With Moore’s law in peril, the search is on for the next computing substrate. Keeping up the pace of progress in an ever more compute and data driven world will likely require a post-silicon invention that can satisfy humanity’s need for faster, smaller, greener and more powerful computers.
In recognition of the size and speed limitations of current compute and memory technology, scientists from the University of Cambridge, the Singapore A*STAR Data-Storage Institute and the Singapore University of Technology and Design are exploring the feasibility of replacing silicon with a material that can switch back and forth between different electrical states. Phase-change materials (or PCMs) as these promising substrates are known can switch between two structural phases with different electrical states, one crystalline and conducting and the other glassy and insulating, in billionths of a second. It’s a technology that could one day enable processing speeds one-thousand times faster than current systems, while using less energy.
As described in the journal Proceedings of the National Academy of Sciences, the researchers created a PCM-based processor using chalcogenide glass, which can be melted and recrystallized in as little as half a nanosecond (billionth of a second) when the correct voltage is applied. The team showed that logic-processing operations can be performed in non-volatile memory cells using particular combinations of ultra-short voltage pulses, which is not possible with silicon-based technology. This works because the PCM devices put logic operations and memory in the same location.
Silicon-based systems are movement-intensive, and that takes time and energy. “Ideally, we’d like information to be both generated and stored in the same place,” said Dr. Desmond Loke of the Singapore University of Technology and Design, the paper’s lead author. “Silicon is transient: the information is generated, passes through and has to be stored somewhere else. But using PCM logic devices, the information stays in the place where it is generated.”
“Eventually, what we really want to do is to replace both DRAM and logic processors in computers by new PCM-based non-volatile devices,” added Professor Stephen Elliott of Cambridge’s Department of Chemistry, who led the research. “But for that, we need switching speeds approaching one nanosecond. Currently, refreshing of DRAM leaks a huge amount of energy globally, which is costly, both financially and environmentally. Faster PCM switching times would greatly reduce this, resulting in computers which are not just faster, but also much ‘greener’.”
A drawback of current PCM devices is that they are not as fast as their silicon-based counterparts. There is also a stability issue affecting the amorphous phase. But the researchers found that by performing logic operations in reverse, putting the crystalline phase first, they can facilitate faster, more stable performance.
PCMs were developed in the 1960s and found their way into optical-memory devices and more recently in electronic-memory applications. They are just now starting to replace silicon-based flash memory in some smart phones. As researchers continue to identify speed enhancements, non-volatile PCM could eventually supplant the more energy-intensive DRAM.