New Power Switches vs. Mature Materials at PCIM Europe

May 18, 2011 by Jeff Shepard

The Wednesday afternoon panel discussion at this year’s PCIM Europe event was organized by Bodo’s Power Systems and designed to focus on the ability of devices such as capacitors and thermal management systems to support the advanced capabilities of new power switches made with SiC and GaN. Panelists represented Cree, CPS Technologies, Infineon, International Rectifier, SBE, SemiSouth, TranSiC/Fairchild Semiconductor and Transphorm.

The discussions ended on a high-energy note during the Q&A time, with a "heated" debate of the relative merits of SiC and GaN power switches. John Palmour, CTO for Advanced Devices at Cree took the first question about the relative defect densities in SiC and GaN. He observed that "defect densities make a difference."

Dr. Palmour continued, "GaN on silicon has a million times the defect density of today’s SiC materials and the GaN devices don’t have proven field reliability while Cree’s SiC devices have a proven reliability of 0.4 FITs based on 127,400,000,000 device hours in the field. That is twice as good as conventional silicon device reliability."

Umesh Mishra, Chairman and CEO of Transphorm observed that since SiC devices are vertical structures, defects in SiC are much more detrimental to performance and reliability than are defects in lateral GaN structures. And he commented, "It is possible to overcome any potential problems with defects by sufficiently derating the GaN devices."

Drs. Mishra and Palmour agreed that the wider band gap of GaN relative to SiC makes it easier to implement the needed levels of derating in practical GaN devices. Dr. Mishra commented, "That solves the problem of derating GaN devices to compensate for defect densities."

Tim McDonald, International Rectifier’s Vice President of Emerging Technologies closed the session with a prediction that, "GaN will be the dominant material for devices rated for below 1,200V and SiC will dominate the higher-voltage area such as 1,700V devices."

In the lead-up to the active Q&A session, Jeff Cassidy, CTO with SemiSouth started the discussions by pointing out that the latest SiC JFETs being developed by his company produce performance levels nearing the theoretical limit for 4H SiC materials. When used in a photovoltaic inverter design, the SiC devices operate at 3-times the frequency of corresponding silicon devices, deliver 3-times the power density and offer improved efficiency. SemiSouth’s SiC devices are offered in modules from Vincotech and Microsemi for each of integration into inverters and other power converters.

Mats Reimark, Sr. Director with the TranSiC division of Fairchild Semiconductor followed with a presentation of the capabilities of his company’s SiC bipolar power transistors. The TranSiC devices feature a positive temperature coefficient for easier paralleling with no secondary breakdown, very good high-temperature capabilities and very fast switching capabilities, 3- to 4-times that of silicon IGBTs.

Ralf Keggenhoff, Sr. Marketing Manager and Head of Application Engineering with Infineon, continued the discussion of the impact of wide band gap devices on power system design. Infineon offers SiC Schottkys with ratings of 600V, 1,200V, and 1,700V and SiC JFETs rated for 1,200V. He observed that "devices such as these are required for today’s high performance systems."

Mr. Keggenhoff also discussed the importance of advanced packaging when using these SiC devices. He reviewed Infineon’s ’dot XT’ technology that features improved interconnections that can increase module lifetimes by 10-times or support a 25% increase in power density. Key features of the technology include low stray inductances and improved thermal interfaces designed to operate at elevated temperatures.

Turning to GaN devices, Transphorm’s Dr. Mishra shared three of the many possible applications for GaN power designs. He showed the results of a "Total GaN" 300W dc-dc boost converter that delivers over 99% efficiency with an operating frequency of 100-kHz.

Dr. Mishra also reviewed a 1.5-kW diode-free pure sine wave motor drive inverter with an efficiency of 98.8%, operating at 100-kHz and a 750-kHz power factor correction module. Finally, he "busted the myth that high-frequency GaN power switches produce more EMI noise than lower-frequency silicon devices" by showing a side-by-side comparison of two 90W power converters. The GaN converter was switching at twice the frequency of the silicon-based design, and the EMI spectrums were essentially the same from 150-kHz up to 30-MHz.

IR’s McDonald presented an analysis of the "requirements for commercially viable GaN devices." IR is interested in wide-spread adoption of the new technology, not niche applications. He pointed out that the "performance/cost of GaN needs to be at least two- to three-times the same ratio for silicon devices, the total EPI plus substrate cost for GaN needs to be similar to that for silicon, the leakage current needs to be under 1 microamp per millimeter and the ratio of the on/off current levels needs to be at least 100,000:1 with truly crack free EPI and yields of greater than 80%"

He reiterated IR’s "possible 650V GaN Roadmap" that projects an improvement from today’s situation where high-voltage GaN offers a two-times improvement in figure of merit versus superjunction silicon MOSFETS to an eight-times improvement in performance by 2015.

Furthermore, McDonald observed that to be a mainstream technology "GaN needs to be produced on large diameter (at least 150mm) wafers and in volumes of greater than 10,000 wafer starts per week. One hundred thousand 150mm wafer equivalents are needed to support 10% of the total power semiconductor market at current utilization rates."

Cree’s Dr. Palmour noted that SiC is an established technology that offers proven performance and reliability. He noted, "Cree’s two largest customers for SiC JBS diodes have commented ’Your parts are much more reliable than the silicon parts we were using’."

According to Dr. Palmour, "Cree’s 80mΩ, 30A SiC MOSFETs are poised to replace silicon MOSFETs and IGBTs in high-efficiency, high-speed power designs." He also noted that the devices are true MOSFETs with simple drive requirements and are scalable to higher currents though paralleling. He closed by stating "it’s a myth that the gate oxides in SiC MOSFETs are unreliable and that the body diodes degrade, we have performed extensive accelerated life-time testing and fine no degradation in today’s devices."

Dr. Mark Occhionero, VP Marketing and Technical Sales with CPS Technologies followed up on the dialog about advanced, high-speed semiconductor devices with a presentation about AlSiC metal matrix composites. He noted that wide band gap devices operate at higher temperatures as well as higher frequencies and higher efficiencies and demand new solutions for thermal management.

Closing out the opening part of the discussions, Dr. Michael Brubaker, CTO and VP of Engineering at SBE offered metallized-polyproplyene (MPP) capacitors are the optional solution for use with wide band-gap semiconductor devices. He enumerated several important attributes including: cost, availability, high breakdown strength, low losses, self healing, long life, linear characteristics and good frequency response as key features.

When used in high-power, high-frequency converters, MPP designs can operate at 105 degrees C coolant temperatures with a minimal temperature rise of 2 degrees at a steady-state ripple current of 150A rms at 20 kHz. Options for higher-frequency and higher-temperature operation are currently being developed by SBE in cooperation with both GE and DuPont.

Then the Q&A part of the dialog began with a question from the floor: "Dr. Palmour, please amplify on your comments related to the performance and reliability impact of the increased defect densities in GaN devices relative to SiC devices." And the discussions ended on a high-energy note during the Q&A time, with a "heated" debate over the relative merits of SiC and GaN power switches.