High Power Semiconductors for Medium Voltage Wind Applications · 2009. 7. 16. · permanent magnet motors and, eventually, also the use of medium frequency conversion from catenary

WIND POWERHigh Power Semiconductors forMedium Voltage Wind Applications

ISSUE 5 – JULY/AUGUST 2009

Also inside this issueOpinion | Market News | PCIM 2009 Review | Power Semiconductors| Products | Website Locator

p01 Cover.qxd:p01 Cover 29/06/2009 09:09 Page 1

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CONTENTS

Power Electronics Europe Issue 5 2009

3

Editor Achim ScharfTel: +49 (0)892865 9794Fax: +49 (0)892800 132Email: [email protected]

Production Editor Elaine GladwellTel: +44 (0)1322 380057

Editorial/Advertisement Administration Clare JacksonTel: +44 (0)1732 886495Fax: +44 (0)1732 886149

Circulation Manager Anne BackersTel: +44 (0)208 647 3133Fax: +44 (0)208 669 8013

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Eastern US Karen C Smith-Kerncemail: [email protected] US and CanadaAlan A KerncTel: +1 717 397 7100Fax: +1 717 397 7800email: [email protected]

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Publisher Ian AtkinsonTel: +44 (0)1732 886495Fax: +44 (0)1732 886149Email: [email protected]

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Circulation and subscription: Power ElectronicsEurope is available for the following subscriptioncharges. Power Electronics Europe: annual chargeUK/NI £60, overseas $130, EUR 120; single copiesUK/NI £10, overseas US$32, EUR 25. Contact:Techmedia International Ltd, Kildonan, St Mary’sRoad, Wrotham, Kent TN15 7AP, Great Britain. Tel: +44 (0)1732 886495. Fax: +44 (0)1732886149. Refunds on cancelled subscriptions willonly be provided at the Publisher’s discretion, unlessspecifically guaranteed within the terms ofsubscription offer.

Editorial information should be sent to The Editor,Power Electronics Europe, PO Box 340131, 80098Munich, Germany.

The contents of Power Electronics Europe aresubject to reproduction in information storage andretrieval systems. All rights reserved. No part of thispublication may be reproduced in any form or by anymeans, electronic or mechanical includingphotocopying, recording or any information storageor retrieval system without the express prior writtenconsent of the publisher.Origination: Elaine GladwellPrinted by: Garnett Dickinson UK.ISSN 1748-3530

PAGE 16

Photovoltaic Converter TopologiesSuitable for SiC-JFETs SiC semiconductors offer very interesting characteristics and can be considered asa future trend in photovoltaic converter technology. The vertical JFET is anexample of a very promising device, mainly due to its relative structural simplicity. Nevertheless, its inherent normally-on characteristic calls for speciallytailored topologies. The article is a short version of PCIM’s best paper. Benjamin Sahan, Samuel V. Araújo, Thomas Kirstein, Lucas Menezes,Peter Zacharias, Kompetenzzentrum für Dezentrale ElektrischeEnergieversorgungstechnik (KDEE), University of Kassel, Germany

PAGE 19

Pros and Cons for Silicon CarbideMOSFETs, JFETs and BJTs The most commonly pursued switches in SiC are compared in terms of deviceperformance, reliability, and cost of manufacturing. The DMOSFET structure offersthe most features, but it can be more expensive to manufacture. Normally-onJFETs can be manufactured at a lower cost and provide excellent characteristics,but will have difficulty winning wide acceptance in the power electronics field.Normally-off JFET devices can also be produced at a lower cost, but sensitivity tomaterials and processing requirements may result in low yields, which can negatethe lower fabrication cost, and provide relatively poor performance compared toother structures. BJTs offer good performance and lower cost of manufacturing,but device stability is yet to be resolved. John W. Palmour, Sei-Hyung Ryu,Qingchun (Jon) Zhang, and Lin Cheng, Cree, Inc.; Durham, USA

