By Peter Weiss
Already common in such gadgets as highway passes for paying tolls on the move, miniature radio-equipped circuit cards may ultimately become as widespread as bar codes. First, however, the cost of such radio frequency identification (RFID) tags must drop by a factor of 10 or more.
Engineers in Belgium have just developed an extraordinarily fast diode—a one-way valve for electric current—that could usher in RFID tags made of plastic rather than the more-expensive silicon of tags today. Paul Heremans of the company IMEC in Leuven and his colleagues describe their novel plastic semiconductor prototype in the August Nature Materials.
RFID tags on items from bakery goods to bank notes could result in instant supermarket checkouts, improved tracking of shipped or warehoused goods, and enhanced protection against counterfeiting, says Klaus J. Dimmler of Organic ID in Colorado Springs, Colo.
To be sufficiently low cost for these widespread applications, RFID tags can’t contain an internal power source. When another device, called a reader, beams an electromagnetic wave of energy at a passive tag, such as the highway-toll device, part of its circuitry, called the rectifier, harnesses the energy. Then the tag sends a signal.
Plastic, or organic, electronics have been too slow for the high-frequency reader signals dictated by industry standards. However, the Belgian team made a sufficiently fast rectifier with the new diode, so “this problem has now been overcome,” says Thomas N. Jackson of Pennsylvania State University in State College in a commentary accompanying the new study.
Many RFID developers are aiming at organic tags operating at 13.56 megahertz (MHz), with a range up to 1 meter. The new diode responds to frequencies up to 50 MHz. To create it, Heremans and his colleagues deposited ultrathin layers of gold, aluminum, and the organic semiconductor pentacene on a piece of glass. The diode’s remarkable speed owes mainly to the pentacene layer being thin enough for signals to traverse extremely quickly, Heremans says.
The structure of this diode makes it awkward to meld with other components of a tag, Dimmler says, so circuits based on the new diode could be difficult to manufacture. Consequently, his company has been striving to build rectifiers that employ organic transistors rather than diodes. Recently, OrganicID engineers attained 15-MHz operation in a transistor-based rectifier, Dimmler says.
However, organic transistors offer little hope for the much higher frequencies needed for tags that operate at distances of several meters, as some of the silicon-based devices do today, he adds.
The Belgian team’s approach is promising, Dimmler says, because it is potentially much faster than any other demonstrated so far. Calculations indicate that versions of the new diode might function at up to 800 MHz.
