50 March/April 2024 | E-Mobility Engineering Nick Flaherty discovers how the world of power semiconductor devices is shifting away from silicon Cascade of power Power semiconductors are used across many areas of e-mobility, with different technologies suitable for each part of a vehicle, depending on the voltage and current requirements, while emerging tech is allowing smaller systems to be implemented. As a result, the choice of power semiconductor now depends on the voltage, type of vehicle – whether e-bike, three-wheeler, truck or high-performance – and offroad equipment with higher current requirements. Similarly, the choice of power semi-conductor varies, depending on its usage in the vehicle platform, from the inverter to the DC-DC converter, the AC-DC charger or other motor-control applications. New materials have emerged over the last decade alongside silicon that shave a wider electron bandgap, hence a wide bandgap (WBG), giving more flexibility in design and higher performance. For example, the traction inverter is a stalwart area for silicon insulated-gate bipolar transistors (IGBT) that run up to 1200 V, and even 1700 V, but field-effect transistors (FETs) built on metal-oxide semiconductors (MOSFETs) with widebandgap silicon carbide (SiC) at 1200 V are becoming increasingly popular. Another WBG material, gallium nitride (GaN), is being used as the level of voltage the devices can take – the ‘withstand voltage’ – increases. Rather than the bipolar transistor structure of the MOSFET, this uses a different structure with a high-energy mobility transistor (HEMT) switching a twodimensional electron gas. Meanwhile, in the AC-DC onboard charger and DC-DC converters, silicon MOSFETs are a mainstream device, but GaN is starting to dominate. It allows much higher switching frequencies for the power conversion, which means smaller, magnetic components are needed, saving space and weight. This becomes significant for light electric vehicle (LEVs) with two or three wheels. All of these are built on wafers of silicon or silicon carbide but are manufactured with significantly different techniques to the silicon chips found in mobile phones or laptops. Key considerations As well as the withstand voltage, the ‘on resistance’ is a key parameter for a power semiconductor and it is measured in milliohms (mΩ). This is the resistance between the drain and the source in any transistor when it is switched on, and it is referred to as RDS(on). The lower this value, the lower the losses and the more efficient the transistor. However, the way to reduce resistance is to increase the size of the transistor as the current requirement increases, which also raises the cost. As a result, device makers have moved from planar transistors that are spread out horizontally to a trench structure, which is built vertically. While this is more complex to make, the trench structure allows many more devices to be built on a silicon wafer, reducing the cost of each individual device. IGBTs have moved from a planar structure to trench, driving down the cost, although large planar devices that withstand voltages up to 3300 V are still used for high-power traction applications, particularly for electric trains. Work is continuing to move silicon-carbide MOSFETs to trench structures as the wafers are an expensive material that is almost as hard, and expensive, as diamond. While there is little drive to do the same for GaN, which uses a GaN layer on top of a silicon wafer, there are some projects working to do just that. The real promise for GaN is the ease of integrating the other control logic needed into one chip, further simplifying
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