PAGE 23

GaN Based Power Conversion GaN based power devices such as HEMTs promise to deliver a figure-of-merit(FOM) performance that is at least an order of magnitude better than state-of-the-art silicon MOSFETs. In addition to reviewing the distinct advantages of a newGaN-on-Si technology platform, this article will also describe how DC/DCconverters built using the new GaN technology platform will enable a new era inhigh frequency, high density, highly efficiency power conversion solutions.Michael A. Briere, ACOO Enterprises LLC/International Rectifier, USA

PAGE 31

Product Update A digest of the latest innovations and new product launches

PAGE 33

Website Product Locator

High PowerSemiconductors forMedium Voltage WindApplicationsWith the increased power levels of modern windturbines, medium voltage generation and powerconditioning have become a viable solution for thistraditionally low voltage application. The continuous development of Bipolar and BiMOSproducts enables the medium voltage convertermanufacturer to select the power semiconductorbased on the application requirements, rather thentrying to optimise the converter around a given deviceor technology. Full story on page 28.

Cover picture supplied by EWEA/ABB Semiconductors

COVER STORY

PAGE 6

Market NewsPEE looks at the latest Market News and company

developments

PAGE 14

PCIM 2009 Review -Higher Efficiencythrough InnovativePower SemiconductorsRenewable energies and transportation can benefitheavily from the application of innovative powersemiconductors, as the keynotes and the best paperof PCIM 2009 have illustrated.

p03 Contents:p03 Contents 29/06/2009 12:25 Page 3

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04_PEE_Issue 5 _2009:04_PEE_Issue 5 _2009 29/06/2009 09:19 Page 1

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OPINION 5


Achim ScharfPEE Editor

A suitablequote as alead in to

the editorsopinion


Europe is a world leader

in the field of Power

Electronics, with well-

known research and

industrial companies and

many large academic

laboratories in all main

European countries. In

the past 26 years, the

EPE Conference has

grown to become the

largest in this field, and it draws regularly the foremost technical

contributors from industry and academia worldwide. The EPE 2009

conference, the 13th International European Power Electronics

Conference and Exhibition, to be held in Barcelona from September

8 to 10, will feature more than 750 papers for presentation.

Renewable energies and a large number of other issues at the

cutting-edge of power electronics will be addressed.

‘Potential of SiC and other Wide Bandgap Semiconductors’ is the

subject of the third SiC User Forum, organised by ECPE, to be held

also in Barcelona from September 10 – 11, complementing the EPE

conference. SiC application examples come from electric drives,

including converters for transportation and power supplies including

inverters for renewable energy. Additionally, insights into recent SiC

and GaN material and device technology - which is the base for

future system development - will be given. This has already been

performed at PEE’s PCIM 2009 Special Session ‘Wide Bandgap

Materials and Devices’, and its sponsored Best Paper Award for the

paper ‘Photovoltaic converter topologies suitable for SiC-JFETs’.

Silicon Carbide (SiC) is characterised by electrical field strength

almost nine times higher than normal Si, allowing the design of

semiconductor devices with very thin drift layers and, as a

consequence, low on-state resistance and reduced switching losses.

In other words, such characteristics can be translated into the

possibility of operating at higher blocking voltages with reduced

losses. Silicon Carbide will also become the semiconductor material

of choice for railways. Energy efficiency is a major strategic asset of

modern traction and auxiliary equipment. Increasing energy prices

will lead to even higher focus on overall energy efficiency of

railways. Key elements of further energy efficiency improvement are

energy storage devices and their optimal use in the traction system,

permanent magnet motors and, eventually, also the use of medium

frequency conversion from catenary voltage to traction system

voltage levels. Thus, we report on the progress in Silicon Carbide

and also Gallium Nitride in this issue.

