Plastic Transistors Formed by Ink-Jet Printing
Peter Singer, Editor-in-Chief -- Semiconductor International, 11/1/2000
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In work to be presented at the upcoming International Electron Devices Meeting (IEDM), to be held Dec. 11-13 at the San Francisco Hilton and Towers, the Cambridge researchers will present details on all-polymer, thin-film transistors fabricated by high-resolution ink-jet printing. The transistors, which have a gate length of 5 µm and a patterned gate electrode, have a high mobility of 0.02 cm2/Vs and high on/off ratios exceeding 105.
Progress such as this makes it increasingly likely that organic - what some might call "plastic" - thin-film transistors will find use in a number of low-cost applications, such as active-matrix LCDs, smart cards, price and inventory tags, and large-area sensor arrays. Not only can they potentially be manufactured at a lower cost than their inorganic cousins, organic TFTs can be printed on almost any substrate because of the low processing temperatures.
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| Cambridge University researchers have developed a technique for making all-polymer thin-film transistors using a high-resolution ink-jet printer. The inset shows the printed source, drain and gate electrode. (Source: Cambridge University) |
The ink-jet printing process developed at Cambridge achieves accurate pattern definition by controlling the flow of ink on the substrate through a pattern of hydrophilic and hydrophobic substrate regions. The team used piezoelectric-type ink-jet heads made by Seiko-Epson. The team consists of Takeo Kawase, Henning Sirringhaus and Richard Friend, all of who work at the Cavendish Lab in Cambridge (Kawase also works at the Epson Cambridge Lab).
Source and drain electrodes were formed by ink-jet deposition of a solution of a conductive conjugated polymer, PEDOT (poly-ethylenedioxythiophene), onto a glass substrate with a polyimide layer that was patterned in advance. The solution exhibits relatively high contact angles of about 70 degrees on the polyimide layer, and small contact angles of less than 20 degrees on the etched aperture area. This contrast allows the solution to flow up to the edge of the channel, but still be confined to the glass substrate.
A self-organizing conjugated polymer and PVP (polyvinylphenol) were spin-coated as a semiconductor and insulator layer, respectively. Finally, a PEDOT gate electrode was printed onto the insulator layer. The photo of the printed transistor (Figure) shows that the gate electrode is well aligned with the channel.
