Semiconductor-free microelectronics using metamaterials: faster, can handle more power

Back to the vacuum-tube future
November 13, 2016

Illustration of semiconductor-free microelectronic device (credit: UC San Diego Applied Electromagnetics Group)

University of California San Diego engineers have made the first semiconductor-free, optically controlled microelectronic device, using metamaterials, with a 1,000 % increase in conductivity when activated by low voltage and a low-power laser.

The discovery may lead to microelectronic devices that are faster and capable of handling more power, and to more efficient solar panels. The work was published Nov. 4 in Nature Communications (open access).

Replacing semiconductors with free electrons in space, similar to how vacuum tubes work (credit: UC San Diego Applied Electromagnetics Group)

Semiconductors impose limits on a device’s conductivity, or electron flow, since electrons are constantly colliding with atoms as they flow through the semiconductor. To remove these roadblocks to conductivity, the engineers replaced semiconductors with free electrons in space, similar to how vacuum tubes work.

However, liberating electrons from materials is challenging. It requires either applying high voltages (at least 100 volts for cold-cathode emitters), or high power lasers, or extremely high temperatures (more than 1,000 degrees Fahrenheit for thermionic emission) — all impractical in making micro- and nanoscale electronic devices.

Scanning electron micrograph images showing the semiconductor-free microelectronic device (top left) and details of the gold metasurface (top right, bottom). (credit: UC San Diego Applied Electromagnetics Group)

To address this challenge, Sievenpiper’s team fabricated a microscale device that can release electrons from a material without such extreme requirements. The device consists of an engineered surface, called a “metasurface,” on top of a silicon wafer, with a layer of silicon dioxide in between. The metasurface consists of an array of gold mushroom-like nanostructures on an array of parallel gold strips.*

The gold metasurface is designed such that when a low DC voltage (under 10 volts) and a compact, low-power infrared laser are both applied, the metasurface generates “hot spots” — areas with a high-intensity electric field — that provide enough energy to pull electrons out from the metal and liberate them into space. (credit: UC San Diego Applied Electromagnetics Group)

Best for very high frequencies or high power devices

“This certainly won’t replace all semiconductor devices, but it may be the best approach for certain specialty applications, such as very high frequencies or high power devices,” said senior author Dan Sievenpiper, a UC San Diego electrical engineering professor.

The team is also exploring other applications for this technology, such as photochemistry and photocatalysis. These may enable new kinds of photovoltaic devices or environmental applications.

This work was funded by the Defense Advanced Research Projects Agency and the Office of Naval Research Defense University Research Instrumentation Program.

Jacobs School | Semiconductor-free microelectronics

* The method uses a combination of photoemission (assisted by localized surface plasmon resonances in the near-IR range) and field emission to inject electrons into the surrounding space (vacuum or gas). The intensity of the electric field at the hot spots can be controlled both electrically (with static bias) and optically (with the incoming laser).

Abstract of Photoemission-based microelectronic devices

The vast majority of modern microelectronic devices rely on carriers within semiconductors due to their integrability. Therefore, the performance of these devices is limited due to natural semiconductor properties such as band gap and electron velocity. Replacing the semiconductor channel in conventional microelectronic devices with a gas or vacuum channel may scale their speed, wavelength and power beyond what is available today. However, liberating electrons into gas/vacuum in a practical microelectronic device is quite challenging. It often requires heating, applying high voltages, or using lasers with short wavelengths or high powers. Here, we show that the interaction between an engineered resonant surface and a low-power infrared laser can cause enough photoemission via electron tunnelling to implement feasible microelectronic devices such as transistors, switches and modulators. The proposed photoemission-based devices benefit from the advantages of gas-plasma/vacuum electronic devices while preserving the integrability of semiconductor-based devices.