Depletion OFETs

Figure 14-27. Drain current-voltage charaeieristics of a doped DH6T OFET sliowiug both accumulation (V cO) and depletion (Fx>0) regimes.

This implies that under conditions that might be typical for high-performance OFETs, the contact resistance is not determined by the Schottky barrier at the interface, but by bulk transport processes in the semiconductor in the vicinity of the contact. Consistent with this interpretation, the contact resistance was found to depend on temperature in the same way as the mobility [107] so that the potential profiles become independent of temperature. This result was explained by invoking the existence of a depletion layer in the vicinity of the contacts. Similar results have been reported using channel length scaling analysis [76]. 

Fig. 6.7. Sschematic models foi C — V measurement of OFETs. (a) shows the connection made to take the characteristic Vos is set to OV and the capacitance is measured between the gate and the shorted source and drain electrodes. In accumulation (b) there are three major contributions to the total capacitance the channel capacitance C oxWL, the gate/source overlap capacitance Cos, and the gate/drain overlap capacitance C gd- In depletion (c), only the overlap capacitance is observed, there is no mobile charge in the channel to contribute to a channel capacitance. The transistor channel is really a distributed RC structure and the lumped R shown is only schematic.

The doped state of PEDOT is almost transparent as the polaron bands absorb in the infrared (IR) region. The de-doped form of PEDOT is dark blue with an absorption maximum located slightly above 600 nm. As PEDOT is highly conducting in pristine state, it caimot be used as channel material in depletion mode OFETs. Instead, the good ion mobility of PEDOTrPSS and other PEDOT analogues makes this material ideal for... 

Besides the transport properties, the contact formation between the source/drain electrodes and the organic semiconductor is crucial for the OFET performance. To illustrate this, balanced charge carrier transport properties ii = i2 in the above evaluated model are assumed and the injection of the complementary charge carrier species is hindered, e.g., by a pronounced injection barrier at the drain/semiconductor contact. The source contact is supposed to inject freely charge carriers of type 1 and out of simplicity they are assumed to be ejected freely at the drain electrode as well. For Vd —> IV g] the channel depletes more and more in the vicinity of the drain electrode and for Vg < Vd the charge carriers of type 1... 

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