Quantum diodes speed silicon

Faster semiconductors based on quantum tunnel diodes could become a commercial reality within five years after US researchers successfully manufactured a commercially viable tunnel diode on conventional silicon. The diodes could be used to replace large and significantly more complex chunks of existing on-chip circuits, reducing component count, signal propagation lengths and delays, and overall power consumption.

Tunnel diodes exploit quantum-mechanical electron tunnelling to substantially boost their electrically current carrying capabilities in relation to low applied voltages. Although tunnel diodes are nothing new—having first been created in the 1960s—they have traditionally relied on the use of exotic materials to achieve the desired diode properties, and researchers have struggled to recreate these in silicon. This has effectively ruled out the use of tunnel diodes in mainstream semiconductor design.

A group led by Ohio State University researchers working in conjunction with the US Naval Research Laboratory and University of California (Riverside), however, now claims to have cracked the problem by producing a silicon tunnel diode that conducts three times more electrical current (per unit surface area of its silicon-based material) than any comparable silicon tunnel diode. The device can conduct 150,000 A/cm².

The tunnel diode is of the inter-band variety whereby it permits electrons to pass back and forth between different energy bands. Its production demanded the development of a new technique for creating silicon structures that contain unusually large quantities of impurities such as boron and phosphorous.

In construction, the researchers layered silicon and silicon-germanium into a structure that measured only a few nanometres high. They then discovered that by changing the thickness of a central spacer layer, where the electrons tunnel, that they could tailor the amount of current that passed through the material. This had to be tempered with a design that prevented the boron and phosphorus from intermixing.

The ability of tunnel diodes to operate in low-power conditions makes them a good choice for power-hungry radio frequency wireless devices as with little power input the diodes could be used to generate a strong output signal.

Another interesting potential application involves medical devices. The diode could support a low power data link that would allow doctors to perform diagnostics on pacemakers and other implants wirelessly, without having wires protruding through a patient’s skin with the associated risk of infection.

“Researchers have long sought to marry tunnel diodes with conventional electronics as a means to simplify increasingly complex semiconductor circuits,” says Paul Berger, professor of electrical engineering and physics at Ohio State University. “[Semiconductor designs] are now worse than the Los Angeles freeway, with wires running back and forth clogging the path of propagating signals. At some point, things are going to come to a grinding halt, and chips won’t run any faster. Because this diode could replace some of the circuits on a typical chip it could potentially simplify chip design without compromising performance.”

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