'Brain-on-a-chip' contains tens of thousands of memristors: Page 2 of 3

June 10, 2020 //By Julien Happich
Researchers from MIT have engineered tens of thousands of artificial brain synapses on a silicon chip a few square millimeters in size.

The new chip (top left) is patterned with tens of thousands of
artificial synapses, or “memristors,” made with a silver-copper
alloy. When each memristor is stimulated with a specific
voltage corresponding to a pixel and shade in a gray-scale
image (in this case, a Captain America shield), the new chip
reproduced the same crisp image, more reliably than chips
fabricated with memristors of different materials.
Credit: Image courtesy of the researchers.

Like a brain synapse, a memristor would also be able to "remember" the value associated with a given current strength, and produce the exact same signal the next time it receives a similar current. This could ensure that the answer to a complex equation, or the visual classification of an object, is reliable—a feat that normally involves multiple transistors and capacitors.

Existing memristor designs, however, are limited in their performance. A single memristor is made of a positive and negative electrode, separated by a "switching medium," or space between the electrodes. When a voltage is applied to one electrode, ions from that electrode flow through the medium, forming a "conduction channel" to the other electrode.

The received ions make up the electrical signal that the memristor transmits through the circuit. The size of the ion channel (and the signal that the memristor ultimately produces) should be proportional to the strength of the stimulating voltage.

Kim says that existing memristor designs work pretty well in cases where voltage stimulates a large conduction channel, or a heavy flow of ions from one electrode to the other. But these designs are less reliable when memristors need to generate subtler signals, via thinner conduction channels.

The thinner a conduction channel, and the lighter the flow of ions from one electrode to the other, the harder it is for individual ions to stay together. Instead, they tend to wander from the group, disbanding within the medium. As a result, it's difficult for the receiving electrode to reliably capture the same number of ions, and therefore transmit the same signal, when stimulated with a certain low range of current.

Kim and his colleagues found a way around this limitation by borrowing a technique from metallurgy. "Traditionally, metallurgists try to add different atoms into a bulk matrix to strengthen materials, and we thought, why not tweak the atomic interactions in our memristor, and add some alloying element to control the movement of ions in our medium," said Kim.

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