In December 2008, the European Union decided to launch the

‘Renewable Energy Road Map’, including the target of producing

20% of total EU energy consumption from renewable energy

sources by 2020. The German BEE (Bundesverband Erneuerbare

Energien) even announced a scenario in January 2009 with a share

of 47% of renewable electrical energy for Germany by 2020. One

important and increasing part of for this renewable share will be

photovoltaic (PV) power. PV has some special characteristics, which

predestinate it to play a major role in the concert of different

renewable energy sources. This issue was evaluated – using

Germany as reference - by the study ‘The Role of Solar Power

Generation in Future Energy Provision Structures – What Value has

Solar Power’. The study provides evidence that wind and solar

energy complement one another in an ideal way, because the solar

peak production in summer correlates with the maximum wind

power production in winter. This demonstrates the need for

increasing solar energy capacity with more and more wind power

plants being installed. Large scale offshore wind farms far from

shore will require innovative energy converters with high power

capacity, rugged design and high efficiency. Power electronic

converters will be key components in the transmission of power to

the distant shore, especially in hybrid, multi-terminal AC/DC

networks. Power electronic converters will have to be adapted to fit

the harsh conditions and strict requirements of offshore installations

and to match the voltage level of the transmission grid. In multi-

terminal networks voltage source converters will have advantages

over traditional grid commutated current source converters.

Achieving high voltage level is a universal challenge for voltage

source converters with IGBT type switching devices. For offshore

wind applications both ruggedness and low losses are of vital

importance, making design for high reliability and high efficiency two

likewise important challenges. In modern wind power applications,

the requirements from the grid operators regarding net quality make

the inclusion of power electronics almost mandatory. Depending on

the applied configuration, the power electronics will directly control

between 20 and 100% of the generated power, where the lower

percentage figures are valid for systems using a Doubly Fed

Induction Generator (DFIG). On the other hand, by using full power

conversion, an electrical decoupling from the generator side to the

line side can be achieved which, in many cases, is a viable solution

although the converter itself will be much larger. To accomplish this,

high power semiconductor devices are needed, but to ensure that

they perform as required, the topology in which they are used and

their ratings must be carefully selected. Our cover story ‘High Power

Semiconductors for Medium Voltage Wind Applications’ addresses

these issues.

Enjoy reading!


Focus on Renewable Energies andInnovative Power Semiconductors

p05 Opinion.qxd:p05 Opinion 29/06/2009 09:36 Page 5

6 MARKET NEWS

Issue 5 2009 Power Electronics Europe

Europe is a world leader in thefield of Power Electronics, withwell-known research andindustrial companies and manylarge academic laboratories inall main European countries. Inthe past 26 years, the EPEConference has grown tobecome the largest in this field,and it regularly draws theforemost technical contributorsfrom industry and academiaworldwide. “Spain has verygood standards in renewableenergies production, powerindustrial applications of powerelectronics and advancedresearch groups in these fields.The Global Energy Councilranked Spain third in terms ofoverall installed wind powercapacity at about 20,000MW,just behind the USA andGermany. According to theNational Commission of Energy(CNE), our country installedmore than 3,000MW ofphotovoltaic power last year“,Honorary Chairman Prof. JoanPeracaula stated. Power Electronics, as an

enabling technology, isbecoming more and more

important and is the basis formany industrial processes, forthe rational use of the energy,for new technologies inindividual and masstransportation, areas that arerapidly growing requiring newconcepts in order to fulfil cost,reliability, and miniaturization,as well as environmentalrequirements. Theimprovements in PowerSemiconductor, together withnew advanced topologies andembedded systems, are pushingPower Electronics towards highswitching frequency andsmaller, cheaper, and moreefficient realisations, openingpossibilities for newapplications. Due to the newrules published by the EC on theelectrical energy production,transport and distribution, andalso to the technical problemsarising from the interconnectionof different kinds of distributedenergy production systems, highprecision and reliablecontrollers are needed. Toaddress these ideas, the EPE2009 will have special tailoredsessions comprising top

industrial expert presentationsand round table meetings todiscuss in depth and focus onfuture developments insemiconductors, materials andcomponents, topologies andembedded systems. TheConference will complement theregular program, with new oremerging topics of particularinterest to the power electronicssystems community that mayalso cut across and beyonddisciplines traditionallyrepresented.Thus, renewable energies and

