silicon

Researchers get spiking neural behavior out of a pair of transistors

The team found that, when set up to operate on the verge of punch-through mode, it was possible to use the gate voltage to control the charge build-up in the silicon, either shutting the device down or enabling the spikes of activity that mimic neurons. Adjustments to this voltage could allow different frequencies of spiking. Those adjustments could be made using spikes as well, essentially allowing spiking activity to adjust the weights of different inputs.

With the basic concept working, the team figured out how to operate the hardware in two modes. In one of them, it acts like an artificial synapse, capable of being set into any of six (and potentially more) weights, meaning the potency of the signals it passes on to the artificial neurons in the next layer of a neural network. These weights are a key feature of neural networks like large language models.

But when combined with a second transistor to help modulate its behavior, it was possible to have the transistor act like a neuron, integrating inputs in a way that influenced the frequency of the spikes it sends on to other artificial neurons. The spiking frequency could range in intensity by as much as a factor of 1,000. And the behavior was stable for over 10 million clock cycles.

All of this simply required standard transistors made with CMOS processes, so this is something that could potentially be put into practice fairly quickly.

Pros and cons

So what advantages does this have? It only requires two transistors, meaning it’s possible to put a lot of these devices on a single chip. “From the synaptic perspective,” the researchers argue, “a single device could, in principle, replace static random access memory (a volatile memory cell comprising at least six transistors) in binarized weight neural networks, or embedded Flash in multilevel synaptic arrays, with the immediate advantage of a significant area and cost reduction per bit.”

Simple voltage pulse can restore capacity to Li-Si batteries

batteries, chemistry, electrochemistry, lithium, Science, silicon / Mike M. / October 19, 2024

The new work, then, is based on a hypothetical: What if we just threw silicon particles in, let them fragment, and then fixed them afterward?

As mentioned, the reason fragmentation is a problem is that it leads to small chunks of silicon that have essentially dropped off the grid—they’re no longer in contact with the system that shuttles charges into and out of the electrode. In many cases, these particles are also partly filled with lithium, which takes it out of circulation, cutting the battery’s capacity even if there’s sufficient electrode material around.

The researchers involved here, all based at Stanford University, decided there was a way to nudge these fragments back into contact with the electrical system and demonstrated it could restore a lot of capacity to a badly degraded battery.

Bringing things together

The idea behind the new work was that it could be possible to attract the fragments of silicon to an electrode, or at least some other material connected to the charge-handling network. On their own, the fragments in the anode shouldn’t have a net charge; when the lithium gives up an electron there, it should go back into solution. But the lithium is unlikely to be evenly distributed across the fragment, making them a polar material—net neutral, but with regions of higher and lower electron densities. And polar materials will move in an uneven electric field.

And, because of the uneven, chaotic structure of an electrode down at the nano scale, any voltage applied to it will create an uneven electric field. Depending on its local structure, that may attract or repel some of the particles. But because these are mostly within the electrode’s structure, most of the fragments of silicon are likely to bump into some other part of electrode in short order. And that could potentially re-establish a connection to the electrode’s current handling system.

To demonstrate that what should happen in theory actually does happen in an electrode, the researchers started by taking a used electrode and brushing some of its surface off into a solution. They then passed a voltage through the solution and confirmed the small bits of material from the battery started moving toward one of the electrodes that they used to apply a voltage to the solution. So, things worked as expected.

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