Silicon compatible diode
ENGINEERS HAVE designed a new diode that transmits more electricity than any other device of its kind, and the inspiration for it came from technology that is 40 years old.
Unlike other diodes in its class, called tunnel diodes, the new diode is compatible with silicon, so manufacturers could easily build it into mainstream electronic devices such as cell phones and computers.
Industry has long sought to marry tunnel diodes with conventional electronics as a means to simplify increasingly complex circuits, explained Paul R. Berger, professor of electrical engineering and physics at Ohio State.
``Computer chips now are worse than the Los Angeles freeway, with wires running back and forth clogging the path of propagating signals,'' Berger said. ``At some point, things are going to come to a grinding halt, and chips won't run any faster.''
Because this diode can replace some of the circuits on a typical chip, it could potentially simplify chip design without compromising performance.
The new diode conducts 150,000 amps per square centimetre of its silicon-based material — a rate three times higher than that of the only comparable silicon tunnel diode. The diode has been described in the journal Applied Physics Letters.
Tunnel diodes are so named because they exploit a quantum mechanical effect known as tunnelling, which lets electrons pass through barriers unhindered. In an effort to build more powerful diodes, researchers have increasingly turned to expensive, exotic materials that are not compatible with silicon, but allow tailored properties not often available in silicon.
Most modern tunnel diodes are `intraband' diodes, meaning they restrict the movements of electrons to one energy level, or `band', within the semiconductor crystal. But the Esaki tunnel diodes were `interband' diodes — they permitted electrons to pass back and forth between different energy bands.
At first, Berger's team tried to develop intraband diodes with silicon technology. But faced with what he called a `materials science nightmare,' they turned instead to Esaki's early tunnel diode technology for inspiration.
To construct a powerful interband diode, Berger's team had to develop a new technique for creating silicon structures that contain unusually large quantities of other chemical elements, or dopants, such as boron and phosphorus.
They layered silicon and silicon-germanium into a structure that measured only a few nanometres, or billionths of a meter, high. Then they discovered that by changing the thickness of a central `spacer' layer, where the electrons are tunnelling, they could tailor the amount of current that passed through the material. This had to be tempered with a design that kept the boron and phosphorus from intermixing.
Berger said that the diode's ability to operate in low-power conditions makes it ideal for use in power-hungry devices that generate radio-frequency signals, such as cordless home telephones and cell phones. With little power input, the diode could generate a strong signal.
One other application that Berger finds particularly interesting involves medical devices. The diode could support a low-power data link that would let doctors perform diagnostics on pacemakers and other implants by remote, without wires protruding through a patient's skin that could cause infections.
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