a large number of other issuesat the cutting-edge of powerelectronics will be addressed atEPE 2009. “The Conference willbe held at the Palau deCongressos/Fira de Barcelona,the perfect venue for largeconferences, which is located inone of the town’s most visitedplaces, due to the proximity ofthe Montjuïc Mountain gardensand important museums. Ninetutorials will be organised onMonday. During the three daysof the main conference, we willhave about 150 papers forlecture sessions developed

during the morning in sixparallel tracks. In the afternoon,dialogue sessions will takeplace. Fields like drives,automotive, custom powersystems and new devices willbe on the focus of the lectureand dialogue sessions. In thelate afternoon, differentworkshops will be held onpower electronics in powersystems, and education“,Conference Chairman Prof.Enrique J. Dede explains. EPE2009,co-organised with ECPE,will offer several industrialsessions, running in parallelwith the regular conferencesessions. Every day, one ‘actualtopic’ will be covered, withkeynotes, coming mainly fromtop experts from the academia,invited lecture sessions withspeakers mainly from theindustry, and aworkshop/roundtablediscussion planned by the endof the day. Keynotes andlectures will highlight the topicthat will be presented duringthe day.

www.epe2009.com

EPE 2009 in BarcelonaThe EPE 2009 conference, the 13th International European Power Electronics Conference and Exhibition, to be held in Barcelona from September 8 to 10, will feature more than 750 papers forpresentation.

Potential of SiC and other Wide BandgapSemiconductorsThis is the subject of the third SiCUser Forum organised by ECPE to beheld in Barcelona from September10 – 11.After the previous Silicon Carbide

(SiC) User Forums organised by ECPE,new power electronic systems withwide bandgap (WBG) componentsand new devices have been reported,which are based on SiC or recently,also on GaN (Gallium Nitride) material.Time has thus, come to continue

the exchange between expertsinvolved in converter and devicedevelopment. Application examplescome from electric drives, includingconverters for transportation andpower supplies including inverters forrenewable energy. Additionally,insights into recent SiC and GaNmaterial and device technology -which is the base for future systemdevelopment - will be given.Internationally renowned experts

have been invited to give an overviewin keynotes, to explain in depth theirresearch and development work intechnical presentations, and to sharetheir knowledge in discussion forums. The SiC User Forum is this way

intended as a platform to shareexperience and ideas, to discuss andfind out which power electronicsystems are predestinated for usageof wide bandgap devices, and how toappropriately design-in those novel,

almost ideal, but also challengingcomponents. SiC User Forum 2009 isscheduled to take place right afterEPE conference 2009 in Barcelona.Prof. Andreas Lindemann (Otto-von-Guericke-University Magdeburg,Germany) will chair the event,together with Prof. José Millan(Centro National de Microelectronica)and Thomas Harder (ECPE).

www.ecpe.org

p06-12 Market News:New Market News Template 29/06/2009 09:51 Page 6

http://www.epe2009.comhttp://www.ecpe.org

Worldwide semiconductorrevenue in the first quarterdeclined to $44.3 billion, down18.8% from $54.5 billion in thefourth quarter, and a decline of33.8% from $66.8 billion in thefirst quarter of 2008. Revenuewas down 36% from the start ofthe present sharp downturn inthe third quarter of 2008. Evenprior to the downturn, thesemiconductor industryexperienced an extendedperiod of lethargy. Quarterlysemiconductor revenue peakedsix quarters ago in the thirdquarter of 2007. “Of the 130+semiconductor suppliers

tracked on a quarterly basis,only six managed to expandtheir revenue in the firstquarter, compared to the fourthquarter of 2008”, said iSupplianalyst Dale Ford. “Even amongthese six suppliers, fourincreased their revenue by only1 to 3%”. Meanwhile, everymajor region of the worldsuffered double-digitpercentage declines insemiconductor revenue in thefirst quarter, compared to thefourth. “Although the firstquarter is typically weak for theglobal semiconductor industry,the sharp declines in

semiconductors during thatperiod, and in the fourthquarter of 2008, reflect theimpact of the global economicdownturn on the worldwidechip business”, Ford observed.Companies headquartered in

the Americas fared the bestduring the downturn, with acombined revenue decline in USdollars of 30.8% since the thirdquarter of 2008. European-headquartered companiessuffered the worst decline, withtheir combined revenues fallingby 44.5% during the sameperiod. Japanese suppliers farednearly as badly as their

