In the energy sector, engineers and designers play a crucial role in making green technologies cheaper and more attractive to consumers. With the UK government legally committed to reaching net zero by 2050, efficiency improvements in the energy sector will need to go hand-in-hand with transformative changes to the consumer’s experience.

For example, imagine charging an electric car battery to 80% of its capacity in five minutes – not much longer than it takes to fill a petrol tank. Much of the range anxiety of a long journey would disappear, along with many of the issues with kerbside charging. This would clearly make electric vehicles (EVs) much more attractive to consumers. Achieving five-minute car charging will require the development of new technology not just for batteries but also chargers. In the UK, to achieve net zero in electricity production will require increased use of renewable sources such as offshore wind farms. Using high voltage DC (HVDC) transmission from turbine arrays to shore would reduce both transmission losses and capital costs. But to use HVDC, the power needs to be stepped up and rectified at the offshore end, and subsequently converted to AC for input into the National Grid.

Current power semiconductor electronics cannot easily perform these transformations at the voltage and current ratings required. Grid voltages must then be stepped down to be distributed throughout neighbourhoods and then our homes. For electric vehicle charging and similar applications, there is at least one more stage with a rectifier to convert AC back to DC, and then further losses in the electronics which regulate battery power output.

Over multiple stages of power conversion and transmission, these small losses add up. For example, if each device is 97% efficient, ie ‘only’ 3% of the energy is lost, when the power has gone through five devices, the efficiency is roughly 86%, so there is a significant energy loss of >14%. If we can increase the efficiency of power conversion, we can substantially reduce the requirement for electricity (green or otherwise) to be generated in the first place. Large substations which use traditional transformers will continue to be important, but the future lies in super-efficient semiconductor power conversion at both the grid level and in our homes.

Have silicon semiconductors reached the end of the road?   
Traditional power electronics are based on the semiconducting properties of silicon. However, as the capacity requirements of these power devices increase, silicon is no longer the ideal material. As the operational voltage increases, to prevent device failure the devices must have thicker and thicker layers of silicon.

Band gap refers to an electric property of a semiconductor linked to its conductivity. Wide band gap (WBG) ‘compound’ semiconductors, ie, those based on semiconducting compounds rather than one chemical element, are the ideal materials for many of these applications. Devices made from WBG materials are already commercially available and being used in our daily lives: the power applications of silicon carbide (SiC) took off when it was used in the first Tesla electric cars, while gallium nitride (GaN) is replacing silicon in fast mobile phone and laptop chargers.

The wider band gaps of SiC and GaN (2.3–3.3 eV and 3.4 eV respectively, compared to 1.12 eV for silicon) mean that, for the same voltage, the semiconductor material in a WBG device can be thinner than in a silicon device. Thinner devices have lower conduction and switching losses, which enable smaller size and greater efficiency. WBG semiconductors can also operate at higher temperatures than silicon. There are fewer temperature-related failures (a major cause of failure in semiconductor devices) so devices can achieve higher reliability and longer lifetime, which also means less e-waste to deal with.

Once any power semiconductor device has been produced it must be ‘packaged’ to turn it from an unprotected semiconductor chip into a robust, practical module which can be integrated into a wider system. Unfortunately, the packaging methods used for silicon power semiconductors cannot be reused for WBG devices.

For example, the ability of advanced WBG semiconductors to operate at higher temperatures means alternative housing and cooling techniques can be explored. WBG materials also open the possibility of placing power electronics in extreme environments, such as under high radiation loads during nuclear decommissioning, where they cannot currently operate, but will also require specialist packaging.

If we can increase the efficiency of power conversion, we can substantially reduce the requirement for electricity (green or otherwise) to be generated in the first place. Large substations which use traditional transformers will continue to be important, but the future lies in super-efficient semiconductor power conversion at both the grid level and in our homes.

How are new semiconductors developed?  
As the climate crisis worsens and consumers demand ever higher performance, the speed of commercialisation for new power electronics is paramount. So far, the development cycles of WBG devices have been much shorter than those of their silicon predecessors. This is mainly due to our improved experience of semiconductor power systems and the lessons learned during the development of silicon devices. Companies and governments now recognise that collaboration between industry and universities is needed to ensure that new semiconductor devices can be commercialised as quickly as possible.

The REWIRE Innovation and Knowledge Centre (IKC), led by Professor Martin Kuball and the author, is a prime example of this type of collaboration. In April 2024, the UK government funded REWIRE to the tune of £11mn. The investment is based across the universities of Bristol, Warwick and Cambridge, and the consortium is working closely with industry in the UK and globally to pioneer the next generation of power semiconductor technologies and devices.

The next generation   
Promising materials with even wider band gaps than SiC and GaN are already being developed. For example, here at Bristol the Kuball group is working on gallium oxide (Ga₂O₃).

According to Kuball: ‘Ga₂O₃ has a band gap of 4.8 eV, even larger than that of GaN and SiC, and has the advantage over many traditional WBG materials of it being easier and cheaper to manufacture into devices. Ga₂O₃ also has a very high critical electric field strength, making it ideal for high voltage devices. However, there are intrinsic issues such as its low thermal conductivity, much lower than that of silicon, which complicate its application. However, new commercial products are being developed.’

The group aims to produce the first fully packaged Ga₂O₃ power semiconductor device, demonstrating a ‘world-record’ 4 kV diode-on-a-chip. A key step in the production of the device will be to increase the ‘breakdown voltage’ towards the material’s theoretical limit. The breakdown voltage is the minimum voltage at which a device will fail, ie no longer behaving like a semiconductor.

One of the main failure mechanisms in current Ga₂O₃ devices is breakdown of the dielectric material isolating the semiconductor. The dielectric is usually Al₂O₃ deposited using thermal atomic layer deposition (ALD). REWIRE is currently working with Oxford Instruments Plasma Technology to develop a better dielectric using plasma ALD.

Nevertheless, there are still some barriers to innovation in the semiconductor industry.

Although SiC is the most well-developed WBG power semiconductor, since the first commercial device came on to the market in 2001, there is still a lot of scope for improving SiC devices with a push towards higher voltages and improved efficiency. However, making a change to an existing ‘qualified’ production line to test a new solution can be prohibitively expensive and time consuming. To address these kinds of challenges, researchers at the University of Warwick are working to produce demonstrator SiC devices which can be used test out new device designs and fabrication techniques more rapidly.

Although most consumers do not know what WBG materials are or where they are used, they are already at the heart of our electricity economy and are set to increase in importance as we transition to a net zero society.

• Further reading: ‘HVDC – life after due diligence and the case for standardisation.’  As HVDC transmission becomes integral to our evolving energy system, asset owners and developers must be acutely aware of the pitfalls and challenges when considering long-term economics. Tim Miles, Managing Director Europe for PSC Consulting, argues that for HVDC, like other transmission assets, due diligence is about digging into the details. But more challenges with this technology are already evident.
• Seagreen, Scotland’s largest and the world’s deepest fixed-bottom offshore wind farm, is now fully operational, generating some 5 TWh of renewable energy, enough to power almost 1.6 million homes annually.