Gallium Oxide-based diode better than SiC diodes

Flosfia Inc developed a schottky-barrier diode (SBD) with a withstand voltage of 531V and an on-resistance of 0.1mΩcm² by using gallium oxide.

The on-resistance is lower than that of commercially-available SiC SBDs, according to Flosfia, a venture firm based in Kyoto. Based on the latest results, the company will ship samples of a product that uses the TO-220 package, which is commonly used for power devices, and has a withstand voltage of 600V at the end of 2015, planning to start volume production by 2018.

Gallium oxide is drawing attention as a material that potentially enables to make higher-withstand-volage, lower-loss power semiconductor devices at a low cost than silicon carbide (SiC) and GaN (gallium nitride), which are being developed as next-generation power semiconductor materials.

Specifically, Flosfia deals with an “α-type” gallium oxide having a structure called “corundum.” It makes an α-type gallium oxide by using the “mist epitaxy” method.

The method was developed by combining the “mist CVD method” developed by Shizuo Fujita, professor at Kyoto University, and the company’s own technologies including a technology to reduce impurity concentration and multi-layer technology. The “mist epitaxy” method eliminates the need for an expensive vacuum unit.

Thickness of foundation layer reduced to lower on-resistance

The new SBD consists mainly of two gallium oxide layers: the “foundation material (layer)” on the lower side and the “semiconductor layer” on the upper side. The foundation layer was formed as a film with a thickness of 20μm or less to lower on-resistance. The resistance of this gallium oxide film can be reduced to 1/100 that of a commercially-available SiC substrate, Flosfia said.

According to Flosfia, gallium oxide has a low thermal conductivity. But the thickness of 20μm or less contributed to lowering thermal resistance.

This time, the foundation and semiconductor layers were formed on a sapphire substrate by using the mist epitaxy method, and then, the sapphire substrate was removed. After that, processes such as etching and metalization were applied to make an SBD.

In addition to the SBD with a withstand voltage of 531V, Flosfia prototyped an SBD having a withstand voltage of 855V and an on-resistance of 0.4mΩcm². The company plans to announce the details of the new SBD at IWGO, an international conference on gallium oxide, which will take place in November 2015.