Wide-bandgap Power IC Technologies: Making Design Decisions More Challenging

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There’s good news for power supply system designers. Wide-bandgap semiconductor IC technologies like gallium nitride (GaN) and silicon carbide (SiC) are poised to grow rapidly, offering designers greater performance in a smaller package. Higher operating voltages, higher operating frequencies (faster switching times), higher temperature tolerances, and lower power dissipation than conventional silicon-based devices like diodes, transistors and field-effect transistors (FETs) are the hallmarks of GaN and SiC power ICs. Wideband gap (WBG) devices are those that offer higher voltage electronic gaps that are greater than 1 electron volt (eV). Silicon has a bandgap of 1.1 eV whereas SiC has a bandgap of 3.3 eV and GaN has a bandgap of 3.4 eV.

1. Introduction

As part of its efforts to increase energy efficiency and double U.S. energy production by 2030, the Dept. of Energy (DoE) has awarded $22 million for WBG semiconductor IC developments. The aim is to merge WGB IC technology with advancements in driving megawatt motors for large-scale power generation and applications. Silicon-based power IC technologies are limited by the inability of silicon to handle higher operating voltages, temperatures and frequencies. The one advantage they have is a lower cost since they’re manufactured using conventional silicon processing steps. One indication of the interest in WBG semiconductor ICs is the U.S. Dept. of Energy (DOE) awarding $22 million in funding for five projects aimed at WGB semiconductor technology with advancements for large-scale motors to increase energy efficiency in high-energy consuming industries, homes, fossil fuels and next-generation electric machines. The funding was made to three U.S. companies and two U.S. universities. To be sure, silicon-based power ICs are making their own performance advances in the form of advanced field-effect transistors (FETs) like high-electron-mobility transistors (HEMTs) and advanced MOSFETs that make use of insulated-gate bipolar transistors (IGBTs). IGBTs are presently the workhorses of the power electronics industry. IGBTs continue to post advantages with higher performance attributes of high-speed switching, smaller size and lower cost. An IGBT combines the simple gate-drive characteristics of a MOSFET with the high-current and low-saturation-voltage capability of a bipolar transistor (figure 1).

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Figure 1: Cross section of a typical IGBT cell which combines the simple gate-drive characteristics of a MOSFET with the high-current and low-saturation-voltage capability of a bipolar transistor (source: Wikipedia).

Recent IGBT market introductions include the S series 1.2-kV devices from STMicroelectronics which boost efficiency in power-switching applications up to 8 kHz. They’re claimed to be the industry’s best low-frequency performers. STMicroelectronics also offers the M Series 650-V IGBTs that boost efficiency in 20-kHz power-switching applications for uninterruptable power supplies (UPSs), solar converters, etc. IGBTs can be laid out in parallel for even higher-voltage applications from modules. The Netherlands’ HVIC Technologies uses 2-kV second-generation X2PT IGBTs from IXYS in a module. And Germany’s Vincotech offers the NPC modules that use Infineon Technologies H5 IGBTs for very high speed at low saturation current and good switching. Fuji Electric offers a 1.2-kV 7th-generation IGBT-based module that operates at temperatures up to 175°C. In the works are 1.7-kW versions. IXYS Corp. uses its Ultra Junction technology to deliver a power IC figure of merit better than the state of the art according to Dr. Nathan Zommer, its CEO and founder. These 650-V devices handle currents from 2 to 120 A, and feature an RDSon as low as 24 mΩ. “With the combination of reduced on resistance and improved switching and thermal performance, these devices offer the best cost performance to our customers, even better than SiC MOSFETs, especially in ruggedness and reliability.” Conventional silicon-based MOSFETs are finding room for designers to make the usual tradeoffs in smaller size, efficiency, lower switching speeds, power density and component count. Recent examples include the STMicroelectronics’ StripFET F7 devices that deliver the highest energy efficiency synchronous rectification. This allows the use of fewer parallel devices for a desired maximum current which helps increase power density and lowers component count. Infineon Technologies, which recently purchased International Rectifier Corp, offers the StrongIRFET DirectFETs which claim to deliver the highest energy efficiency in space-constrained applications like power tools and E-bikes (figure 2).

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Figure 2: International Rectifier’s StrongIRFET DirectFETs deliver the highest energy efficiency in space-constrained applications like power tools and E-bikes (Source: International Rectifier Corp.).

Even the manner of packaging has improved with power MOSFETs. For example, Fairchild Semiconductor’s Dual Cool 88 package gives power conversion engineers an alternative to bulky D2 packages at one-half the size, higher power density and efficiency and easier cooling (figure 3).

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Figure 3:   Improved power MOSFET packaging like this Dual Cool 88 package from Fairchild Semiconductors gives engineers an alternative to bulky D2 packages at one-half the size, higher power density and efficiency and easier cooling. (Source: Fairchild Semiconductor Corp.).

