Organic field-effect transistor
[1] One of the benefits of OFETs, especially compared with inorganic TFTs, is their unprecedented physical flexibility,[2] which leads to biocompatible applications, for instance in the future health care industry of personalized biomedicines and bioelectronics.
[3] In May 2007, Sony reported the first full-color, video-rate, flexible, all plastic display,[4][5] in which both the thin-film transistors and the light-emitting pixels were made of organic materials.
The concept of a field-effect transistor (FET) was first proposed by Julius Edgar Lilienfeld, who received a patent for his idea in 1930.
[6] He proposed that a field-effect transistor behaves as a capacitor with a conducting channel between a source and a drain electrode.
[14][15] The concept of a thin-film transistor (TFT) was first proposed by John Wallmark who in 1957 filed a patent for a thin film MOSFET in which germanium monoxide was used as a gate dielectric.
In 1986, Mitsubishi Electric researchers H. Koezuka, A. Tsumura and Tsuneya Ando reported the first organic field-effect transistor,[18][19] based on a polymer of thiophene molecules.
One common feature of OFET materials is the inclusion of an aromatic or otherwise conjugated π-electron system, facilitating the delocalization of orbital wavefunctions.
The field is very active, with newly synthesized and tested compounds reported weekly in prominent research journals.
Another popular OFET material is pentacene, which has been used since the 1980s, but with mobilities 10 to 100 times lower (depending on the substrate) than rubrene.
[25] The major problem with pentacene, as well as many other organic conductors, is its rapid oxidation in air to form pentacene-quinone.
However if the pentacene is preoxidized, and the thus formed pentacene-quinone is used as the gate insulator, then the mobility can approach the rubrene values.
The ON/OFF voltage is different for devices grown by those two techniques, presumably due to the higher processing temperatures using in the vapor transport grows.
If a larger positive bias is applied, the band bending in the opposite direction occurs and the region close to the insulator-semiconductor interface becomes depleted of holes.
If there is zero bias, the electrons are expelled from the surface due to the Fermi-level energy difference of the semiconductor and the metal.
The performance of OFETs, which can compete with that of amorphous silicon (a-Si) TFTs with field-effect mobilities of 0.5–1 cm2 V−1 s−1 and ON/OFF current ratios (which indicate the ability of the device to shut down) of 106–108, has improved significantly.
Currently, thin-film OFET mobility values of 5 cm2 V−1 s−1 in the case of vacuum-deposited small molecules[29] and 0.6 cm2 V−1 s−1 for solution-processed polymers[30] have been reported.
As a result, there is now a greater industrial interest in using OFETs for applications that are currently incompatible with the use of a-Si or other inorganic transistor technologies.
The active FET layer is usually deposited onto this substrate using either (i) thermal evaporation, (ii) coating from organic solution, or (iii) electrostatic lamination.
The horizontal lines indicate the comparison guides to the main OFET competitors – amorphous (a-Si) and polycrystalline silicon.
[37] The device structure comprises interdigitated gold source- and drain electrodes and a polycrystalline tetracene thin film.