European counterparts,suffering a contraction of 43.5%.On a positive note, iSuppli’s

latest semiconductor forecastpredicts that the first quarter of2009 will represent the bottomof the semiconductor marketdecline and that revenues inthe fourth quarter of 2009 willexceed those in the fourthquarter of 2008. On asequential basis, revenue willrise by 7.1% in the secondquarter, by 10.4% in the thirdquarter and by 4.9% in thefourth quarter.

www.isuppli.com/news.aspx

MARKET NEWS 7


From Paris to Tokyo, from the biggest broad-line suppliers to the most modest boutiques, from high-fliers

serving hot markets to low-profile players plodding away in slow-growth segments, virtually no

semiconductor company was immune from the miserable conditions in the global semiconductor

industry during the first quarter, according to iSuppli Corp.

No Escape for Global Chip Suppliersin Miserable First Quarter


http://www.isuppli.com/news.aspx

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Digital power conversion is entering its third generation,and sales of digital controller ICs have achieved a significantmilestone with cumulative shipments exceeding five billionunits by 2010. As a result of the introduction of the newthird-generation controller ICs, the digital landscape will berecast in 2009, market researcher Darnell expects.“The digital power market is being driven by a growing

number of factors and is enabling new power architecturesthrough digital control techniques; the migration of digitalcontrol into nearly all application segments; the adoption ofdigital control in high-volume and cost-sensitive consumerdevices; the realisation of adaptive control techniques incost-effective controllers; the shift from predictive toproactive real-time power systems diagnostics with digitalpower; and neural-based digital controller chips that willresult in power supplies that can ‘learn’ and improve theirperformance over time”, stated analyst Linnea Brush.What does this mean for power supply and digital

controller IC companies? First, the digital powermanagement and control market is not only alive, it is justentering its adolescence. Its biggest growth spurts areimmediately ahead, and maturity is still years away. “This isalways an exciting time for any market, since thegroundwork has already been established and companiesdon’t have to ‘make a case’ for the technology anymore.Even though the major players are established, the way isnow opened for companies to differentiate themselves in

MARKET NEWS 9


Digital Controller Market is Adolescentspecific application segments and product lines”,Brush said. Digital power management andcontrol is on the cusp of widespreadimplementation, and despite a slower economy,the technology developments are not only likelyto continue, but are likely to enable the veryefficiencies and cost-effectiveness that customersare looking for. The next couple of years shouldsee the emergence of an even more-establishedmarket for digital control products. Like switch-

mode regulation, digital control is not a limitedtechnology. It has applications in embedded andexternal AC/DC power supplies, isolated andnon-isolated DC/DC converters, telecomrectifiers, and lighting ballasts. Most importantly,digital has penetrated nearly all applicationsegments, from high-performance computing tohigh-volume consumer products.

www.darnell.com

Texas InstrumentsAnnounces LicenseAgreement with Power-OneTexas Instruments announced an agreement with Power-One, which expands the market for TI’s analog and digitalpower management products that support the PMBusprotocol. The non-exclusive Field of Use licenseagreement for Power-One’s digital power technologypatents will benefit original equipment manufacturerswho use TI’s point-of-load controllers. The license alsoextends to TI power modules, but not to other merchantpower supplies using TI controllers. The PMBus standard specification, developed by a

consortium of 40 power supply, telecommunicationsequipment and semiconductor providers, defines a digitalcommunication protocol that enables power converters tobe configured, monitored and maintained according to aset of more than 100 commands. PMBus providesincreased reliability and system intelligence. Designerscan set a power supply’s operating parameters, monitoroperation and perform corrective measures in responseto faults or if the power shuts a system down. TI offersseveral power management controllers that supportPMBus. For example, the four-output, multi-phaseUCD9240 and dual-output UCD9220 power systemcontrollers are fully configurable via an easy-to-usegraphical user interface (GUI) for monitoring, control andmanagement for DC/DC point-of-load power conversion.

