Transistors Image
An important part of the work has been conducted on one of the world ́s most outstanding transmission electron microscopes, Arwen, at Linköping University. (Image: Magnus Johansson)

Scientists at Linköping University (Linköping, Sweden) have described a method to manufacture transistors using gallium nitride and aluminum nitride that have the ability to withstand voltages as high as 1800 volts.

Gallium nitride is a semiconductor used for efficient light-emitting diodes. It may, however, also be useful in other applications, such as transistors, since it can withstand higher temperatures and current strengths than many other semiconductors. These are important properties for future electronic components, not least for those used in electric vehicles.

Gallium nitride vapor is allowed to condense onto a wafer of silicon carbide, forming a thin coating. The method in which one crystalline material is grown on a substrate of another is known as “epitaxy.” The method is often used in the semiconductor industry since it provides great freedom in determining both the crystal structure and the chemical composition of the nanometer film formed. The combination of gallium nitride and silicon carbide, (both of which can withstand strong electric fields), ensures that the semiconductors are suitable for applications in which high powers are needed.

The fit at the surface between the two crystalline materials, gallium nitride and silicon carbide, is, however, poor. The atoms end up mismatched with each other, which leads to failure of the transistor. This has been addressed by research, which subsequently led to a commercial solution, in which an even thinner layer of aluminum nitride was placed between the two layers.

The engineers noticed by chance that their transistors could cope with significantly higher field strengths than they had expected, and they could not initially understand why. The answer can be found at the atomic level — in a couple of critical intermediate surfaces inside the components. They discovered a previously unknown epitaxial growth mechanism that they 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, gallium nitride and aluminum nitride, on silicon carbide 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 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.