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First GaN vertical diode with 3000V breakdown

Using a new technique known as void-assisted separation, Hitachi Cable has developed gallium nitride power devices with low dislocation densities and a very high breakdown voltage

Hitachi Cable says it has succeeded in the trial manufacture of the world's first GaN vertical diode with a reverse breakdown voltage of 3,000V or higher and a low on-resistance of about 1mΩcm2 in the forward direction.

The firm achieved this development through joint research with Toru Nakamura Laboratory in the Research Centre for Micro-Nano Technology of Hosei University and the Hitachi Central Research Laboratory.

In recent years, public interest in power devices as a way to conserve energy has been escalating. Power devices are semiconductors such as diodes and transistors with the ability to convert and control electricity. They are used in a wide variety of fields, including consumer electronics in houses, automobiles, railroad vehicles, and power plants.

Conventional power devices have mainly used silicon. To further reduce energy consumption, however, there has been active development of power devices using new materials which exceed the performance limits of silicon.

The latest device is a GaN vertical p-n diode with an electrode diameter of 400 to 800µm in which GaN epitaxial layers are grown on Hitachi Cable's freestanding GaN substrate using MOCVD.

Appearance of GaN substrates for power devices

Thanks to the VAS (void-assisted separation) method developed by Hitachi Cable, it was possible to create a stable device with a low dislocation density of 106cm-2.

The firm also confirmed that the dislocation defects in the freestanding GaN substrate were only edge dislocations and mixed dislocations ; nowhere on the entire surface of the substrate is there a "core" area where dislocation defects are densely located.

The voltage resistant test using sample diodes confirmed a high reverse breakdown voltage of 3,000V or higher, demonstrating that the dislocation defect in the freestanding GaN substrate is not a killer defect which significantly degrades the performances of power devices. This test also demonstrated that the epitaxial layers grown on the freestanding GaN substrate have a breakdown field which is close to the ideal GaN value (3.3 - 3.8MV/cm).

The test also indicated that the on-resistance of this diode in the forward direction is about 1mΩcm2, smaller than the sum of the resistances at each layer of the diode. This means that conductivity modulation, which lowers the resistance, occurs when an electric current is injected.

Conductivity modulation is an excellent effect which can increase the performance of a device beyond its material's inherent performance limits. While it is often used in silicon power devices, it has rarely been seen in compound semiconductors, which have short carrier lifetimes.

In terms of this diode, Hitachi's theoretical analyses have suggested that the light generated at a p-n junction is re-absorbed by a diode, and that conductivity modulation, which increases carriers, occurs due to the absorption. This time, Hosei University experimentally demonstrated the light-emitting properties of diodes.

These findings demonstrated that the freestanding GaN substrate produced by the VAS method is capable of making possible power devices with a much higher performance index than with conventional materials such as silicon and SiC. High-efficiency power devices based on this development are expected to reduce the power consumed by equipment and facilities in the future.

A schematic cross-section of a sample diode created in this study

Hitachi Cable is committed to working on the sales of freestanding GaN substrates to be used in optical devices as well as to the expansion of its compound semiconductor business by putting more effort into expanding the sales of freestanding GaN substrates for power devices and GaN epitaxial wafers grown using the MOVPE method.

Hosei University is planning to present these research findings at the 73rd JSAP (The Japan Society of Applied Physics) Autumn Meeting 2012, held in the Johoku area of Ehime University and the Bunkyo Campus of Matsuyama University from Tuesday, September 11th to Friday, September 14th (lecture numbers: 12p-F2-15 and 12p-F2-16).
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