2. GaN Power ICs are Here

A WBG technology like GaN on silicon substrates is beginning to bear fruit and may open the door for power ICs over the next few years. Many GaN-based devices with performance improvements are coming out of the lab and some like diodes, transistors, and FETs are appearing on the market. GaN technology power ICs are coming down in size as well, opening up more market applications. Efficient Power Conversion Corp. has shown off its EPC2037 enhancement-mode GaN (eGaN) FETs in a tiny 0.9-mm by 0.9-mm package. It can handle 100 V 1-A uses with an RDS(on) of 550 mΩ (with 5V applied to the gate). The newest Cree company, Wolfspeed Inc., has unveiled plastic-packaged CGHV2706MP GaN HEMT FETs for low-cost 50-V 60-W applications. They’re supplied in 4.5-mm by 6.5- mm surface-mount (SMT) packages to suit long-term evolution (LTE) phone, cell-based station, radar, public-safety radio, and other communications applications. Panasonic Corp., for example, claims it has developed GaN diodes that can not only operate at high currents four times greater than  SiC ones, but also has a low threshold voltage. The diodes can handle 7.6 kA/cm2, require a low turn-on voltage of 0.8 V and feature a low on resistance of just 1.3Ω/cm2. To achieve this performance, Panasonic used a hybrid structure of GaN diodes with a trenched p-GaN layer which can be removed selectively during processing by an n-type layer. GaN Systems Inc. is marketing the G S66540C 650-V 100-A GaN transistor based on the company’s proprietary Island Technology die design that produces devices with fast switching speeds of 100 V/ns with very low thermal losses. Part of the company’s portfolio of enhancement-mode HEMT family, it makes use of a patented GaNPX packaging and Drive Assist technology which together with the Island Technology provides GaN HEMTs with a 45 times improvement in switching and conduction performance over silicon MOSFETs and IGBTs. You can also purchase prototype samples of a GaN FET in modular form like the 80-V 10-A LGM5200 from Texas Instruments. Aimed at applications that require increase power density and efficiency in space-constrained high-frequency industrial and telecom applications, the module contains two GaN FETs in a half-bridge configuration and a high-frequency driver, it is housed in a quad no-lead (QFN) package.

3. SiC Forging Ahead

Wolfspeed Inc. recently offered samples of its C2M1000170J SiC MOSFET that handles 1.7 kV. Optimized for this rating in an SMIT package, it has an avalanche rating greater than 1.8 kV and an RDS(on) of 1 Ω. Its small footprint with a wide creepage distance of 7 mm between the source and drain terminals suits it for auxiliary power supplies with high-power inverters, uninterruptable power supply (UPS) equipment, wind-energy converters, and traction power systems. Cree recently put on the market a 900-V SiC MOSFET. The 3CM0065090J has 65 mΩ of on resistance and is available in TO-247-3, TO-220-3 and D2Pak-7L SMT packages.
One of the newest SiC MOSFETs is the SCT20N120 from STMicroelectronics (figure 4). This 1.2-kV device has an RDS(on) of 290 mΩ all the way up to the 200°C maximum operating junction temperature. Housed in STMicroelectronics’ HiP227 package that’s compatible with the industry standard TO-247 power package, it permits switching frequencies 3X higher than similarly rated silicon IGBTs.

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Figure 4:  STMicroelectronics’ 1.2-kV SiC MOSFET device has an RDS(on) of 290 mΩ all the way up to the 200°C maximum operating junction temperature. The SCT20N120 permits switching frequencies 3X higher than similarly rated silicon IGBTs. (Source: STMicroelectronics).

HEMT structures are finding their way into GaN and SiC power devices (figure 5). Such devices incorporate a junction between two materials with different as the channel instead of a doped region (as is generally the case for MOSFETs). HEMT transistors can operate at higher frequencies (millimeter wave frequencies) than ordinary transistors, for mobile phone communications, satellite TV receivers, voltage converters and radar equipment. An HEMT structure (Figure 5) is used in the unmatched and recent 50-V 50-W GaN transistor from Cree that provides high-performance broadband solutions for RF and microwave applications up to 4 GHz, including narrow-band UHF, L-band, S-band and multi-octave bandwidth amplifiers. The CGHV40050 exhibits high-efficiency, wide-bandwidth, and high-gain performance.

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Figure 5: An HEMT transistor can operate at higher frequencies (millimeter waves) than ordinary transistors, for mobile phone communications, satellite TV receivers, voltage converters and radar equipment. (Source: Wikipedia).

4. GaN or SiC?

In the short term over the next 4-5 years, GaN will fulfill many power IC applications in the 600-V to 1.2-kV range. Two voltage options are being targeted, 600 V and 1.2 kV. Present applications include solar inverters, electric and hybrid electric vehicles (EVs and HEVs), uninterruptable power supplies (UPSs), communications, and RF solid-state power amplifiers. However in the longer term, SiC IC power technology is poised to gain some ground, particularly for applications that require very high temperatures and extreme ruggedness, very low switching losses, rolling stock uses like trains, transportation, very high voltages of 2 kV or more and geophysical applications like oil and gas drilling. Some companies have already introduced SiC MOSFETs that can reduce switching losses by more than 70% compared with IGBT MOSFETs. Analyst Pallavi Madakasira, lead of the study “Breaking Down the GaN Electronics Market” at Lux Research in the U.S. points out that GaN on silicon substrates offer performance improvements over other approaches like GaN on GaN substrates and GaN on SiC substrates, the latter two which “will limit their adoption.” She sees the transportation and renewable energy/smart-grid sectors as key markets, reaching sales of $350 and $380 million, respectively.  She forecasts the overall GaN IC power-conversion market to grow to $1.1 billion by 2024. A similar assessment of the growth of GaN power IC technology is made by France’s Yole Développment in conjunction with KnowMade, a Yole partner, based on intellectual property (IP) dynamics. “The future of GaN devices also depends on the growing global patent landscape and coming mergers and acquisitions” explains Nicolas Baron CEO of KnowMade (figure 6).

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Figure 6: The growing global patent landscape and company mergers and acquisitions will determine the future growth of GaN power ICs. (Source Yole Développment and KnowMade).

Yole has already forecasted dynamic growth for both GaN and SiC power devices. It sees competition for applications from both technologies setting up a question mark of “how will GaN and SiC technologies cohabitate?” In Yole’s “nominal” market scenario, it estimates that the market size for GaN power ICs will be $303 million by 2020.

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