Silicon power MOSFETs are approaching their theoreticalperformance limits for both on-resistance and gatecharge. Although silicon switches will continue to ekeout gains, the increases will come more slowly and besmaller. Enter wide-bandgap materials: SiC (siliconcarbide) and GaN (gallium nitride). Both technologieshave for several years found use in RF amplifiers.Advances in process manufacturing are driving downthe costs and increasing the power capability of powerswitches employing these materials, and proponents of both claim thatthey will supplant silicon MOSFETs in switched-mode-power-supplydesigns in which power density and efficiency are key attributes. Silicon power MOSFETs are approaching their theoreticalperformance limits for both on-resistance and gatecharge. Although silicon switches will continue to ekeout gains, the increases will come more slowly and besmaller. Enter wide-bandgap materials: SiC (siliconcarbide) and GaN (gallium nitride). Both technologieshave for several years found use in RF amplifiers.Advances in process manufacturing are driving downthe costs and increasing the power capability of powerswitches employing these materials, and proponents of both claim thatthey will supplant silicon MOSFETs in switched-mode-power-supplydesigns in which power density and efficiency are key attributes.

The size of a power supply’s componentsdetermines its power density; thelargest components are the inductorsand capacitors, and the MOSFET isa distant third. Capacitors and inductorsshrink with higher switching frequency,which is topping out for siliconMOSFETs. Switching losses dueto the gate charge would cancel anyfurther increase in switching speed.As for the MOSFET itself, shrinkingthe die of a silicon MOSFET unfortunatelyincreases the on-resistance.MOSFETs have traditionally measuredtheir performance figure of merit as theon-resistance times the gate charge. Forsilicon devices, one parameter tradesoff against the other. Decreasing on-resistanceusually comes from increasingthe area of the device, but that increasein area comes with an increase in gatecharge and, hence, capacitance, whichdecreases the device’s speed.

Wide-bandgap materials have loweron-resistance by an order of magnitudethan silicon materials, with no correspondinglygreat increase in gate charge.The problems have been the difficultyand expense of working at high powerwith GaN and SiC materials. GaN andSiC transistors have for years founduse in RF amplifiers, and SiC Schottkydiodes commonly work as rectifiers inhigh-voltage power supplies becausethey have a fraction of the reverse-leakagecurrent of silicon diodes. In addition,SiC has a high thermal conductivity,so an increase in temperature doesn’tdegrade the device’s switching parameters.SiC devices target high-voltageapplications that require a blocking voltageof 1200 to 1700V. At these voltages,silicon IGBTs (insulated-gate bipolartransistors) are more common switchingdevices than MOSFETs, but IGBTs aremore expensive, and fewer engineers arefamiliar with IGBTs’ design constraints.SiC FETs share some of the same drivecharacteristics as silicon MOSFETs,such as being normally off, and can takeadvantage of MOSFETs’ relatively largebase of design engineers. Wide-bandgap materials have loweron-resistance by an order of magnitudethan silicon materials, with no correspondinglygreat increase in gate charge.The problems have been the difficultyand expense of working at high powerwith GaN and SiC materials. GaN andSiC transistors have for years founduse in RF amplifiers, and SiC Schottkydiodes commonly work as rectifiers inhigh-voltage power supplies becausethey have a fraction of the reverse-leakagecurrent of silicon diodes. In addition,SiC has a high thermal conductivity,so an increase in temperature doesn’tdegrade the device’s switching parameters.SiC devices target high-voltageapplications that require a blocking voltageof 1200 to 1700V. At these voltages,silicon IGBTs (insulated-gate bipolartransistors) are more common switchingdevices than MOSFETs, but IGBTs aremore expensive, and fewer engineers arefamiliar with IGBTs’ design constraints.SiC FETs share some of the same drivecharacteristics as silicon MOSFETs,such as being normally off, and can takeadvantage of MOSFETs’ relatively largebase of design engineers.

Among available SiC powerMOSFETs is Cree’s recently introducedSiC Figure 1 ).On-resistance increases by only 20% atthe maximum operating temperature of150°C. The devices have a gate charge ofless than 100 nC throughout the input-voltagerange, simplifying gate-driverequirements, and the devices have aforward-voltage drop of less than 2V ata 20A load current, reducing losses. Creealso offers the Z-Rec rectifier, a companionfamily of SiC power diodes, whichhas essentially no reverse recovery at600, 650, and 1200V breakdown voltages.The devices have current ratingsof 1 to 20A at 600V, 4 to 10A at 650V,and 5 to 20A at 1200V. The devices arealso available in chip form with currentsof 10 and 25A at 1700V. Among available SiC powerMOSFETs is Cree’s recently introducedSiC CMF20120D series. The $93.75devices have a 1200V blocking voltageand a maximum on-resistance of 110mΩ at a drain current of 20A and a gateto-source voltage of 20V ().On-resistance increases by only 20% atthe maximum operating temperature of150°C. The devices have a gate charge ofless than 100 nC throughout the input-voltagerange, simplifying gate-driverequirements, and the devices have aforward-voltage drop of less than 2V ata 20A load current, reducing losses. Creealso offers the Z-Rec rectifier, a companionfamily of SiC power diodes, whichhas essentially no reverse recovery at600, 650, and 1200V breakdown voltages.The devices have current ratingsof 1 to 20A at 600V, 4 to 10A at 650V,and 5 to 20A at 1200V. The devices arealso available in chip form with currentsof 10 and 25A at 1700V.

