Bridging the gap. Memristors can integrate with existing computing hardware. (Image: J. J. Yang, HP Labs)

In the 18 months since the “missing link of electronics” was discovered in Hewlett-Packard’s laboratories in Silicon Valley, California, memristors have spawned a hot new area of physics and raised hope of electronics becoming more like brains.

Now the same team have upgraded a standard silicon chip with a layer of memristors to show that the novel component can play nicely with existing computing hardware.

That suggests it may not be long before they reach the market. And that in turn is good news for manufacturers, who need to find a new way to keep computer power growing: the methods that have shrunk computers in recent years look to have reached their limits.


Decreasing returns

Memristors behave a bit like resistors, which simply resist the flow of electric current. But rather than only respond to present conditions, a memristor can also “remember” the last current it experienced.

That’s an ability that would usually require many different components. “Each memristor can take the place of 7 to 12 transistors,” says Stan Williams, head of HP’s memristor research. What’s more, it can hold its memory without power. By contrast, “transistors require power at all times and so there is a significant power loss through leakage currents”, Williams explains.

A memristor’s memory of its last current can be read by watching how it creates a new memory as it responds to a new current. They are made from a double layer of semiconducting titanium dioxide – the pigment in white paint.

Memristor array

Williams and his colleagues patterned 10,000 memristors on top of an ordinary CMOS chip.

“The biggest technical challenge for building the hybrid circuit was the fact that the CMOS circuit had a highly irregular surface,” says Williams. Even bumps only one ten-thousandth of a millimetre high would be too large, so complex physical and chemical polishing was required.

The memristor array is made from a criss-crossing grid of 100-by-100 conducting wires. Each junction between two wires sandwiches titanium dioxide and acts as a memristor. A series of copper connections link the guts of the original chip with its new memristive top coat.

Flexible chip

That new memristor array can take over some tasks of the CMOS circuit beneath.

“The new hybrid system lifts the data-routing network and the switches out of the CMOS plane,” says Williams. “This will greatly free up the space on the CMOS layer for more devices, effectively increasing the density of circuits,” he says, without the need to shrink the transistors any further.

Adding the memristors has given a fixed silicon chip similar properties to a field-programmable gate array, a kind of chip that can be physically reconfigured on the fly to prototype new circuit designs without building many fixed designs, says Williams.

It has also proved that memristors can integrate with standard silicon hardware.

Learning machines

The similarities between memristive circuits and the behaviour of some simple organisms suggests the hybrid devices could also open the way for “neuromorphic” computing, says Williams, in which computers learn for themselves, like animals.

Wei Lu, a memristor researcher at the University of Michigan in Ann Arbor, says the new device is a “significant step forward” in the development of nanoelectronic circuits. Showing that nanoscale memristors can be made on top of existing chips “will certainly stimulate additional interest from the semiconductor industry and academia”, he says.

The speed of such hybrid devices needs to improve, Lu adds. “However, continued development of the memristor devices may fundamentally change the way we design and fabricate integrated circuits in the future.”

Journal reference: Nano Letters, DOI: 10.1021/nl901874j