How’s your phone battery doing? As consumers we generally want that juice to last between charges, so we can avoid that anxious niggle of a dwindling battery icon when an important call or email is on the cards. So what are researchers doing to address our plight? Could the answers lie in less leaky transistors and more energy-efficient materials?
Getting into shape
Transistors are at the heart of electronic devices, with potentially millions crammed onto each chip, and their role is to control flow of electrons through their structure, explains Prof John Boland, director of CRANN.
He likens the process to stopping and starting the flow of water through a hose by pressing on it and releasing. But the problem is that transistors can be “leaky”, which does your battery life no good at all, he notes.
One approach to reducing this leakiness is to change the shape of the transistor so it can be “pinched” more efficiently.
“The ideal way to do this would be to pinch it off from all sides and completely choke the flow,” explains Prof Boland.
“So you would need to have the gate completely wrapped around the device – in other words you would have a nanowire of silicon that is surrounded by a gate. Those are the kinds of experimental structures we are looking at now in CRANN.”
But there’s still an issue with the silicon. While it has been a driving force in electronics for decades, its relative lack of ability to let electrons through is becoming an issue, according to Boland, and the search is on for new materials with properties that could better suit the demands of mobile devices.
Gallium arsenide and indium gallium arsenide don't exactly trip off the tongue as easily as the stalwart silicon, but they could potentially fit the bill here because they have a far greater mobility for electrons, explains Prof Boland, who is professor of chemistry at Trinity College Dublin.
“These materials won’t lose as much power in flowing charge through the device itself,” he says. “They require less power to operate and hence they are much more efficient.”
Gallant gallium
Being able to reduce the energy consumption and loss per transistor, even by a small amount, could translate into big savings when you think about the sheer numbers of transistors in use, explains Dr Paul Hurley from Tyndall National Institute in Cork.
“If you take one transistor and you save a few nanowatts, that doesn’t sound very much – but then consider there may be millions of transistors on a single chip, and then you can multiply that by the billions of electronic devices in use around the world each day.”
Tyndall is part of the Focus (Future Oxides and Channel Materials for Ultimate Scaling) research collaboration with Dublin City University, Queen's University Belfast and the University of Texas at Dallas.
Funded under the US-Ireland R&D Partnership Programme, they are looking at using germanium and indium gallium arsenide as the semiconductor materials in the transistors instead of silicon, and finding that the alternative materials are proving their mettle.
“By using these materials you could make the transistors go faster,” says Dr Hurley. “But it also means you can use the same amount of current, achieve the same speed, but you can do it with a lower supply voltage, and that’s where the gain could be for mobile devices and battery life.”
Cramming in more transistors is another important aspect, he notes. “As we reach the limits of transistor scaling – you can’t get smaller than the separation between atoms – how you get more transistors per unit area remains critical for the industry and the consumer.”
One way to do it is to go up, explains Dr Hurley, who hopes to start research into three-dimensional integration of transistors.
“That would mean you get more transistors per unit area without making them smaller, and this could be the next big thing for integrated circuits in mobile devices like phones and tablets.”
Great graphene
And what about graphene, the wunderkind atom-thick carbon material discovered in 2004? Could a new generation of more energy-efficient and cheap transistors glide in on its coat-tails? It's not so straightforward, according to Prof Boland.
“Graphene looks great on paper, but as soon as you put it into a device its mobility drops,” he explains.
“That’s often the challenge with a nanomaterial – as an individual freestanding material it has wonderful properties, but it needs to be incorporated into a device structure and as soon as you do that you tend to impact on the properties of the material.
“So one approach being investigated at CRANN is to position a spacer layer between the graphene and the surrounding device structure to minimise this influence.”
Researchers at CRANN are also investigating other two-dimensional materials to see if they could have more attractive properties for transistors, explains Prof Boland.
And they are separately looking to develop a nano-scale switch that reduces leakiness by “closing” the transistor more securely to stop the flow of electrons.
He foresees that such switches, which are as yet still a long way from market, might help to reduce leakage by shutting down parts of circuitry in a device when they aren’t being used.
“You could imagine a single one of these switches would turn off hundreds or even thousands of normal silicon transistors and prevent all those transistors from leaking,” he explains.
“Having this extra level of hierarchy might add functionality but most importantly it would add battery life.”