www.ti.com/digitalpower-pr


http://www.darnell.comhttp://www.ti.com/digitalpower-pr

IMS Research expects the overallpower discrete market to contractby over 25%, or $2.8 billion, in2009. Power rectifier revenues areanticipated to fall by almost 30%and those of power MOSFETs byalmost 26%. Revenues for powerdiscretes are not expected toregain the level of 2008 until2014. The global powersemiconductor market was worth$13.9 billion in 2008, 0.7% higherthan in 2007.However, the global power

module market grew over 16% in2008, to be worth over $3 billion.Most revenue growth came fromstandard IGBT modules and IPMs.Power module sales to therenewable energy sector grew by83% in 2008, the largestpercentage growth of allapplications. The renewable energysector is forecast to dip slightly in2009, but will continue to growstrongly over the medium term,because of aggressive goals onrenewable energy in Europe, USAand China for solar and wind powergeneration. Standard IGBT modulesaccounted for 45% of powermodule revenues in 2008.Industrial motor drives remain thelargest application sector within thepower module market in 2009,worth almost $1.5 billion.Nevertheless, the motor drivemarket is envisaged to be the worsthit application for power modules in2009. IMS Research believes that

shipments of low voltage motordrives in Q1, 2009 have droppedby some 25% from Q1, 2008.There are several reasons behindthis downward revision. Mostnotably, machinery markets inEurope, Asia, and the Americas areperforming worse than predicted,with many OEMs experiencingdramatic declines in order intakeduring the last quarter of 2008 andthe first quarter of 2009. However,many suppliers are now starting tosee a slow increase in orders fromthe market and a gradual pick-up indemand is anticipated for Q3.Another reason for the market

contraction is due to ongoingproblems with capital lending. Tightlending conditions are having animpact on previously strong growthsectors such as renewable energy.Several component suppliers andwind turbine manufacturersexhibiting at the HannoverIndustrial Automation Showmentioned that while demand forwind projects is still abundant,many of the projects haveencountered significant delays dueto restricted lending by banks. Thishas, in turn, caused problems withsecuring funding for large scale andvery costly wind and solar farmprojects. Also global uninterruptible

power supply (UPS) revenues inthe first quarter of 2009 fell bynearly 25% from the same quarter

last year, to under $1.5 billion. Thismarks the second consecutivequarter of year-on-year decline.Regionally, EMEA showed thesharpest pullback because ofsevere economic difficulties inmost countries, but notably inSpain and Russia. This wasexacerbated by an unfavorablecurrency exchange with the Euroand Pound weakening significantlyagainst the US dollar. TheAmerican and Asian markets werenot far behind, all falling by over20%. In 2008, Infineon Technologies

remained the leading supplieroverall to the global powersemiconductor (discrete andmodule) market for the sixthconsecutive year. It increased itsshare to 10.2%. Vishay led in

power discretes, just overtakingSTMicroelectrics, increasing itsshare by 0.5% in a decliningmarket. “Although the global powerdiscrete market declined by over$340m in 2008, some suppliers,such as Infineon and Vishay,performed admirably tooutperform the market”, IMSAnalyst Josh Flood comments.Mitsubishi remained the leadingglobal supplier of power modulesin 2008, accounting for over 28%of the market. However, thestrength of the Yen against the USDollar in 2008 has played asignificant part in the marketrankings. Hitachi performedimpressively in 2008, leaping from10th to 6th position.

www.imsresearch.com

Power Semiconductor MarketDeclines Strongly

10 MARKET NEWS



http://www.imsresearch.com

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Train makers have perceived SiliconCarbide (SiC) as a highly valuabletechnology for their next-gen powerproducts (higher breakdown voltage,reduction of passives volume,system size shrinking, weightreduction, simplification andreduction of cooling systems,decreasing of transformer size andweight…) However, the current SiCindustry is focusing on the 600V-1200V range applications to benefitfrom the huge potential market size.3.3, 6.5 and even 10kV+ havealready been demonstrated andtransportation industry is nowexpecting to find commercialproducts in a very near future,market researcher YoleDeveloppement predicts.

