Introducing One of the Best Thin-film Transistors Ever

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Photo: University of Alberta

Wide-band gap metal-oxide thin-film transistors (TFTs) have never been quite as popular as the ubiquitous metal-oxide semiconductor field-effect transistors (MOSFET). One of the main issues with TFTs has been that they are limited to n-type semiconductor materials that can only move negative charges through them, limiting their electrical output.

While different architectures have been investigated to overcome this, the problem has remained that there just haven’t been p-type wide-band inorganic semiconductor materials that have done the trick. The result has been that TFTs have been limited to low-power applications, such as display screens.

Now researchers at the University of Alberta in Canada say they have come up with a design that will take nearly any wide-band metal-oxide n-type semiconductor used in thin-film transistors and create a p-type channel, or inversion layer, through which positive charges can travel without the need of some new semiconductor material. As proof of their design, the Canadian researchers have created a TFT capable of conducting both electrons (negative charges) and holes (positive charges) resulting in far greater electrical output.

In research described in the journal Nature Communications, the device achieved among the highest electrical performances ever recorded for a TFT in terms of current, power and transconductance densities. The researchers claim that their new transistor has power-handling capabilities at least 10 times greater than commercially produced thin-film transistors.

“It’s actually the best performing [TFT] device of its kind—ever,” said materials engineering professor Ken Cadien, a co-author on the paper, in a press release.

The developed TFT actually is a tunneling junction transistor that behaves like a bipolar transistor.

Tunneling is the name give to when an electron passes right through a material. This occurs when the material used to keep the electrons from passing through becomes too thin, which we are beginning to see with the extreme miniaturization that now exists in MOSFET devices.

While this electron tunneling is a problem for MOSFET devices—creating leaks that make it difficult to completely start and stop the flow of electrons—it is actually a way for tunneling junction transistors (TJT) to stop and restart the flow of electrons. While the MOSFET device operates by raising or lowering the energy state to turn the flow of electrons on and off, a TJT maintains a high energy state and manages to get the on-off effect by changing the probability that the electrons will materialize on the other side of the gate. This change in probability is brought on by the TJT using the gate to control the electrical thickness of the barrier.

“Usually tunneling current is considered a bad thing in MOSFETs and it contributes to unnecessary loss of power, which manifests as heat,” explained Gem Shoute, lead author on the article, in press release. “What we’ve done is build a transistor that considers tunneling current a benefit.”

As with all transistors, this latest design has three electrical contacts: a source, a drain and a gate. And between the gate and the semiconductor material sits an oxide layer. In terms of materials to fill out these components, the Canadian researchers have used the intrinsically n-type semiconductor zinc oxide and deposited it on a p-type silicon (p-Si) wafer. A thin high-k oxide  was used as the oxide layer at the gate. This oxide layer has a large enough electrical field to effectively control the electrical thickness of the barrier, creating a tunneling junction transistor design.

The researchers have filed a patent on their transistor and are continuing their research by trying it out in a fully flexible medium to create devices that can be applied to biomedical imaging or renewable energy.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
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