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GaN transistors use single crystal of diamond as substrate

Technology News |
By Nick Flaherty

The company claims the direct bonding of a multi-cell GaN-HEMT to a single-crystal diamond substrate is a world’s first, improving the power efficiency of high-power amplifiers in mobile communication base stations and satellite communications systems.

GaN-on-Diamond HEMT view from above and cell structure.

Mitsubishi Electric aims to refine the GaN-on-Diamond HEMT prior to its commercial launch sometime around 2025. The company handled the design, manufacture, evaluation and analysis of the GaN-on-Diamond HEMT and AIST developed the direct bonding technology. A part of this achievement is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Today, most existing GaN-HEMTs that use a diamond substrate for heat dissipation are created using a GaN epitaxial layer foil from which silicon substrate has been removed and onto which diamond is deposited at high temperature.

HEMTs are then fabricated on the diamond substrate of the flattened GaN wafer. However, because the thermal expansion coefficients of GaN and diamond are different, the wafer can warp during the manufacturing process, making it difficult to fabricate large multi-cell GaN-HEMTs.


Cross-sectional view of new GaN-on-Diamond HEMT.

During this research a silicon substrate was removed from a multi-cell GaN-HEMT that was fabricated with a silicon substrate; the back surface of the GaN-HEMT was then polished to make it thinner and flatter, after which it was bonded directly onto a diamond substrate using a nano adhesion layer. A multi-cell structure was used for the parallel alignment of eight transistor cells of a type found in actual products. This enabled the fabrication of a multi-cell GaN-on-Diamond HEMT using a single-crystal diamond as the substrate, with high heat dissipation (thermal conductivity of 1,900 W/mK).

This superior heat dissipation suppresses temperature degradation, decreasing the temperature rise of the GaN-HEMT from 211.1ºC to 35.7ºC during operation. This improves output per gate width from 2.8 W/mm to 3.1 W/mm as well as raising power efficiency from 55.6 percent to 65.2 percent, report the researchers, achieving significant energy savings.

Mitsubishi Electric – www.MitsubishiElectric.com


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