July 2005

From the Lab: Nanotechnology

Faster plastic-based semiconductors and a step toward lighter, nonsteel auto engines

By Monya Baker (editor)

Plastic That Performs
Organic transistors get up to speed

Context: Organic transistors, which are made from semiconducting plastics, are cheap to manufacture. Although they enable thin, bendable electronics, so far they can't implement the fastest, most efficient circuit designs, because the plastics can't transport electrons. Instead, they rely on a flow of positive charges, or electron "holes," to pass current, which limits their use. Now, in a surprising discovery, Lay-Lay Chua at the University of Cambridge and Peter Ho at the National University of Singapore have shown that the transistors' inability to move electrons is due not to the plastic itself but to an interaction with other materials in the transistor.

Methods and Results: In a transistor, current passes through a semiconductor under the control of a gate electrode. The gate electrode is separated from the semiconductor by an insulator, typically silicon dioxide. In conventional silicon transistors, electrons pass through the semiconductor without interacting with the insulator. But in most plastic semiconductors, atoms at the interface between the insulator and the plastic trap electrons, so they can't flow through the transistor. By carefully designing an alternative insulator to replace silicon dioxide, Chua, Ho, and colleagues demonstrated that organic semiconductors can indeed conduct electrons. The discovery could make for simpler, higher-quality organic transistors that can implement the most commonly used designs.

Why It Matters: Organic transistors can be built using relatively cheap fabrication technologies such as ink-jet printing. The new insulator should let such cheap transistors perform many more tasks. The first application that beckons: electronically active tracking labels known as radio frequency identification (RFID) tags. With the new understanding of organic transistors provided by Chua and his colleagues, fast and cheap plastic electronics could soon be as ubiquitous as ink.

Source: Chua, L.-L., et al. 2005. General observation of n-type field-effect behaviour in organic semiconductors. Nature 434:194-9.