Engineering 101

Researchers Build Functional Transistor Integrated with Ferroelectric RAM

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As our demand for computer chips grows, with some devices using 200 or more chips, the manufacturing conundrum is how to make them smaller and more powerful. Since a computer chip processes and stores information using two different devices, engineers know that combining these devices into one or putting them next to each other, would allow more space on a chip, making it faster and more powerful.

Recently, Purdue University engineers have developed a way that the millions of tiny switches used to process information—called transistors—could also store that information as one device.

The method accomplishes this by solving another problem: combining a transistor with higher-performing memory technology than is used in most computers, called ferroelectric RAM.

Researchers have been trying for decades to integrate the two, but issues happen at the interface between a ferroelectric material and silicon, the semiconductor material that makes up transistors. Instead, ferroelectric RAM operates as a separate unit on-chip, limiting its potential to make computing much more efficient.

A team led by Peide Ye, professor of Electrical and Computer Engineering at Purdue, discovered how to overcome the mortal enemy relationship between silicon and a ferroelectric material.

“We used a semiconductor that has ferroelectric properties. This way two materials become one material, and you don’t have to worry about the interface issues,” Ye said.

The result is a so-called ferroelectric semiconductor field-effect transistor, built in the same way as transistors currently used on computer chips.

The material, alpha indium selenide, not only has ferroelectric properties, but also addresses the issue of a conventional ferroelectric material usually acting as an insulator rather than a semiconductor due to a so-called wide “band gap,” which means that electricity cannot pass through and no computing happens.

Alpha indium selenide has a much smaller band gap, making it possible for the material to be a semiconductor without losing ferroelectric properties.

Mengwei Si, a Purdue postdoctoral researcher in electrical and computer engineering who built and tested the transistor, found its performance was comparable to existing ferroelectric field-effect transistors, and could exceed them with more optimization. Si and Ye’s team also worked with researchers at the Georgia Institute of Technology to build alpha indium selenide into a space on a chip, called a ferroelectric tunneling junction, which engineers could use to enhance a chip’s capabilities.

In the past, researchers hadn’t been able to build a high-performance ferroelectric tunneling junction because its wide band gap made the material too thick for electrical current to pass through. Since alpha indium selenide has a much smaller band gap, the material can be just 10 nanometers thick, allowing more current to flow through it.

More current allows a device area to scale down to several nanometers, making chips more dense and energy efficient, Ye said. A thinner material—even down to an atomic layer thick—also means that the electrodes on either side of a tunneling junction can be much smaller, which would be useful for building circuits that mimic networks in the human brain.

Source: Purdue University

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