Researchers are repurposing decades-old technology to build faster gadgets for the future, creating nanoscale vacuum tubes that could dramatically improve the speed and efficiency of personal electronics and solar panels.
Vacuum tubes were originally used in the earliest digital electronic computers back in the 1930s and 1940s, before being replaced by transistors composed of semiconductors, which can can be manufactured much smaller, making today's computers, smartphones, and tablets possible.
But transistors have their limits in size and speed too, and we're getting closer than ever to reaching them. Now scientists from UC San Diego have gone back to the vacuum tube idea - and this time they've made them at tiny sizes and with far more efficient technology.
"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," says lead researcher and electrical engineer, Dan Sievenpiper.
While transistors remain one of the most important inventions of the 20th century - and much smaller and more energy-efficient than the original vacuum tubes - scientists are now struggling to make them any tinier or more powerful than they already are.
What's more, electron flow through transistor semiconductor materials like silicon is slowed as electrons collide with atoms, and semiconductors also have what's called a band gap - where a boost of external energy is needed to get electrons moving.
The main advantage new nanoscale vacuum tubes have over semiconductor-based transistors is that they carry currents through air, rather than a solid material, and could be be much faster as a result.
The vacuum tube design (left), electric field enhancement (middle), and electric field distribution (right) of the new nanoscale structure. Credit: UC San Diego
Freeing up electrons to carry currents through the air normally takes a large voltage or a powerful laser, both of which are difficult to do at the nanoscale, and which hampered the progress of early vacuum tubes.
To solve this problem, the team created a layer of special mushroom-style structures made of gold - known as an electromagnetic metasurface - and placed it on top of a layer of silicon dioxide and a silicon wafer.
When a low-powered voltage (less than 10 volts) and a low-powered laser are applied to this metasurface, it creates 'hot spots' with high-intensity electric fields, giving the structure enough energy to free the electrons from the metal.
In testing, this enabled the researchers to achieve a 1,000 percent (or 10-fold) increase in conductivity compared with nanoscale vacuum tubes without the metasurface.
Right now, it's just a proof-of-concept demonstration, and there's a lot more work to be done to make the system practical for use in actual devices. But in the future, different metasurfaces could be designed to meet specific needs, such as new kinds of solar panels, the researchers suggest.
"Next we need to understand how far these devices can be scaled and the limits of their performance," says Sievenpiper.
Here's the team explaining their findings:
The research is published in Nature Communications.