After spending much of the 20th century languishing in development hell, electric cars have finally hit the roads in a big way. Automakers are working feverishly to improve range and recharge times to make vehicles more palatable to consumers.
With a strong base of sales and increased uncertainty about the future of fossil fuels, improvements are happening at a rapid pace. Oftentimes, change is gradual, but every so often, a brand new technology promises to bring a step change in performance. Silicon carbide (SiC) semiconductors are just such a technology, and have already begun to revolutionise the industry.
Mind The Bandgap
A graph showing the relationship between band gap and temperature for various phases of Silicon Carbide.
Traditionally, electric vehicles have relied on silicon power transistors in their construction. Having long been the most popular semiconductor material, new technological advances have opened it up to competition. Different semiconductor materials have varying properties that make them better suited for various applications, with silicon carbide being particularly attractive for high-power applications. It all comes down to the bandgap.
Electrons in a semiconductor can sit in one of two energy bands – the valence band, or the conducting band. To jump from the valence band to the conducting band, the electron needs to reach the energy level of the conducting band, jumping the band gap where no electrons can exist. In silicon, the bandgap is around 1-1.5 electron volts (eV), while in silicon carbide, the band gap of the material is on the order of 2.3-3.3 eV. This higher band gap makes the breakdown voltage of silicon carbide parts far higher, as a far stronger electric field is required to overcome the gap. Many contemporary electric cars operate with 400 V batteries, with Porsche equipping their Taycan with an 800 V system. The naturally high breakdown voltage of silicon carbide makes it highly suited to work in these applications.
It All Adds Up
The benefits of the wider band gap semiconductor flow on to other design factors, too. Thanks to higher breakdown voltage and lower on-resistance, at 1200 volts, a SiC part can have a die size 20 times smaller than a comparable silicon part. This smaller size then helps increase switching speed, further reducing losses which end up as heat. If that weren’t enough, silicon carbide parts can deal with junction temperatures up to 200 C, over and above the 150 C typical of traditional silicon parts.
Breakthroughs in processes have enabled the production of silicon carbide wafers of suitable quality for high-power use.
Until recently, however, silicon carbide wasn’t viable as a semiconductor technology, due primarily to production issues. Thanks to advances in manufacturing techniques, it’s now possible to create wafers using a single-crystal growth process, with acceptable yields for cost effective production.
All these performance gains place SiC technology in the box seat to revolutionize electric vehicle technology. ST Microelectronics give the example of a traction inverter, the hardware that takes power from the battery and drives an EV’s motor. Able to handle higher voltages in a smaller package, and able to deal with more heat, SiC semiconductors enable the device to be downsized on the order of 70% and have lower cooling requirements. Additionally, with the lower on-resistance and switching resistance, less power is wasted as heat; enabling the vehicle to be more efficient and drive further on a single charge.
The technology also has applications in the charging side of things. SiC parts promise to deliver more compact chargers, capable of delivering a fast charge with lower losses. As electric vehicles continue to proliferate, the demand for fast chargers will skyrocket, so any space and efficiency gains will pay dividends. Any electricity not lost in the charging process doesn’t have to be delivered across an already-strained electricity grid, after all.
Looking To The Marketplace
Tesla’s Model 3 features an inverter built with silicon carbide technology, increasing efficiency and reducing cooling requirements.
These devices have already hit the market in a big way. Tesla’s Model 3 was one of the first vehicles on the road to use the technology, with its main inverter packing 24 SiC MOSFET modules sourced from ST Microelectronics. It’s likely that, due to the production ramp up of their mass-market model, Tesla were using the vast majority of ST’s production in 2018. Since then, similar hardware has been rolled out to the Model S and Model X Long Range models, with silicon carbide inverters and other improvements helping push the vehicle’s maximum range up to 370 miles.
Other automakers are rushing to get on the bandwagon, with the Renault-Nissan-Mitsubishi alliance also signing an agreement to use parts from ST. Meanwhile, Bosch are also ramping up to produce components at their new Dresden plant. It’s currently unclear where these parts will end up, but with Bosch’s long history as a Tier 1 automotive supplier, it’s likely they’ve got a significant customer base at their fingertips.
Eventually most, if not all, electric vehicles will make the switch to silicon carbide technology in the coming years. Vehicles using SiC hardware will have the edge in packaging, power efficiency, range, and performance, and it’s unlikely vehicles using traditional silicon hardware will be able to compete effectively in the medium term. While silicon parts will still have a place in digital and low-voltage subsystems, it’s highly likely that silicon carbide will take the reigns in the power electronics of the electric car moving forward.