GaN Boosts MOSFET Figure of Merit

Mar 1, 2010 12:00 PM
Sam Davis, Editor in Chief

The first of two GaN MOSFET introductions from International Rectifier can switch up to 5 MHz, deliver up to 30 A output, and have a figure of merit better than their silicon cousins.

Tim McDonald, International Rectifier's vice president of Emerging Technologies, notes that silicon-based HEXFETs, Trench MOSFETs, and Super Junction MOSFETs have evolved over three decades, gaining widespread use in power supplies. However, silicon-based MOSFET performance is reaching its limit (Fig. 1), a realization that spurred the company on a search for semiconductors materials that could boost MOSFET performance. That search, which began five years ago, has led International Rectifier to gallium nitride (GaN).

McDonald explained that GaN was one of the semiconductor materials IR initially tried, but its use in power applications required a substrate on which to grow GaN layers. Bulk GaN, SiC, and sapphire wafers have cost, volume, availability, and size drawbacks. To address these problems, IR developed a silicon substrate process with a low defect density, high uniformity, and device reliability. The manufacturing cost associated with wafer fabrication was another hurdle that needed to be cleared before GaN devices reached commercial viability. The result is the development of GaNpowIR devices, which run on a standard CMOS processing line. Besides lower costs, this process scales well for large volumes.

This pioneering GaN-based power device technology platform is the result of IR's research and development on its proprietary GaN-on-silicon epitaxial technology. The high-throughput, 150-mm GaN-on-Si epitaxy, combined with subsequent device fabrication processes, is compatible with the company's silicon manufacturing facilities.

The GaNpowlR platform meets industry standards for quality and robustness. Extensive intrinsic reliability testing has already been done and will continue, allowing the company to characterize failure modes of GaN-based power devices.

The GaNpowIR technology platform provides designers with improvements in key application-specific figures of merit (FOM), and a roadmap has been set targeting improvements up to a factor of 10 compared with state-of-the-art silicon-based technology platforms. This dramatically increases performance and cuts energy consumption for end applications in a variety of application segments, such as computing, communications, automotive, and appliances.

International Rectifier's first GaN-based product (released in Feb. 2010) is the iP2010 integrated power stage (Fig. 2). Together with the iP2011 — which is due for a summer 2010 release — the devices form a family intended for multiphase and point-of-load (POL) applications including servers, routers, switches and general purpose dc-dc converters. The product manager is John Lambert.

The Table 1 lists the characteristics of the iP2010 and iP2011, which both include monolithic GaN-based MOSFET's with a 30-mΩ-nC figure of merit. Both products include a sophisticated, ultra-fast matching PowIRtune™ driver IC that is designed for operation with the depletion-mode GaNpowIR FET's, assuring minimal dead time (~1 ns).

The combination enables high performance without compromised figure of merit, which is common in enhancement-mode GaN FETs. Mounted in a flip-chip package, these devices reduce parasitics to enhance operation at higher frequencies.

The package for the iP2010 (Fig. 3) has the PowIRtune driver IC mounted internally. Available in an LGA package with a 7.7 × 6.5 × 1.7-mm footprint, the iP2010 and iP2011 packages are optimized for very low power loss and are RoHS compliant.

Both devices exhibit high peak efficiency, with more than 90% at a 1.2-MHz switching frequency. Fig. 4 is plot of efficiency vs. load current. It is unlikely that even state-of-the-art silicon MOSFETs can achieve 90% efficiency when switching at 1.2 MHz. The result is that the iP2010/iP2011 can operate up to two or three times higher than conventional frequencies, thus occupying a smaller footprint.

Fig. 5 is the circuit for a two-phase converter capable of delivering 2×30 A. It consists of two iP2010s, an IR3623 controller IC, an IR3610 negative voltage generator IC, and a p-channel FET for sequencing and protection. As seen in Fig. 6, the GaNPowIR configuration occupies less board space than either a conventional dc-dc converter or a DrMOS design. Fig. 7 shows a demonstration board of a dc-dc converter using the iP2010.

IR believes its family of GaN-based MOSFETs for dc-dc converters will enable a new era in high-frequency, high-density,and high-efficiency power-conversion solutions. Switching up to 5 MHz, the iP201x dramatically reduces the value and size of a dc-dc converter's output capacitors and inductors, which benefits space-sensitive applications.

The devices can also be configured to operate at lower switching frequencies, for applications that require the highest possible efficiencies. Future introductions will include GaN-based power FET products with even better figures of merit (Fig. 8) than the present 30-mΩ-nC.

CHARACTERISTICS OF IR'S GaN-BASED MOSFET PACKAGES

PART NUMBER	PACKAGE	Vin RANGE (V)	Vout RANGE (V)	Iout MAX (A)	SWITCHING FREQUENCY RANGE (MHz)	FIGURE OF MERIT (mΩ-nC)
iP2010TRPBF	7.7 × 6.5-mm LGA	7 to 13.2	0.6 to 5.5	30	0.25 to 3.0	30
iP2011TRPBF	7.7 × 6.5-mm LGA	7 to 13.2	0.6 to 5.5	20	0.25 to 5.0	30

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