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New method for making higher voltage GaN transistors


Transmorphic epitaxial growth of AlN nucleation layers on SiC substrates for high-breakdown thin GaN transistors

A new method to fit together nanometer thin layers of semiconductors could lead to a new type of transistor for high-power electronic devices. The result was published in Applied Physics Letters by scientists at Linköping University and SweGaN, a spin-off company from materials science research at LiU.

When GaN vapour is allowed to condense onto a wafer of SiC to form a thin coating, the fit at the surface between the two crystalline materials is poor, which can lead to failure of the transistors. The commercial solution is to add an even thinner layer of aluminium nitride between the two layers.

Engineers at SweGaN noticed by chance that their transistors could cope with significantly higher field strengths than they had expected, and they could not initially understand why. They found the answer at the atomic level - in a couple of critical intermediate surfaces inside the components.

Researchers at LiU and SweGaN, led by LiU's Lars Hultman and Jun Lu, present in Applied Physics Letters an explanation of the phenomenon, and describe a method to manufacture transistors with an even greater ability to withstand high voltages.

The scientists discovered a previously unknown epitaxial growth mechanism that they have named ‘transmorphic epitaxial growth’. It causes the strain between the different layers to be gradually absorbed across a couple of layers of atoms.

This means that they can grow the two layers, GaN and AlN, on SiC in manner so as to control at the atomic level how the layers are related to each other in the material. In the laboratory they have shown that the material withstands high voltages, up to 1800 V. If such a voltage were placed across a classic silicon-based component, sparks would start flying and the transistor would be destroyed.

"We congratulate SweGaN as they start to market the invention. It shows efficient collaboration and the utilisation of research results in society. Due to the close contact we have with our previous colleagues who are now working for the company, our research rapidly has an impact also outside of the academic world", says Lars Hultman.

The research has been funded by research grants from the Knut and Alice Wallenberg Foundation and from the CoolHEMT programme, part of the EU Horizon 2020.

'Transmorphic Epitaxial Growth of AlN Nucleation Layers on SiC Substrates for High-Breakdown Thin GaN Transistors' by Jun Lu et al; Appl. Phys. Lett. 115, 221601 (2019)

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