How Vacuum Tubes, New Technology Might Save Moore's Law

The transistor is one of the most profound innovations in all of human existence. First discovered in 1947, it has scaled like no advance in human history; we can pack billions of transistors into complicated processors smaller than your thumbnail. After decades of innovation, however, the transistor has faltered. Clock speeds stalled in 2005 and the 20nm process node is set to be more expensive than the 28nm node was for the first time ever. Now, researchers at NASA believe they may have discovered a way to kickstart transistors again -- by using technology from the earliest days of computing:  The vacuum tube.

No, really.  Stop laughing.

Once upon a time, vacuum tubes were the fundamental unit of computing, vital to the function of nearly all electronics before the advent of solid-state transistors. Radios, telephones, early recording and sound reproduction equipment, and the first computers (including ENIAC) relied on vacuum tubes to function. Vacuum tube technology improved slowly over the years (early computers could suffer tube failures every day or so), but once the solid state transistor had been invented the writing was on the wall for these devices. Vacuum tubes ran hot when they operated, which meant a computer with a few thousand tubes soon became a furnace. ENIAC contained 17,648 tubes and consumed between 150-175KW of power (reports vary).  

                                                                                                                                                            Credit: IEEE Spectrum

The transistor quickly outstripped even the fastest vacuum tube technology, but today is slamming into the fundamental limits of our manufacturing technology. The simplest way to explain the myriad reasons why transistors aren't scaling as they used to is this:  We have hit the limits of perfection. We need perfect feature sizes at sizes so tiny, we can't build a laser small enough to etch them. We need to control the level of dopants (deliberate impurities in the silicon substrate) to within 10s of atoms. Heisenberg himself is tugging at the sleeve of Intel's finest engineers and clucking disapprovingly.

How Vacuum Transistors Could Save Moore's Law

It turns out that when you shrink a Vacuum  transistor to absolutely tiny dimensions, you can recover some of the benefits of a vacuum tube and dodge the negatives that characterized their usage. According to a report in IEEE Spectrum, vacuum transistors can draw electrons across the gate without needing a physical connection between them.  Make the vacuum area small enough, and reduce the voltage sufficiently, and the field emission effect allows the transistor to fire electrons across the gap without containing enough energy to energize the helium inside the nominal "vacuum" transistor. According to researchers, they've managed to build a successful transistor operating at 460GHz -- well into the so-called "Terahertz Gap," which sits between microwaves and infrared energy. The "gap" refers to the fact that we have a limited number of devices that can generate this frequency and only a handful of experimental applications for this energy band.


Right now, these structures are in the highly experimental stages. Researchers have built a handful of transistors that can reach these enormous operating frequencies, but have yet to tackle the considerable challenge of integrating millions of them. The current 460GHz transistor runs at 10V, though prototypes have been shrunk down to 1-2V. It's not clear what scale these vacuum transistors have been built at, either -- but clearly the research team behind them sees long-term potential in everything from communication hardware to (a long ways down the line) CMOS and microprocessors.  

Vacuum transistors aren't going to reinvent computing in the next 2-3 years, but long term they could emerge as a way to avoid some of the quantum tunneling and electromigration effects that currently challenge modern microprocessors. That's a huge advance for a transistor structure with nothing in between the source and drain.