From European Space Agency: “ESA probing into post-silicon, wide bandgap landscape”

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From European Space Agency

13 February 2019
Crystal of gallium nitride

What comes next after silicon? Some seven decades from the invention of the silicon chip this question is often asked, especially in the space sector: demands for extreme high performance have highlighted silicon’s inherent limitations, raising interest in a new range of ‘wide bandgap’ semiconductor materials.

A material’s ‘bandgap’ is the space between atomic shell layers that dictates the amount of energy needed to get electrons moving and make that material conductive. But electrons can also be shifted into the conduction band by heat; so, for instance, silicon-based semiconductors become unusable for temperatures from around 180°C and up.

Wide bandgap materials such as gallium nitride, silicon carbide and diamond can attain much higher operating temperatures, potentially enabling much higher current density designs running at higher voltages. Electrons also move faster through them, delivering faster device speeds. And as an added benefit they are robust against the effects of radiation – a particular issue in space.

GaN on silicon carbide wafers, produced as part of ESA’s GREAT2 project

ESA recognised the potential of the wide bandgap realm for space at an early stage, founding the ‘GaN Reliability Enhancement and Technology Transfer Initiative’ (GREAT2) in 2008 for gallium nitride (GaN) microwave devices, at a time when it was used mainly for high-performance LEDs and the lasers of Blu-ray players.

Leading research institutes were brought together with manufacturers to set up an independent European supply chain to manufacture high-quality GaN radio frequency devices for space applications.

GaN wafer quality check

“The promise of these materials makes them highly strategic,” explains Andrew Barnes, heading GREAT2. “We need independent European supply chains because if we were totally dependent on a foreign source that became subject to export restrictions then the whole industrial sector would be compromised, along with our competitiveness.”

The initial focus of GREAT2 was on GaN-based microwave power transistors and integrated circuits, as building blocks for high-performance solid-state power amplifiers, to bolster the competitiveness of the European telecommunications industry – the single largest and most commercial space sector.

ESA Biomass mission depiction

“These same power transistors shall in the near future be offered on the open market after completion of a project funded by ESA’s European Component Initiative, which is the first time we’ve achieved that,” adds Andrew. “Such power transistors are also being investigated for use in the next-decade Galileo Second Generation.

“At the same time we’re in the process of qualifying some of the foundry processes used on a more systematic basis, following European Cooperation for Space Standardization rules. We’re also seeking to develop additional standards in terms of screening and testing and insertion into space systems – this is essential for wider industrial take-up of the technology, and we’re making steady progress.”

GaN Single Chip Front End. Natanael Ayllon, ESA payload engineer, showing a prototype transmit/receive module on a single gallium nitride chip.

GREAT2’s focus has now moved to high voltage power converters and amplifiers for higher frequencies, operating at millimetre wavelength. The Ninth Wide Bandgap Workshop hosted at ESA’s Harwell centre in the UK took stock of the general progress made and looked ahead to the next steps towards the wide bandgap future.

“This includes improving production quality of GaN ‘epitaxy’ – meaning crystal growth – and moving to higher frequency foundry processes for space ready parts, utilizing 100 nanometre gate lengths or lower for GaN,” explains Andrew. Work is also ongoing to test the space radiation robustness of GaN power transistors for DC-DC convertor applications.

Diamond Light Source, located at the Harwell Science and Innovation Campus in Oxfordshire U.K.

ESA ECSAT-The European Centre for Space Applications and Telecommunications (ECSAT) is ESA’s facility in the United Kingdom. It is based at the Harwell Campus in Oxfordshire

“We’ve also worked to boost their general robustness. We found, for instance, that some of the passive components used on GaN monolithic microwave integrated circuit parts weren’t performing so well in radiation testing as expected, and this came down to the legacy of their design, originally these capacitor designs were based on legacy parts built in gallium arsenide. By tuning design parameters, i.e. adjusting the dielectric layer stack, we have now achieved a big improvement.”

Other wide bandgap materials such as diamond are also under investigation, in their own right and also as a ‘substrate’ backing material for GaN devices to give improved thermal performance.

Laser alignment for heavy ion radiation testing of GaN power devices at IMEC by ON Semiconductor. ESA

Andrew comments: “Diamond has a similar crystalline structure to GaN, and excellent thermal conductivity properties. The University of Bristol presented work done in integrating GaN with diamond. The result is a fivefold increase in heat dissipation, allowing even higher power densities – diamond is a GaN’s best friend.”

The workshop also peered further into the future, with discussions looking forward to GaN-based optoelectronic devices, diamond-based integrated circuits and electric thrusters and the 3D printing of wide bandgap materials, as a means of revolutionising device manufacturing.

25mm diameter diamond substrates growing in 100mm chemical vapor deposition reactor. University of Bristol

See the full article here .

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The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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