Watch Out, Transistors: Memristors Gaining Traction
Michael Alba posted on January 27, 2018 |
Professor Themis Prodromakis and his team have put forth exciting new research into memristor memory cells. (Image courtesy of University of Southampton.)
Professor Themis Prodromakis and his team have put forth exciting new research into memristor memory cells. (Image courtesy of University of Southampton.)

The first computers were made of a now-outdated technology called vacuum tubes. Though still in use in some niche applications, vacuum tubes have largely been replaced by solid state transistors, a few billion of which are hiding behind these very words.

But just as vacuum tubes gave way to new technology, transistors too may soon face obsolescence. The shadow hanging over them is a device called a memristor, and that shadow looms a bit larger thanks to recent research from the University of Southampton.

“This is a really exciting discovery, with potentially enormous implications for modern electronics,” researcher Themis Prodromakis said.

What Is a Memristor?

First things first, lets define what a memristor is. Theorized in 1971 by electrical engineer Leon Chua, a memristor is a circuit element whose resistance changes depending on the amount of current that flows through it. The memristor fills a gap left by the holy trinity of electrical devices—the resistor, capacitor and inductor—as seen in this diagram:

The memristor relates electric charge (time integral of current) to flux (time integral of voltage), completing the conceptual symmetryof resistor, capacitor, inductor and memristor. (Image courtesy of Parcly Taxel.)
The memristor relates electric charge (time integral of current) to flux (time integral of voltage), completing the conceptual symmetryof resistor, capacitor, inductor and memristor. (Image courtesy of Parcly Taxel.)

To give an intuitive idea of how a memristor behaves, let’s start with the common analogy for a resistor: as water (electricity) flowing through a pipe. The wider the pipe, the more water can flow through, and the lower the resistance. A memristor is like a pipe that can grow and shrink in width—when water flows through in one direction, the pipe grows (the resistance decreases), and when water flows the other way, the pipe shrinks (the resistance increases). When you turn off the water (cut the power), the pipe retains its width (resistance) until you turn it back on. In fact, this “memory” of its resistance is what gives the memristor its name.

The memristor was merely theoretical for the first few decades of its existence, but in 2008, Hewlett-Packard researchers published a paper, The missing memristor found, which demonstrated memristor behavior in nanoscale systems. Since then, there’s been renewed interest and research in commercial applications of memristors, such as computer memory.

The Southampton Research

The recent research examines the multibit memory operation of a type of memristor built with metal oxides. The researchers developed variants of a two-terminal metal-insulator-metal (MIM) resistive to random access memory (ReRAM) cell. These consisted of a TiO2 solid electrolyte with different material used as interface barriers for a bilayer structure. That’s a bit much to take in, so here’s a picture:

Illustration of one of the bilayer configurations tested by the researchers. (Image courtesy of Scientific Reports.)
Illustration of one of the bilayer configurations tested by the researchers. (Image courtesy of Scientific Reports.)

The research found that introducing the second layer in addition to the TiO2 electrode resulted in an increase in the attainable resistive states of the device—in other words, an increase in how much information it can store. The researchers set a record of 92 distinct resistive states in one of their devices, capable of 6.5 bits of information storage. Furthermore, the team set out a method for programming the devices as multibit memory elements.

“Through this study, we were able to demonstrate for the first-time solid-state ReRAM operating as analogue memory cells with up to 5.5-bits capacity,” concluded the research paper. “While ReRAM technologies have been mainly promoted for high-spatial density storage and corollary applications, our work demonstrates the new prospects arising from high-capacity memory.”

So is this the nail in the coffin of the transistor? Not quite—but nonetheless, it’s a step in the right direction, according to Prodromakis.

“Memristors are a key enabling technology for next-generation chips, which need to be highly reconfigurable yet affordable, scalable and energy efficient,” he said. “At the same time, this technology is ideal for developing novel hardware that can learn and adapt autonomously, much like the human brain.”

For more breaking electronics research, read How to Make Waterproof Graphene Circuits.


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