Transistors are everywhere — in your computer, car, phone and refrigerator — but they're not shrinking fast enough to satisfy our hunger for ever-faster devices. A new kind of light-based transistor might just fix that.
At its most straightforward, a transistor — or field effect transistor, to give the microscopic device found in all your electronics its proper name — is just a tiny switch. It has three terminals: a source, a drain, and a gate. By varying the voltage (or the field, as you may've guessed) at the gate, it's possible to control the current that flows from the source to the drain. Simple. Throw enough together, and you can create complex circuits to carry out logical operations — like a computer chip.
But making these transistors smaller is tough. They already measure just nanometres in size, made of ultra-thin layers of silicon that is scattered — the technical term is doped — with other atoms to imbue them with their switching abilities. But as they're made smaller, it's harder to control how the atoms are added, and that can result in an unpredictable switch. Which is bad when you want something to work reliably.
Now though, as Technology Review reports, a team of researchers from the University of North Carolina in Charlotte has developed a new kind of transistor that controls the current that flows through it not using an applied voltage but with light instead. Think of it as a smart microscopic wire: When it's illuminated it allows electrons to flow, when it's dark nothing passes through. While that alone may not sound too useful, the team points out that the devices can be made smaller than field effect transistors because they don't require doping in the same way. In turn, that should allow more to be squeezed into the same space, allowing speeds to increase in a a way that satisfies our thirst.
A schematic of a field effect transistor on the left, and the new light-based transistor on the right.
The science behind the transistor itself is pretty simple. Materials have been known to be photoconductive — that is, conduct more or less electricity depending on their illumination — for a while now. What the team has done is create a device which uses a ribbon of such a material — in this case cadmium and selenium — that's just a couple of atoms thick. In tests where the illumination was controlled using laser beams, the team has found that they conduct around a million times more current when on than off, which is broadly comparable to regular transistors. The research is published on arXiv.
That all sounds to good to be true — and for now it is, because there are some major hurdles to overcome before light-based transistors can be used in anger. There are decades of engineering work applied to building large grids of conventional transistors and the electronic networks required to switch them. Swapping to a light-based control system is relatively unexplored, with plenty of questions: How do you send light to each transistor? How much power will that use? How fast can the switches be flicked on and off?
There might not be many answers right now, but we sure look forward to hearing about them in the future.