Researchers Solve a Roadblock in an Unusual Approach to Chip-Making: A Return to Vacuum Tubes

Researchers from Shanghai Jiao Tong University and Shaoxing University have called for a return to vacuum tubes — solving one of the fundamental problems that prevents them being shrunk down to a size where they can be used in integrated circuits like microprocessors and microcontrollers.

"We've worked on this problem for years, because everyone knows that if you could make vacuum tubes work again at the chip scale, the speed advantage would be enormous," the team, under corresponding author Yuelin Wang, say of the research. "The reason previous attempts failed was always the same — the gate leaked. By controlling the electron supply at the cathode instead of trying to catch electrons in mid‑air, we finally got rid of that leakage. Seeing the same device work as an amplifier, a differential pair, and even a NAND gate on the test bench was the moment we realized this approach actually has a future."

Vacuum tubes, also known as an electron tube or thermionic valve, were mainstays of electronics from their invention by John Fleming in 1904 through to the development of solid-state semiconductor alternatives — with the 1960s marking the turning point away from vacuum tubes to transistors. Today, modern electronics are made up of thousands, millions, or even billions of transistors built on a nano-scale, crammed into compact silicon chips to power everything from smartphone and TVs to autonomous vehicles.

The chip industry is rapidly running into scaling problems, though. Moore's Law, an observation by Intel co-founder Gordon Moore that the number of transistors in a leading-edge part trends towards a doubling roughly every two years, is running into fundamental physical limits as component sizes shrink. While most people thinking of vacuum tubes today will imagine the finger-sized ones powering amplifiers, researchers believe that vacuum tube shrunken down to the nano-scale could be the answer.

The problem isn't in making small vacuum tubes, oddly enough, but in making them work properly. Previous attempts have demonstrated the potential for significant speed boosts, as electrons are free to flow at the speed of light in a vacuum, but production was stymied by a leakage problem: instead of hitting the tiny anode, the electrons tended to crash into the gate instead. The researchers' solution: a reversal of roles, in which the gate is used to modulate electron concentration inside the cathode rather than blocking or diverting electrons on their way to the anode.

In testing, the tiny tubes — a cathode-modulated vacuum/air-channel electron tube, or CMVET, to give the technology its proper name — showed leakage orders of magnitude lower than previous attempts, and worked as components in common-source, differential, and cascade amplifiers, as well as NAND and NOR gates.

It'll be a while before the technology reaches a computer near you, though. The team admits that there's another fundamental issue in play: the prototype parts are non-saturating, meaning that the current keeps rising as the anode voltage increases — in contrast to traditional MOSFETs, which plateau at a certain point. Resolving this, the researchers say, is the team's next goal.

The team's work has been published in the journal Microsystems & Nanoengineering under open-access terms.