Diode developed for faster electronics

Engineers have designed a new diode that transmits more electricity - conducting 150,000 A per square centimetre. Unlike other diodes in its class, called tunnel diodes, the new diode is compatible with silicon, so manufacturers could build it into mainstream electronic devices such as mobile phone and computers.

Paul R Berger, professor of electrical engineering and physics at Ohio State University designed the diode with a team of engineers from Ohio State, the Naval Research Laboratory and University of California, Riverside.

Tunnel diodes are so named because they exploit a quantum mechanical effect known as tunnelling which lets electrons pass through barriers unhindered. The first tunnel diodes were created in the 1960s and led to a Nobel Prize for physicist Leo Esaki in 1973.

Since then, in an effort to build more powerful diodes, researchers have increasingly turned to expensive, exotic materials that aren't 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.

"Essentially, we trade one nightmare for another," Berger said with a laugh. "Mother Nature doesn't want that much dopant in one place, but the doping problem was one that we felt we could tackle."

They layered silicon and silicon-germanium into a structure that measured only a few nanometres or billionths of a metre, 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 suitable for use in power-hungry devices that generate radio-frequency signals, such as cordless home telephones and mobile phones. With little power input, the diode could generate a strong signal.

One other application 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 wire protruding through a patient's skin.