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Simple material gives electronic devices a power boost

22 Oct 2015  | Walt Mills

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A team of researchers from Penn State has found a way to improve the performance of the transistor by using a technique to incorporate vanadium oxide, a functional oxide, into the electronic devices. The researchers knew that vanadium dioxide had an unusual property called the metal-to-insulator transition. In the metal state, electrons move freely, while in the insulator state, electrons cannot flow. This on/off transition, inherent to vanadium dioxide, is also the basis of computer logic and memory.

"It's tough to replace current transistor technology because semiconductors do such a fantastic job," said Roman Engel-Herbert, assistant professor of materials science and engineering, Penn State. "But there are some materials, like vanadium oxide, that you can add to existing devices to make them perform even better."

Crystal structures in VO2

A schematic of the crystal structures in VO2, showing the motion of the vanadium (black arrows) with respect to the oxygen ions across the metal-insulator transition. VO2 acts like an insulator at low temperatures but like a metal at near room temperature. Image: Lawrence Berkeley National Laboratory

The researchers thought that if they could add vanadium oxide close to a device's transistor it could boost the transistor's performance. Also, by adding it to the memory cell, it could improve the stability and energy efficiency to read, write and maintain the information state. The major challenge they faced was that vanadium dioxide of sufficiently high quality had never been grown in a thin film form on the scale required to be of use to industry, the wafer scale.

Although vanadium dioxide, the targeted compound, looks simple, it is very difficult to synthesise. In order to create a sharp metal-to-insulator transition, the ratio of vanadium to oxygen needs to be precisely controlled. When the ratio is exactly right, the material will show more than four orders-of-magnitude change in resistance, enough for a sufficiently strong on/off response.

The Penn State team reports in the online journal Nature Communications that they are the first to achieve growth of thin films of vanadium dioxide on 3in sapphire wafers with a perfect one to two ratio of vanadium to oxygen across the entire wafer. The material can be used to make hybrid field effect transistors, called hyper-FETs, which could lead to more energy efficient transistors. Earlier this year, also in Nature Communications, a research group led by Suman Datta, professor of electrical and electronic engineering, Penn State, showed that the addition of vanadium dioxide provided steep and reversible switching at room temperature, reducing the effects of self-heating and lowering the energy requirements of the transistor.

The implementation of vanadium dioxide can also benefit existing memory technologies, a quest that Penn State researchers are actively pursuing.

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