An international team of researchers has developed a low-power gas sensor chip that can operate at room temperature, making possible the development of personal air-quality monitoring devices that we could carry around with us.
In research described in the journal Science Advances, the team of researchers fabricated a chemical-sensitive field-effect transistor (CS-FET) platform based on 3.5-nanometer-thin silicon channel transistors. The platform, which is highly sensitive but consumes a small amount of power, can detect a wide range of different gases.
FET-based chip sensors are among a small group of low-power chip-scale gas sensors; the others include resistive ceramic, metallic, and nanoelectromechanical sensors. But the FET-based sensors have proven to be head and shoulders above the other approaches in terms of miniaturization, power consumption, and sensitivity.
In FET-based gas sensors, gas interacts with the amplifying region of the transistor, causing a modulation in the current of the transistor’s channel.
“The CS-FET gas sensor utilizes nanoscale silicon on an insulator as the underlying transistor, just like the fully depleted silicon on insulator (FD-SOI) technology used for low power digital electronics,” explains Ali Javey, a co-author of the paper and a professor at the University of California Berkeley, in an e–mail interview with IEEE Spectrum.
In the video below, A CS-FET equipped micro-drone shows the response of the sensor to H2, a colorless, odorless, flammable gas that is a safety concern for oil and gas industries.
FD-SOI technology operates by wrapping a gate around three sides of a 3-D channel to better control the transistor. This is accomplished by fabricating the channel so it’s very thin. This allows the gate to "fully deplete" the channel, removing all the free charges and lowering how much current leaks through the device when it's supposed to be off.
This ultrathin element of a CS-FET sensor provides enhanced sensitivity when configured into a "junction-less" transistor technology, according to Javey, who is also a faculty scientist at the Lawrence Berkeley National Laboratory. The “chemical gate,” or sensing layer, is based on ultra-thin layers of noble metals. The thin layers provide enhanced response time and recovery. Importantly, Javey notes, silicon devices by themselves do not respond to gases. But by choosing the proper chemical gate, it’s possible to obtain high selectivity.
In the CS-FET, gas interaction happens largely with the ultra-thin sensing layer (chemical gate), but “resistance” change happens in the silicon transistor underneath. This separation of the interaction and resistance changes the process and enables room temperature measurements of many reactive and toxic gases.
In traditional resistive ceramic gas sensors, the thick ceramic material can interact with a target gas by losing or gaining oxygen, leading to an increase or decrease in its resistance. This process can happen only at temperatures of 100 to 200° Celsius.
What’s more, says Javey, “This design provides highly sensitive and selective multi-gas sensing on a single chip and at low power, making it very easy to integrate in mobile consumer electronics.” That may be aided by its potential for easy manufacturability.
“This technology is based on traditional silicon and one of the biggest advantage of this is in mass production and much-needed broad area deployment of sensors,” says Javey. “Most of the techniques used in fabricating these sensors are very familiar to the silicon IC industry.”
But before these sensors get into our mobile phones to tell us if we’re in highly polluted air, some engineering still has to occur.
“Packaging design and long-term durability tests are the engineering issues that need to addressed [before the sensors are used] in personal air-quality monitoring,” says Javey. “We are currently working on long-term stability tests for the sensors in the current work, as well as expanding the scope of the CS-FET sensor platform to newer applications in personal consumer electronics as well as healthcare.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.