Semiconductor: How silicon carbide gets a boost
High-efficiency semiconductors with silicon carbide play a key role in the energy transition. Production capacities are also being expanded in Germany.
Silicon carbide wafers during production.
(Bild: Infineon Technologies AG)
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E-cars, trains and industrial drives consume less electricity, solar modules and wind turbines can feed in more - thanks to a single material: silicon carbide (SiC). The semiconductor unfolds its potential wherever direct and alternating current are transformed - between PV modules and the power grid, between batteries and electric motors, or between the grid and home storage systems. Considerable amounts of electricity flow in the process. Energy is lost with each change. SiC reduces these conversion losses. Depending on the application, experts expect losses to be reduced by up to 30 percent. That quickly adds up to several megawatt hours of savings.
The secret of silicon carbide is its wide bandgap (see box below). As a result, silicon carbide can tolerate higher voltages than silicon. This allows the use of thinner semiconductor layers with lower resistance and correspondingly lower power dissipation. And because of its rigid crystal structure, it also withstands higher temperatures and dissipates heat better. Both of these factors make cooling easier. "Wide bandgap semiconductors, together with state-of-the-art power electronics, are the key technologies for the energy and transport transition," says Stefan Reichert from the Fraunhofer Institute for Solar Energy Systems (ISE).
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In addition, silicon carbide enables higher switching frequencies. The advantages of this can be seen in inverters. These "chop" the direct current into many small portions, so to speak, and reassemble them into alternating current. And the higher the chopping frequency, the smaller the respective current portions and the smaller the components required for this.
Silicon carbide for inverters
Whereas a 100-kilowatt inverter for a photovoltaic system weighed more than a ton in 2008, modern 125-kW units weigh well under 100 kilograms. Such devices with an efficiency of 99 percent have already been on the market for a few years.
Researchers at ISE have now developed a compact 250-kW inverter that feeds directly into the medium-voltage grid instead of the low-voltage grid. This would normally require a large, separate transformer. The advantage of medium voltage: "The higher voltage leads to smaller currents and thus smaller conductor cross-sections," says Reichert. He says the devices are particularly suitable for inner-city applications, where existing systems have to be upgraded in a tight space.
In insulators, the electrons are firmly bound to the atoms of the crystal lattice. In metallic conductors, they can move freely. Semiconductors lie in between: With energy from outside, for example in the form of heat, light or an electrical voltage, bound electrons become free. In the process, they jump from a lower energy level (valence band) to a higher one (conduction band). In silicon, the band gap is relatively small at 1.1 electron volts (eV).
This is advantageous for solar cells because light from the visible spectrum is sufficient to lift charge carriers from the valence to the conduction band. SiC and GaN, on the other hand, have a band gap that is three times as high at 3.2 to 3.4 eV. This is an advantage for power electronics because diodes made of this material only break through at a much higher voltage, i.e. conduct current. The maximum current density that the component can withstand without breaking down is also greater because of the large band gap.
