56 March/April 2024 | E-Mobility Engineering The d-mode approach is also used by Transphorm. The 2DEG is an incredibly fast channel that spontaneously forms at the interface between the GaN and a thin AIGaN layer. Its electron density is among the highest naturally occurring in semiconductors. It also offers high mobility, at 2000 cm2 /V·s, which is twice that of state-of-the-art silicon (Si) and silicon-carbide (SiC) devices. This 2DEG channel at the AlGaN/GaN interface forms spontaneously with no need for external gate bias, so the device is normally-on and needs a negative gate bias to deplete the channel and turn off. It is a depletion mode (d-mode) device, but power electronic systems need normallyoff devices for fail-safe operations. The question then is how to make the lateral GaN HEMT normally-off. This is where cascode and e-mode technologies part ways. In the cascode technology, the GaN HEMT is untouched and the 2DEG channel is free to maximise its unparalleled combination of high mobility and charge density. Transphorm’s cascode approach pairs the GaN HEMT with a lowvoltage, normally-off Si MOSFET to achieve normally-off operation. This gives a positive threshold of 2.5-4.0 V, depending on power level, topology and system architecture. This pairs the normally-on GaN HEMT with a highly reliable, highly performant, normally-off, low-voltage Si MOSFET in a cascode configuration. This ensures fail-safe, normally-off operations and is backward-compatible with today’s silicon technology. Silicon carbide With silicon carbide it is the material that is key, rather than the structure of the devices. Some are built on an engineered silicon wafer with a layer of insulator (silicon on insulator, or SOI) on the surface. These are now moving from 150 mm- to 200 mm-diameter wafers to increase the volume of devices that can be produced, reducing costs. Others use silicon infused with carbide throughout. This creates a material that is as hard as diamond, making it more difficult to work with, but with better thermal and electrical characteristics, especially for trench structures. However, these devices struggle with the move from 150 mm to 200 mm for higher-volume production. With a SiC-based MOSFET, the RDS(on) resistance only moves by a factor of around 1.13 between 25 C and 100 C, but with a typical, Si-based MOSFET such as the CoolMOS C7 from Infineon, it changes by a factor of 1.67. This means the operating temperature has much less of an impact on power loss and can therefore be much higher. The cascode approach is also used with SiC devices alongside silicon MOSFETs for lower-power applications. Various devices developed by UnitedSiC and now Qorvo combine a junction FET (JFET) with a specially-designed silicon MOSFET with versions at 650 V, 1200 V and 1700 V, and currents up to 120 A. The advantage of this approach is that the cascode devices can be used with existing gate drivers and can be a drop-in for existing designs, delivering a lower RDS(on). The future The world of power semiconductor devices is shifting from silicon to wide bandgap materials. Device-makers are aiming to make the transition as simple as possible, using the same gate drivers as silicon. But the use of WBG devices is also opening up higher-frequency operation for smaller magnetics and smaller, lighter power designs, particularly in DC-DC and AC-DC e-mobility applications. With GaN and SiC technologies maturing and coming down in price, adoption is growing, and the technologies are increasingly dominating the design and development of e-mobility powertrain and power systems. Deep insight | Power semiconductors GaN is an increasingly popular power technology throughout a vehicle (Image courtesy of GaN Systems/Infineon Technologies)
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