation. We found that both the linear
and saturation field-effect mobilities and threshold voltages of F<sub>16</sub>CuPc-based OTFTs increased with increasing channel
length. The linear and saturated field-effect mobilities were gate-bias dependent in all devices with different channel
dimensions. Moreover, the contact resistance between ITO S/D electrodes and F<sub>16</sub>CuPc and channel resistance of
F<sub>16</sub>CuPc were investigated using the gated-transfer length method. The results of our experiments suggest that the
contact resistance between ITO electrodes and F<sub>16</sub>CuPc plays an important role in current-voltage characteristics.
Additionally, abnormal increases in saturated field-effect mobility at channel lengths below 10 μm were observed in our
experimental devices and were attributed to short channel effects. Such non-ideal effects of the present F<sub>16</sub>CuPc-based
OTFT devices were investigated in detail. In summary, we found that the oxygen-plasma-treatment ITO bottom contact
S/D electrode-based F<sub>16</sub>CuPc OTFT devices were very durable and suitable to make large area transistor arrays with
complicated integrated circuits by photolithography techniques.