Also in the market, Rohm Semiconductorrecently announced itsSCS1xxAGC series of SiC Schottkydiodes, which maintains low forwardvoltage over a wide operating-temperaturerange. For example, the 10A-ratedSCS110AGC part has forward voltageof 1.5V at 25°C and 1.6V at 125°C. Theshort typical reverse-recovery time of 15nsec enables high-speed switching andminimizes switching loss. Prices rangefrom $6.38 for the 6A version to $24.60for the 20A version. Rohm will this yearfollow up its SiC diodes with SiC FETs;it also plans to release 600V transistorsin Japan with 1200V SiC FETs in 2012.SemiSouth also makes SiC Schottky-dioderectifiers for solar-inverter andpower-factor-correction applicationsand offers a SiC power JFET, the normallyon 1200V SJEP120R085.

The nearly $100 price of SiCMOSFETs may condemn them toniche markets, but keep in mind that,when silicon MOSFETs debuted morethan 30 years ago, their prices werealso around $100 (in 1970s dollars),yet they now sell for a few dollars each.John Palmour, chief technology officerat Cree, expects the same downwardtrend in price for SiC devices, in largepart because of the room for manufacturabilityand manufacturing efficiency(Reference 1). However, it’s also likelythat SiC devices sell at their currentprice because that’s what the marketwill bear. Power efficiency has a price,and more efficient power supplies payoff in energy savings.

GaN devices have also had initialsuccess as RF switching devices, generallyat lower voltages than those of SiC.Manufacturers grow the GaN deviceson sapphire substrates, involving highmanufacturing costs. The breakthroughfor GaN came with the ability to grow GaN structures on silicon. Initial GaN-on-silicon devices all operated at lessthan 100V, targeting use in the datacom-power-conversion market. GaN’shigher switching speed and efficiencyallow dc/dc converters to operate in themegahertz region, saving space, reducingthe need for heat sinks, and conservingpower. International Rectifierwas the first company to offer GaN-on-silicon power switching devices tothe commercial market with the introductionof the IP2010 in early 2010.The device has a blocking voltage of20 to 40V and targets point-of-loaddc/dc converters. The company’s technologyoperates in depletion, normallyon, mode but hides this characteristicfrom designers because the companyoffers the parts as complete driver stagesrather than as discrete devices. Forexample, rather than quote a blockingvoltage for the FETs, InternationalRectifier quotes the GaNpowIR stageby its input-voltage range of 7 to 13.2V,output voltage of 0.6 to 5.5V, outputcurrent of 30A, and operating frequencyas high as 3 MHz. The drivers sell for$9 (2500).

EPC (Efficient Power Conversion)also offers GaN devices, for whichit uses the trademarked name eGaN(enhanced-mode GaN). The companyrecently introduced the EPC2010 FET,which has a drain-to-source voltage of 200V, a maximum on-resistance of 25mΩ with 5V applied to the gate, anda pulsed-current rating of 60A. UnlikeInternational Rectifier’s GaN power-conversion-stage device, the EPC2010is a discrete transistor. The drive issimilar to a silicon power MOSFET,but some challenges in driving oneexist. For example, because of the highswitching frequencies, an eGaN circuitis sensitive to layout. The devicealso tolerates only a narrow range ofgate voltages. To ensure that it’s onrequires 4.5V, but it can tolerate only6V. Considering the power transientsyou can expect in a power-converterenvironment, 1.5V is a narrow rangeof operation. Because the threshold islower, you must drive it even harderwhen the gate gets close to ground toensure that it stays below 1.4V ratherthan the 2.5V threshold you wouldencounter in a silicon MOSFET.

“There’s no true body diode in aGaN FET,” says Alex Lidow, founderand chief executive officer of the company.“There’s no reverse recovery loss,which is a performance gain. But whenyou do leave the FET on, it still has aforward drop of greater than 1.5V, soyou have to be careful about dead time.None of these [drawbacks] are insurmountable,but you have to be careful.”

