Diamond is considered as the ultimate semiconductor for power electronics applications. Even though diamond semiconductor devices have been realised worldwide, they are still prototypes or proof of concept devices. It is however necessary to understand how they operate for full use of their benefits in power converters. In this thesis, we focused the analysis on pseudo-vertical diamond Schottky diodes. Firstly, static and switching characterisations were achieved. They allowed us to extract device characteristics to integrate them in power converters and analyse their switching abilities. Management of diodes in power converters was then studied. These studies allowed us to propose device structure modifications to improve diode performance and their integration in power converters. Finally, a theoretical analysis of a diamond Schottky diode performance in a power converter was performed, with comparison to the performances of a SiC Schottky diode. Highlights include the specificities of diamond devices and the benefits they might bring to power electronics applications.

Gaetan Perez will be defending his thesis Monday 9th July at 10am in the amphithéâtre Bergès (RDC), Bâtiment GreEn-ER, 21 avenue des Martyrs, 38031 Grenoble. 

Following proof of concept of diamond power transistors, the GreenDiamond European collaboration is investigating for the first time reliability issues. Results obtained by 6 partners of the consortium are very encouraging. See our latest IEEE paper!

Due to the novel nature of the diamond-based devices under development, there is currently no standard package identified for use with this semiconductor. The main focus of the GreenDiamond WP7 task is the analysis of advanced materials suitable for developing the required packages. WP7 aimed also to design and develop high quality/cost-effective solutions for the study and optimization of high temperature (300ºC) and high blocking voltage (15kV) packages that are adapted to diamond-based semiconductor devices.

To achieve this objective, a fixed package topology based on the SOT-227 industry standard was chosen. The reasons are that the SOT-227 structure allows the packaging of high power single die devices and that it shows the same constitutive elements as multi-chip high-power modules. On the lower part of the proposed package structure, there is a metallic baseplate providing mechanical support, thermal conduction and heat spreading capability towards the final heat extraction system (heatsink, cold-plate, etc.). Pure Cu has been selected as the reference baseplate material, but more advanced options with best matched CTE values have been assessed such as CuMo alloy and Cu-diamond MMC (metal matrix composite). On top of the baseplate, a ceramic substrate provides thermal conduction and electrical isolation between the active device and the external circuit frame. Here, AlN and Si3N4 DCB (Direct Copper Bonded) substrates have been considered, showing good thermal properties and relatively low CTE values. On top of the Cu pads and tracks of the ceramic substrate, the active device is fixed with a die-attach layer. In this case, AuGe solder alloy (~360ºC melting point) and sintered Ag particles have been considered. Particular attention was paid to the Ag sintering solution, analyzing the effect of the processing conditions (pressure, temperature, time) on the final die-attach layer properties. Finally, the semiconductor die and its topside interconnections (wire-bonding) are protected with an encapsulant. This element shows the main limitations in terms of maximum working temperature and three silicone-based references have been analyzed for their use in SOT-227 packages in the range of the 250ºC operating temperature.

UCL hosted the latest GreenDiamond General Assembly, as the project nears its second reporting period. The meeting was productive and full of enthusiam as the first diamond MOSFET chips progress through to advanced fabrication stages! Stay tuned for upcoming results and publications.

The General Assembly wasn’t all work either. The consortium had the opportunity to meet the infamous founding father of UCL, Jeremy Bentham, discover the Grant Museum and enjoy a fabulous meal and music at Sarastro in the West End. Some of the best things on offer in London!

Two partners of the GreenDiamond project, Institut Néel and G2ELab, have published a paper about comprehensive electrical analysis of metal/Al2O3/O-terminated diamond. This paper gives a full picture of the different mechanisms involved in the gate oxide leakage current and Fermi level pinning in diamond MOSCAP. This basic understanding was crucial to reach one of the main objective of the GreenDiamond project: the achievement of reliable and robust gate oxide for diamond MOSFET.

The paper is available in Applied Physics Letters October 26th 2017 (DOI: 10.1063/1.4997975).

GreenDiamond partners from Belgium, France and the UK attended the 4th French-Japanese Workshop on Diamond Devices, held at OIST in Okinawa, Japan (30th October – 1st November 2017). This was an opportunity to discuss challenges and advances in the diamond power device field with colleagues from across the world.

Thank you to Stoffel Janssens (OIST), Satoshi Koizumi (NIMS) and Julien Pernot (Institut Néel) for organising!

Attendees of the annual Warwick De Beers Diamond conference were the lucky audience to a talk by Alex Pakpour-Tabrizi, GreenDiamond partner from UCL!

Members of the GreenDiamond team have travelled to the other side of the world to present some of the work done by the consortium!

At the international New Diamond and Nano Carbons conference in Cairns (Australia), Nicolas Rouget (CNRS) gave an invited talk entitled Diamond Devices for Power Electronics and Shannon Nicley (IMEC)  and Verena Zurbig (IAF) both presented their latest work optimising the growth parameters of n-type diamond for improved electronic and crystalline quality, on of the key development milestones on the roadmap to the full exploitation of diamond’s exceptional properties for power electronics.

Another six months have flown by and the time for the fourth GreenDiamond General Assembly has come!

We were so busy working that we forgot to take a group photo, but here is a small portion of the consortium!

All the partners were present, including our new industrial partner Indra, replacing WinInertia. Updates were detailed to the consortium for each work package and upcoming deliverables. The project is now almost half way through and still full of energy!

Thank you to Richard Seddon and Isabel Obieta for hosting us at Tecnalia  in beautiful San Sebastian.

And on a more creative note, our very own GreenDiamond poet, Loto Oluwasayo (PhD student, Institut Néel, France) has written a short piece about his work:

MY ECHANTILLON*

My echantillon, I waited long for you to come.
When you came , my joy was to the fullest
You needed a dip to cleanse off your dirt
You enjoyed the warmth/hot bath
And with a nozzle, you were properly dried

My echantillon, in that small cylinder you rest so safe
But you needed a heat in tube to glow for value
You were changed and you never remain the same
I cherish you and to lose you I cannot imagine.

My echantillon, small but valuable
Yellowish, bluish or transparent you could be
Men long to have you but for your price.
I found you and will not let you go.

*échantillon means sample in French.