Microplasma transistors for extreme environments, like nuclear reactors

March 21, 2014

University of Utah electrical engineers test a microplasma transistor by applying a voltage through four electrodes touching the surface of the transistor. Plasma is a charged gas that conducts electricity, seen here as a pink glowing light. Unlike typical transistors, the Utah microplasma transistor uses an air gap that conducts ions and electrons from a helium plasma once a voltage is applied. The new devices are designed to withstand ionizing radiation. (Credit: Dan Hixson, College of Engineering, University of Utah)

University of Utah electrical engineers fabricated the smallest plasma transistors that can withstand the high temperatures and ionizing radiation found in a nuclear reactor.

Such transistors someday might enable smartphones that take and collect medical X-rays on a battlefield, and devices to measure air quality in real time.

“These plasma-based electronics can be used to control and guide robots to conduct tasks inside the nuclear reactor,” says Massood Tabib-Azar, a professor of electrical and computer engineering.

“Microplasma transistors in a circuit can also control nuclear reactors if something goes wrong, and also could work in the event of nuclear attack.”

The most commonly used type of transistor is called a metal oxide semiconductor field effect transistor, or MOSFET.

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 (but not plasma TVs, which are different). These microscale devices are about 500 microns long, or roughly the width of five human hairs. They operate at more than 300 volts, requiring special high-voltage sources. Standard electrical outlets in the United States operate at 110 volts.

The new devices designed by the University of Utah engineers are the smallest such 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 also can 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.”

Creating a microplasma transistor

Schematically show perspective and cross-sectional views
of a microplasma field effect transistor (credit: Pradeep Pai and Massood Tabib-Aza/IEEE Electron Device Letters)

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 4-inch glass wafer. A layer of silicon then was 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 unique 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.

In another five years, the devices could be used to detect and identify aerosol pollutants based on the color emitted when the substance passes through the device. “These chemical sensing devices could be used to quantitatively monitor air quality in real time and enable researchers to construct an accurate air-quality map.”

In the nearer-term, these new transistors could be used to generate X-rays to draw fine lines in silicon to pattern microscale devices for the electronics industry without having to use very heavy lenses and X-ray beam shaping devices, he adds.

A study of the new transistor appeared online March 20 in the journal IEEE Electron Device Letters. The study was funded by the Defense Advanced Research Projects Agency.

Abstract of IEEE Electron Device Letters paper

This letter presents the smallest microplasma field effect transistor (MOPFET) reported to date. The MOPFET has a gaseous (atmospheric pressure He) channel and operates in the sub-Paschen breakdown regime, where the channel breakdown voltage depends (nearly) linearly on the channel length. The gate field effect is explained by noting that the channel ionization depends on the primary electron density that is controlled by both VDS and VG; negative VG increased the channel electron density lowering the channel breakdown voltage (VDS-B), whereas positive VG attracted the channel electrons and reduced their density for ionization in the channel increasing the VDS-B. A simple empirical model using Townsend breakdown criteria is developed to include the effect of the gate electric field in VDS-B.