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Could Silicon Replace GaAs In Gunn Diodes?

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Silicon breakthrough could make microwave technology much cheaper and better, say researchers at the University of Waterloo

Researchers at the University of Waterloo in Canada have found a new way to generate microwaves using stretched silicon nanowires, a breakthrough that could dramatically cut costs and improve devices such as sensors in self-driving vehicles, they say.

"Until now, this was considered impossible," said C.R. Selvakumar, an engineering professor at the University of Waterloo who proposed the concept several years ago.

Microwaves are typically generated using Gunn diodes made from GaAs. When voltage is applied to GaAs and then increased, the electrical current running through it also increases "“ but only to a certain point. Beyond that point, the current decreases, This is known as the Gunn Effect that results in the emission of microwaves. The Effect emanates from the transfer of electrons between two different energy subbands. GaAs has a direct bandgap of very low effective mass and an indirect subband of high effective mass which lies ~300"‰meV above the former.

Lead researcher Daryoush Shiri, a former Waterloo doctoral student who now works at Chalmers University of Technology in Sweden, used computational nanotechnology to show that the same effect could be achieved with silicon.

The computer models showed that if silicon nanowires with diameter of 3.1nm were stretched with 3 percent strain as a voltage of 5000"‰V/cm was applied to them, the Gunn Effect, and therefore the emission of microwaves, could be induced.

"With the advent of new nano-fabrication methods, it is now easy to shape bulk silicon into nanowire forms and use it for this purpose," said Shiri.

The stretching mechanism could also act as a switch to turn the effect on and off, or vary the frequency of microwaves for a host of new applications that haven't even been imagined yet.

"This is only the beginning," said Selvakumar, a professor of electrical and computer engineering. "Now we will see where it goes, how it will ramify."

Shiri also collaborated with researchers Amit Verma, Reza Nekovei, Andreas Isacsson and M.P. Anantram at universities in the United States and Sweden.

'Gunn-Hilsum Effect in Mechanically Strained Silicon Nanowires: Tunable Negative Differential Resistance' by Daryoush Shiri et al; Scientific Reports volume 8, Article number: 6273 (2018)

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