A memristor is a device whose resistance depends on its past history – more precisely, it depends on the flux of electric charge through the device. In this sense, a RRAM is a device that shows memristive behaviour, and can be thought of as a specific type of memristor.
Why are RRAMs and memristors of interest?
RRAM devices may help overcome some of the bottlenecks that we are currently facing in microelectronics. As we shrink the size of the transistors that make up semiconductor memories further and further we run into problems of fabrication difficulty, power dissipation and switching speed. RRAM devices can be packed much more densely. fabricated in 3D arrays, and have very low switching energies and fast switching speeds.
In addition, devices whose state depends on their past history behave in some ways similarly to neurons – RRAMs and memristors can thus be used to fabricate very high density neural networks.
What is the UCL Resistive RAM?
We have developed, and filed a patent on, a RRAM device based wholly on the Si/SiO2 system. Unlike competing technologies, it does not rely on the diffusion of metal ions, can be fabricated only from n- and p- type silicon and silicon oxide, and operates in ambient conditions. Resistance contrast is up to 5 orders of magnitude, switching time 90ns or shorter, and switching energy is 1pJ/bit or lower. Scanning Tunneling Microscopy suggests that the individual switching elements may be as small as 10nm
How does the UCL RRAM compare to existing technologies?
The UCL RRAM devices switch 100 times faster than Flash memory, but need to be made in an area that is 1 million to 8 million times smaller.
Devices operating in this way are known as ‘memristors’, and the development of a silicon oxide memristor is a huge step forward because of the potential for its incorporation into silicon chips.
“My work revealed that a material we had been looking at for some time could in fact be made into a memristor,” said Mehonic, at the UCL Department of Electronic and Electrical Engineering.
“The potential for this material is huge. We can programme the chips using the cycle between two or more states of conductivity.
“We’re very excited that our devices may be an important step towards new silicon memory chips.”
The ReRAM chip can be designed to have a continuously variable resistance that depends on the last voltage that was applied – an important property that allows the device to mimic how neurons in the brain function.
Unlike other silicon oxide chips currently in development, the chip does not require a vacuum to work, and is therefore potentially cheaper and more durable.
The design also raises the possibility of transparent memory chips for use in touch screens and mobile devices.
The researchers, who have filed a patent on the device, are also exploring using the resistance properties of their material not just for use in memory but also as a computer processor.
ReRAM memory chip (credit UCL/Adnan Mehonic)