Recognizing an opportunity,National Semiconductor recently introduced a 100V half-bridge gatedriver for use with eGaN FETs inhigh-voltage power converters (Figure2 ). The LM5113 high- and low-sideGaN-FET driver regulates the high-sidefloating bootstrap-capacitor voltageat approximately 5.25V to driveeGaN power FETs without exceedingthe maximum gate-to-source voltagerating. The LM5113 also featuresindependent sink and source outputsfor flexible turn-on strength withrespect to the turn-off strength. A0.5Ω-impedance pulldown path providesa fast, reliable turn-off mechanismfor the low-threshold-voltageeGaN power FETs. The LM5113 alsointegrates a high-side bootstrap diode,further minimizing PCB (printed-circuit-board) real estate, and providesindependent logic inputs for the high- andthe low-side drivers, enablingflexibility for use in a variety of bothisolated- and nonisolated-power-supplytopologies. Recognizing an opportunity,National Semiconductor recently introduced a 100V half-bridge gatedriver for use with eGaN FETs inhigh-voltage power converters (). The LM5113 high- and low-sideGaN-FET driver regulates the high-sidefloating bootstrap-capacitor voltageat approximately 5.25V to driveeGaN power FETs without exceedingthe maximum gate-to-source voltagerating. The LM5113 also featuresindependent sink and source outputsfor flexible turn-on strength withrespect to the turn-off strength. A0.5Ω-impedance pulldown path providesa fast, reliable turn-off mechanismfor the low-threshold-voltageeGaN power FETs. The LM5113 alsointegrates a high-side bootstrap diode,further minimizing PCB (printed-circuit-board) real estate, and providesindependent logic inputs for the high- andthe low-side drivers, enablingflexibility for use in a variety of bothisolated- and nonisolated-power-supplytopologies.

As for the future of GaN powerdevices, International Rectifier andEPC have both preannounced products for release by year-end with drain-to-source blocking voltages of 600V.International Rectifier’s new device(Figure 3 ) will depart from its originalGaN-based power-converter stageand be a discrete switching device thatpairs a low-voltage silicon FET in a cascodeconfiguration in series with a GaNHEMT (high-electron-mobility transistor),meaning that “you get normally offoperation in a three-terminal devicewhose drive requirements are the same as a typical silicon-based power [device],”says Tim McDonald, vice president ofemerging technologies at the company.Further, you can drive it with standardgate drivers, with no special considerationsabout voltage limitations, overvoltage,or reliability, he adds. As for the future of GaN powerdevices, International Rectifier andEPC have both preannounced products for release by year-end with drain-to-source blocking voltages of 600V.International Rectifier’s new device() will depart from its originalGaN-based power-converter stageand be a discrete switching device thatpairs a low-voltage silicon FET in a cascodeconfiguration in series with a GaNHEMT (high-electron-mobility transistor),meaning that “you get normally offoperation in a three-terminal devicewhose drive requirements are the same as a typical silicon-based power [device],”says Tim McDonald, vice president ofemerging technologies at the company.Further, you can drive it with standardgate drivers, with no special considerationsabout voltage limitations, overvoltage,or reliability, he adds.

GaN-power-device vendor Transphormmade its first public announcementsat this year’s Applied PowerElectronics Conference. It plans to introducefully qualified 600V devices withon-resistances as low as 180 mΩ by theend of the year. The company uses bothsilicon and SiC substrates for its GaNdevices, building early versions of its partson SiC, which has a crystal structurethat’s closer to GaN, and troubleshootingthe process on SiC before it moves it tosilicon. The device structure will be similarto International Rectifier’s approach,cascoding a low-voltage silicon FET inseries with a high-voltage GaN HEMT(Reference 2).

“We looked at the problem of creatinga normally off device in GaNand decided that the state of the artfor GaN-gate construction limited themaximum positive voltage to 6V,” saysCarl Blake, Transphorm’s vice presidentof marketing. “We viewed this[limitation] as a serious problem, whichwould limit the practical use of high-voltageGaN devices, so [we] decidedto package two dice in a single packageusing the proven silicon technologyas the current controller and the newGaN as an improved voltage-blockingdevice.” This approach enables power-design engineers to use availablecontrollers and drivers and to focuson rapid performance improvement,he explains.

EPC has also preannounced 600VeGaN devices. According to EPC’sLidow, no fundamental limitation existsfor GaN devices’ blocking voltage, andGaN will rival and cost less than SiC for high-speed devices. It has come asa surprise to him that new applicationshave started using GaN simply as a silicon-MOSFET replacement. One suchapplication is RF envelope tracking,he says. Lidow likens this applicationto a dc/dc power supply except that itfollows the modulated signal up anddown, creating an envelope that alwaysbiases the transistor a couple of voltshigher than what it needs to generatean amplitude signal. This techniqueeliminates the heat that goes to waste inthe transmitter. He says that the use ofGaN enables power supplies to changetheir voltages when operating at 100MHz, adding, “The most expensive partof an RF system is the transmitter, andnow it has unloaded all of the wasteenergy that’s heating it up, allowing youto either pump out twice the power oreliminate some heat sinks.”

Lidow sees solar microinverters asother likely applications. The approximately250W inverters find use inhomes or businesses and compete inefficiency with 3-kW central inverters,which are currently more efficient thanthe lower-power, lower-voltage microinverters.The microinverters’ simultaneousrequirements of high frequency,high voltage, and high power make theuse of GaN HEMTs attractive.

Many industry participants believethat the second-source market for thesenew parts will mimic the early stagesof the silicon MOSFET market andthat many companies will jump in withtheir own takes on the technologies. Forexample, Microsemi plans to partnerwith EPC to develop a high-reliabilityversion of EPC’s GaN HEMT.

You can reachTechnical EditorMargery Connerat 1-805-461-8242and .