Extreme electronics

Print edition : March 06, 2015

Scanning electron microscopy image of a representative device. Photo: Journal of Applied Physics

INCREASINGLY, industries are beginning to require electronics that can operate reliably in a harsh environment, including in extreme temperatures above 200° Celsius. While conventional cooling systems can help to some extent, in some applications, cooling may not be possible. The situation could also be such that it is more convenient to operate electronic devices hot to improve system reliability or reduce cost.

A team of researchers from the University of California, Riverside, and Rensselaer Polytechnic Institute, New York, has discovered that molybdenum disulfide (MoS), a semiconductor, could be a promising material to make thin-film transistors for extreme-temperature applications. The research findings have been reported in a recent paper published in the Journal of Applied Physics.

“Our study shows that molybdenum disulfide thin-film transistors remain functional to high temperatures of at least 500 Kelvin [220° C],” said Alexander Balandin of UC-Riverside, the lead researcher. “The transistors also demonstrate stable operation after two months of aging, which suggests new applications for molybdenum disulfide thin-film transistors in extreme-temperature electronics and sensors.”

MoS can be sourced from the mineral molybdenite, which is an abundant, naturally occurring material and is commonly used as an additive in lubricants. For manufacturing flexible, thin-film transistors, MoS synthesised by chemical vapour deposition has been found to be a promising material.

MoS belongs to a family called van der Waals materials, which have characteristic layered crystal structure with atomic layers weakly bonded to each other through an interatomic force termed as “van der Waals interactions”. The weak connection between atomic sheets enables exfoliation of such materials layer by layer, similar to the process used for obtaining graphene by peeling thin sheets off chunks of graphite. The layered structure also suggests that extremely thin and high-quality layers can also be produced by chemical vapour deposition on industrial scale.

“Although devices made of conventional large-band-gap semiconductors, such as silicon carbide or gallium nitride, hold promise for extended high-temperature operation, they are still not cost-effective for high volume applications,” Balandin said.

“A single-layer MoS shows a band gap of 1.9 electronvolt, which is larger than that of silicon and gallium arsenide. This is beneficial for the proposed application.” The presence of a larger band gap means that a device can be easily switched on and off, a crucial property for a transistor’s operation.

MoS has recently attracted a lot of interest for device applications, but Balandin’s team is the first to investigate the material’s potential for high-temperature electronics.

According to Balandin, practical application of MoS transistors in control circuits or sensors at high temperatures requires operation longer than one month.

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