55 Power semiconductors | Deep insight This concentration of electrons is called a two-dimensional electron gas (2DEG). Further processing forms a depletion region under the gate, and metal layers are added to connect the three terminals, Gate, Drain and Source. A cross-section of this structure is shown in the GaN transistor image (above). This structure is repeated many times to form a power device. The end result behaves in the same way as silicon MOSFETs. “A native e-mode device at lower voltages looks just like a MOSFET except much faster, but that’s where the issues come in. In the motor there is the isolation, and there is a rule of thumb that you can’t switch the motor winding faster than about 10 V/ns because then the insulating resin tends to accumulate charge and breaks down, plus at high frequencies there have been problems with the bearing getting pitted from sparks,” says Lidow. “While 10 V/ns for a 48 V motor drive is not a limitation, for a 800 V drive that’s 80 V/ns, so at high voltages there are problems with high frequencies. This can be addressed with multi-level drives. When you do that you only have the switching losses of the lowervoltage levels, so there would be eight segments for an 800 V motor. While this is more components, the GaN devices are significantly lower cost,” he explains. A project at the University of Bern and the University of Texas at Austin has shown that the sweet spot for driving an 800 V motor is with 200 V devices for the power density, rather than cost, and these 200 V devices are mass-market. However, the multi-level design needs more isolators with multiple outputs. “GaN is more reliable than silicon and that is becoming more widely accepted,” Lidow says. “The wide bandgap means it is far less sensitive to temperature effects. This means these devices last a long time and are more reliable at operating temperatures than room temperature, so the wear-out mechanisms are so much less.” GaN Systems/Infineon Infineon’s GaN semiconductors are highly efficient for power conversion in the voltage range of up to 600 V. The CoolGaN 600 V GaN HEMT family uses an enhancement-mode design, based on the hybrid-drain-embedded gate-injection structure, which provides the highest level of robustness. The HEMT devices can operate at high switching frequencies without compromising efficiency. Many enhancement-mode GaN HEMTs also add a diode to emulate a typical, insulated MOSFET gate. The CoolGaN Integrated Power Stage (IPS) family is available in various configurations, including single-channel and half-bridge. While the half-bridge configuration combines two 600 V GaN power transistors with dedicated gate drivers in small-outline 27-pin and 28-pin packages, the single-channel setup uses only one 600 V GaN power transistor merged with tailor-made GaN drivers in a 21-pin package. GaN Systems, now part of Infineon Technologies, also developed an enhancement-mode GaN-on-silicon power transistor. This implements a patented Island Technology cell layout for high-current die performance and yield, and it has attracted significant interest in automotive designs. This is coupled with custom packaging called GaNPX, which enables low inductance and low thermal resistance in a small package with a bottom side cooled transistor that offers very low junction-to-case thermal resistance. GaN depletion mode The D3GaN (Direct Drive D-Mode) power switch, developed by VisIC Technologies, integrates a patented, high-density, lateral GaN power transistor into a ‘normally-off’ product with extremely low RDS(on). The 650 V D3GaN technology has been implemented into an isolated, high-power package that includes integrated safety functions to ensure safe operation during system startup and shutdown, while having no impact on the switching performance of the GaN transistor with an RDS(on) of 22 mΩ. This package has a small footprint of 19.7 mm x 13.6 mm, including the gull-wing leads, with top-side cooling for a heat sink specifically for EV designs. E-Mobility Engineering | March/April 2024 The cross-section of an e-mode GaN transistor (Image courtesy of EPC)
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