Plastic diodes to shape future circuits

Jan. 12, 2006
Most plastics don't conduct electricity, though that hasn't stopped researchers from trying to build plastic computer chips.

Some groups have coaxed modest numbers of electrons through plastics using quantum mechanical effects, but only by painstakingly manipulating individual molecules at cryogenic temperatures.

But a group of Ohio State University researchers have gotten around the problems of working with single molecules and cryofluids. They layer plastic atop conventional circuit materials, with titanium oxide sandwiched in between, an idea borrowed from earlier research on plastic solar cells. An inexpensive process called spin casting applies the plastic. Drops of a liquid-plastic solution are deposited on a surface, which is spun at high speed to spread a thin, even coating.

A careful look at the results of one of the experiments revealed something unusual — a tiny blip in an otherwise smooth graph line charting the amount of electrical current passing through the material. At low voltages, the current spiked then returned to normal. On closer inspection, the plastic was exhibiting "negative differential resistance," a phenomenon in which the current decreases over a particular range of increasing voltage. It turns out that a semiconductor device called a tunnel diode shows the same behavior. Tunnel diodes, so named because they transmit electricity by quantum tunneling, let electrons pass through barriers unhindered.

Other research groups have tried to marry polymer tunnel diodes to titanium oxide without success. The reason may be that the Ohio State group deposited a layer of pure titanium on a chip then oxidized it, instead of depositing titanium oxide all at once. The diodes work at room temperature on just 1.5 Vdc and consume little power. The group concedes plastic probably won't replace silicon entirely. More likely are lightweight, portable electronics that combine silicon and plastic. The plastic circuits would let devices such as smart cards bend slightly and run on less power.

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