Some seven decades after the invention of the silicon chip, the question is often asked, what comes next? 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.
Crystal of gallium nitride
Wide bandgap materials such as gallium nitride (GaN), silicon carbide and diamond can attain much higher operating temperatures than silicon, which becomes unusable at 180°C. This potentially enables much higher current density designs, running at higher voltages. Electrons also move faster through them, delivering faster device speeds. As an added benefit they are robust against the effects of radiation, a particular issue in space.
The European Space Agency (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 GaN microwave devices, at a time when it was used mainly for high-performance LEDs and the lasers of Blu-ray players.
GREAT2
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.
“The promise of these materials makes them highly strategic,” explains Andrew Barnes, who heads 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.”
Prototype GaN amplifier
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.
“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 Barnes.
GREAT2 has now moved attention to high voltage power converters and amplifiers for higher frequencies, operating at millimetre wavelength.
The future of wide bandgap materials
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.
Commenting on the future of wide bandgap, Barnes expanded: “This includes improving production quality of GaN ‘epitaxy’ (meaning crystal growth) and moving to higher frequency foundry processes for space ready parts, utilising 100nm gate lengths or lower for GaN.”
Work is also ongoing to test the space radiation robustness of GaN power transistors for DC-DC converter applications.
Diamond
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.
Barnes 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.
Diamond substrates
