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Tiny microplasma transistor is able to function in extreme environments

20 March 2014

University of Utah engineers have fabricated the smallest plasma transistors that can withstand high temperatures and ionizing radiation.

A microplasma transistor is tested by applying a voltage through four electrodes touching its surface (photo: Dan Hixson, College of Engineering, University of Utah)
A microplasma transistor is tested by applying a voltage through four electrodes touching its surface (photo: Dan Hixson, College of Engineering, University of Utah)

The Defense Advanced Research Projects Agency (DARPA) funded study was conducted by University of Utah's Professor Massood Tabib-Azar and doctoral student Pradeep Pai. A paper describing the project appears online Thursday, March 20 in the journal IEEE Electron Device Letters.

"These plasma-based electronics can be used to control and guide robots to conduct tasks inside a nuclear reactor," says Professor Tabib-Azar. "Microplasma transistors in a circuit can also control nuclear reactors if something goes wrong, and also could work in the event of nuclear attack."

Plasma-based transistors, which use charged gases or plasma to conduct electricity at extremely high temperatures, are employed currently in light sources, medical instruments and certain displays under direct sunlight. These microscale devices are about 500 microns long, and operate at more than 300V.

The new devices designed by the University of Utah engineers are the smallest microscale plasma transistors to date. They measure 1 micron to 6 microns in length, or as much as 500 times smaller than current state-of-the-art microplasma devices, and operate at one-sixth the voltage.

They can also operate at temperatures up to 1,450 degrees Fahrenheit. Since nuclear radiation ionizes gases into plasma, this extreme environment makes it easier for plasma devices to operate

"Plasmas are great for extreme environments because they are based on gases such as helium, argon and neon that can withstand high temperatures," says Tabib-Azar. "This transistor has the potential to start a new class of electronic devices that are happy to work in a nuclear environment."

A conventional transistor is made with two active layers, one on top of the other. Electricity flows through one of the layers, called the channel. The other layer, called the gate, controls current flowing in the channel. If sufficient voltage is applied to the gate, the transistor turns on.

For the new study, Tabib-Azar and Pai deposited layers of a metal alloy to form the gate on a 4in glass wafer. A layer of silicon was then deposited on top of the gate.

Unlike typical transistors, the Utah microplasma transistor 'channel' is an air gap that conducts ions and electrons from the plasma once a voltage is applied. To achieve this design, the team etched away portions of the silicon film using a chemically reactive gas. This etching process leaves behind cavities and empty spaces to form the transistor's channel and expose the gate underneath. The channel tested in this new study was 2 microns wide and 10 microns long, and helium was used as the plasma source.

"Although the length scales are much smaller here, we came up with an innovative way to make these structures three-dimensional," Tabib-Azar says. "We are currently connecting these devices to form logic gates and computing circuits that we will test in our experimental nuclear reactor at the University of Utah."

Traditional MOSFETs require metal to connect circuits, but the Utah microplasma devices will use a plasma-based connection to enable communication. As a result, these circuits will only be operational when powered up and will disappear otherwise, making them suitable for defence applications.

These plasma devices could also be used as an X-ray imaging source in the next five years, according yo Tabib-Azar. Because the device dimensions are so small, X-ray images from a wounded soldier in the field could be collected on a smartphone equipped with transistors that also generate the X-rays.

Other applications might be the detection and identification of aerosol pollutants based on the colour emitted when the substance passes through the device. Such chemical sensing devices could be used to monitor air quality in real time and enable researchers to construct an accurate air-quality map.

In the nearer-term, these new transistors might be used to generate X-rays to draw fine lines in silicon to pattern microscale devices for the electronics industry. The technique allows you to do the same thing you would with laser printing, according to Tabib-Azar, but instead you can use these tiny X-ray sources to print on a silicon wafer. This gives engineers the ability to do X-ray lithography without having to use very heavy lenses and X-ray beam shaping devices.

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