Ultra-low-power digital memory for portable devices

The technology could give future portable devices much longer battery life between charges.

‘I think anyone who is dealing with a lot of chargers and plugging things in every night can relate to wanting a cell phone or laptop whose batteries can last for weeks or months,’ said electrical and computer engineering professor Eric Pop.

Flash memory used in mobile devices stores bits as charge, which requires high programming voltages and is relatively slow. Industry has been exploring faster but higher power phase-change materials (PCMs) as an alternative. In PCM memory, a bit is stored in the resistance of the material, which is switchable.

Pop’s group lowered the power per bit to 100 times less than existing PCM memory by focusing on size; rather than the metal wires standard in industry, the group used carbon nanotubes.

‘The energy consumption is essentially scaled with the volume of the memory bit,’ said graduate student Feng Xiong. ‘By using nanoscale contacts, we are able to achieve much smaller power consumption.’

To create a bit, the researchers place a small amount of PCM in a nanoscale gap formed in the middle of a carbon nanotube. They can switch the bit ‘on’ and ‘off’ by passing small currents through the nanotube.

‘Carbon nanotubes are the smallest known electronic conductors,’ Pop said. ‘They are better than any metal at delivering a little jolt of electricity to zap the PCM bit.’

Nanotubes also boast an extraordinary stability, as they are not susceptible to the degradation that can plague metal wires. In addition, the PCM that functions as the actual bit is immune to accidental erasure from a passing scanner or magnet.

The low-power PCM bits could be used in existing devices with a significant increase in battery life. Currently, a smart phone uses about 1W of energy and a laptop runs on more than 25W. Some of that energy goes to the display, but an increasing percentage is dedicated to memory.

‘Any time you’re running an app or storing MP3s or streaming videos, it’s draining the battery,’ said graduate student Albert Liao. ‘The memory and the processor are working hard retrieving data. As people use their phones to place calls less and use them for computing more, improving the data storage and retrieval operations is important.’

Results from the research are to be published in an upcoming issue of Science magazine and online in the 10 March Science Express.