Moore's law states that the number of transistors on a microprocessor chip will double every two years or so — which has generally meant that the chip's performance will, too.
The semiconductor industry has released a research road map every two years to coordinate what its hundreds of manufacturers and suppliers are doing to stay in step with the law — a strategy sometimes called More Moore. It has been largely thanks to this road map that computers have followed the law's exponential demands.
Top-of-the-line microprocessors currently have circuit features that are around 14 nanometres across, smaller than most viruses. But by the early 2020s, says Paolo Gargini, chair of the road-mapping organization, “even with super-aggressive efforts, we'll get to the 2–3-nanometre limit, where features are just 10 atoms across. Is that a device at all?” Probably not — if only because at that scale, electron behaviour will be governed by quantum uncertainties that will make transistors hopelessly unreliable. And despite vigorous research efforts, there is no obvious successor to today's silicon technology.
The industry road map released next month will for the first time lay out a research and development plan that is not centred on Moore's law. Instead, it will follow what might be called the More than Moore strategy: rather than making the chips better and letting the applications follow, it will start with applications — from smartphones and supercomputers to data centres in the cloud — and work downwards to see what chips are needed to support them. Among those chips will be new generations of sensors, power-management circuits and other silicon devices required by a world in which computing is increasingly mobile.
The question now is what will happen in the early 2020s, when continued scaling is no longer possible with silicon because quantum effects have come into play.
Quantum computing, which promises exponential speed-up for certain calculations, or neuromorphic computing are possibilities.
A different approach, which does stay in the digital realm, is the quest to find a 'millivolt switch': a material that could be used for devices at least as fast as their silicon counterparts, but that would generate much less heat. There are many candidates, ranging from 2D graphene-like compounds to spintronic materials that would compute by flipping electron spins rather than by moving electrons. “There is an enormous research space to be explored once you step outside the confines of the established technology,” says Thomas Theis, a physicist who directs the nanoelectronics initiative at the Semiconductor Research Corporation (SRC), a research-funding consortium in Durham, North Carolina.
Unfortunately, no millivolt switch has made it out of the laboratory either. That leaves the architectural approach: stick with silicon, but configure it in entirely new ways. One popular option is to go 3D. Instead of etching flat circuits onto the surface of a silicon wafer, build skyscrapers: stack many thin layers of silicon with microcircuitry etched into each. In principle, this should make it possible to pack more computational power into the same space. In practice, however, this currently works only with memory chips, which do not have a heat problem: they use circuits that consume power only when a memory cell is accessed, which is not that often. One example is the Hybrid Memory Cube design, a stack of as many as eight memory layers that is being pursued by an industry consortium originally launched by Samsung and memory-maker Micron Technology in Boise, Idaho.