Power/Performance Bits: Jan. 7

Ferroelectric FET
Researchers at Purdue University developed a ferroelectric transistor capable of both processing and storing information.

The ferroelectric semiconductor field-effect transistor is made of alpha indium selenide, which overcomes the problem of ferroelectric materials not interfacing well with silicon. “We used a semiconductor that has ferroelectric properties. This way two materials become one material, and you don’t have to worry about the interface issues,” said Peide Ye, a professor of Electrical and Computer Engineering at Purdue.

Researchers have created a more feasible way to combine transistors and memory on a chip, potentially bringing faster computing. (Purdue University photo/Vincent Walter)

Alpha indium selenide also has a much smaller band gap than typical ferroelectric materials, making it possible for the material to be a semiconductor without losing ferroelectric properties. It can be 10 nanometers thick without compromising the current flow. In tests, its performance was comparable to existing ferroelectric field-effect transistors, and could exceed them with more optimization, the team said.

With researchers at the Georgia Institute of Technology, they built alpha indium selenide into a space on a chip, called a ferroelectric tunneling junction, which could further improve its capabilities.

GaN thermal properties
Researchers at the University of Illinois propose a way to predict the thermal properties of gallium nitride semiconductors made with four different fabrication methods. The team hopes this will help chip manufacturers find ways to better diffuse the heat that leads to device damage and decreased device lifespans.

The team tested the thermal conductivity of gallium nitride grown using the four most technologically important fabrication techniques: hydride vapor phase epitaxy, high nitride pressure, vapor deposition on sapphire and vapor deposition on silicon. They measured thermal conductivity, defect density and the concentration of impurities of each material.

“The composition and atomic structure of the surface used to grow the crystals influences the number of defects in the final product,” said Can Bayram, an electrical and computer engineering professor at Illinois. “For example, crystals grown on silicon surfaces produce a semiconductor with many defects – resulting in lower thermal conductivity and hotter hotspots – because the atomic structures of silicon and gallium nitride are very different.”

The team found that silicon – the most economical of all of the surfaces use to grow gallium nitride – produces crystals with the highest defect density of the four popular fabrication methods. Deposition on sapphire makes a better crystal with higher thermal conductivity and lower defect density, but this method is not nearly as economical. The hydride vapor epitaxy and high nitride pressure techniques produce superior products in terms of thermal properties and defect density, but the processes are very expensive, Bayram said.

“Using our new data, we were able to develop a model that describes how defects affect the thermal properties of gallium nitride semiconductors,” Bayram said. “This model provides a means to estimate the thermal conductivity of samples indirectly using defect data, which is easier than directly measuring the thermal conductivity.”

“We are trying to create a higher efficiency system so that we can get more out of our devices – maybe one that can last 50 years instead of five,” Bayram added. “Understanding how heat dissipates will allow us to reengineer systems to be more resilient to hotspots.”

Thermoelectric perovskites
Researchers at Queen Mary University of London, University College London, and Consiglio Nazionale delle Ricerche investigated using halide perovskites as thermoelectric materials for converting heat energy into electricity.

The team focused their research on thin films of the halide perovskite caesium tin iodide to test its ability to create electrical current from heat. The researchers found they were able to improve the materials’ thermoelectric properties through a combination of methods, which involved partial oxidation and the introduction of additional elements into the material.

“For many years halide perovskites have been suggested as promising thermoelectric materials. But whilst simulations have suggested good thermoelectric properties real experimental data hasn’t met these expectations,” said Oliver Fenwick, lead Royal Society University Research Fellow and Lecturer in Materials Science at Queen Mary University of London.

“In this study, we successfully used ‘doping’ techniques, where we intentionally introduce impurities into the material, to tweak and improve the thermoelectric properties of caesium tin iodide, opening up options for its use in thermoelectric applications.”

Halide perovskites could be cheaper than other common thermoelectric materials, too.

“The thermoelectric materials we currently have are expensive, and some even contain toxic components,” Fenwick added. “One of the largest growth areas for thermoelectric technology is for domestic, commercial or wearable applications, so there’s a need to find cheaper, non-toxic materials that can also operate well at low temperatures, for these applications to be fully realized. Our research suggests the halide perovskites could, with some fine-tuning, fill this void.”