High-Frequency Flexible Organic Thin-Film Transistors

Organic thin-film transistors (TFTs) are potentially useful for flexible electronics applications, as they can typically be fabricated at temperatures no higher than approximately 100 °C and thus not only on glass, but also on polymeric substrates and even on paper. An important TFT performance parameter is the transit frequency, which is the highest frequency at which a transistor is able to switch or amplify electrical signals. The transit frequency of organic TFTs depends mostly on the contact resistance and on the lateral TFT dimensions (channel length, gate-to-contact overlaps). A better understanding of the factors that determine the contact resistance of organic TFTs has made it possible to fabricate organic TFTs that have a contact resistance within three orders of magnitude of the theoretical limit and an experimentally measured transit frequency of 21 MHz (at 3 V). Any significant further reduction of the contact resistance of organic TFTs to the levels commonly achieved in inorganic field-effect transistors now hinges mainly on the elimination of the Fermi-level pinning induced by metal-induced gap states at the metal-semiconductor interfaces. In terms of decreasing the lateral TFT dimensions, flexible organic TFTs with channel lengths and gate-to-contact overlaps below 1 μm were recently demonstrated that display reasonable static characteristics (turn-on voltage of 0 V, subthreshold swing below 100 mV/decade, on/off current ratio greater than 10⁹), but suffer from a relatively large contact resistance that limits the transit frequency to 36 MHz (at 3 V). One of the challenges in pushing the transit frequency of flexible organic TFTs to 100 MHz and beyond will be to minimize all three parameters (contact resistance, channel length, gate-to-contact overlaps) simultaneously.