Access resistance, OFET

Top contact OFETs (Fignre 2.4.7a) generally exhibit the lowest contact resistances. This is likely becanse of the increased metal-semicondnctor contact area in this configuration. A major contribution to contact resistance in the top contact configuration is access resistance (see Figure 2.4.8a). Access resistance resnlts from the requirement that charge carriers must travel from the source contact on top of the film down to the accnmnlation layer (the channel) at the semicondnctor-insnlator interface and then back np to the drain contact to be extracted. 

In order to minimize access resistance, the thickness of the organic semiconductor layer should not be too large. However, some researchers have proposed that access resistance is less than might be expected for top contact OFETs becanse the contact metal penetrates the film down to the accnmnlation layer (perhaps dne to large peak-to-valley ronghness of the semicondnctor film or the natnre of the metal deposition process) [11]. This scenario is shown in Fignre 2.4.8(b). 

Increasing the purity of organic crystals and reducing the contact resistance in OFETs is another challenging direction of future experimental work, which will help to extend the temperature range where the intrinsic polaronic transport can be studied. The development of more advanced techniques for purification of molecular materials will enable the expansion of the intrinsic transport regime to much lower temperatures, where the effects of quantum statistics and polaron-polaron interactions should become experimentally accessible. 

Alternatively, a local non-contact potentiometry of the transistor channel allows one to measure the potential distribution V (jc) across the entire OFET [70], unveiling thereby possible contact resistances. Due to the local determination of the surface potential, the voltage drops at the source and drain electrodes are accessible. By... 

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