Mitsubishi Electric, Infineon andHitachi are the top three companiessupplying IGBT-based powermodules in the 1.7, 2.5, 3.3, 4.5 and6.5kV range for the rail transportationbusiness. “We do estimate that thisbusiness will generate about $175million revenue in 2009. Thisbusiness is benefitting from quitegood dynamism, thanks to theopening of new markets in China,Eastern Europe and SouthernAmerica. We anticipate an averagegrowth-rate of 5 to 6% to 2012“,analyst Philippe Roussel stated.

SiC business is now ramping upwith an estimated 2008 market sizeof about $22 million at device level,mainly thanks to the SiC Schottky

diode (SBD) business. Leading SiCdevice makers such as Cree orInfineon are sharing the market withthat product, but new entrants(Mitsubishi Electric, Rohm, Denso,Fuji, Hitachi, STM, Microsemi…) arechallenging them, developing newproducts and related technologies.“The Holy Grail is now to get areliable and affordable SiC switch.MOSFET is the most studied devicebut BJT and JFET are exhibiting verypromising results, and some SMEsare proposing very pertinent demoproducts (SemiSouth, TranSiC,GeneSiC…). The current applicationsthat shape this market are currently

in the 600 to 1200V and in the 6 to20A maximum range“, Rousselanalysed.

The main train makers such asBombardier, Alstom, MitsubishiElectric, Siemens, Toshiba, Ansaldo-Breda, GETS, Hitachi or Fuji Electricare requesting much higher voltage.They typically address systems witha minimum of 1.7kV with a productrange going through 3.3kV and upto 6.5kV (and even 10kV+ for somespecific solutions). The currentstatus of the SiC technology hasalready proved that this voltagerange is reachable, but nocommercial product is yet proposed

on the market. “We see three mainreasons explaining it: The currentSiC switch technologies are stillsuffering from reliability issues andreduced life-time; the thick epitaxiallayer (20μm and more) needed tohandle the depletion region in suchhigh voltage devices is not perfectlystabilised from a technology point-of-view and is very cost-sensitive tothe epitaxial growth rate; and thetotal available market of thissegment is perceived as insufficientto drive important developments“,Roussel concluded.

www.yole.fr

SiC Devices in Rail Transportation

12 MARKET NEWS


Joint Venture for Electric and HybridVehiclesMagna Electronics, anoperating unit of MagnaInternational Inc. andSemikron, announced theformation of a 50/50 jointventure to develop andproduce power electronics forfuture electric and hybridvehicle applications.

“This joint venture withSemikron, a global player

across multiple industries,provides us with anexperienced and strong partnerin the field of powerelectronics”, said MatthiasArleth, VP Magna ElectronicsEurope. “In combination withour experience, we are wellpositioned to anticipate thechallenges of the market andexceed customer requirements

for electric and hybrid vehiclecomponents and systems”.“With Magna Electronics wehave a partner which is a well-known and well-respectedsupplier in the automotiveindustry. Magna’s experienceand capabilities will enable usto make best use of our know-how and our innovations in thissector of the industry”,

commented Peter Frey, GeneralManager of SemikronInternational. ”Powerelectronics is a key technologyto assure future mobility withelectric and hybrid vehicles, the answer to increasingemissions and limited naturalresources”.

www.semikron.com


http://www.semikron.comhttp://www.yole.fr

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14 PCIM 2009 REVIEW


In December 2008, theEuropean Union decided tolaunch the ‘Renewable EnergyRoad Map’, including thetarget of producing 20% oftotal EU energy consumptionfrom renewable energysources by 2020. The GermanBEE (BundesverbandErneuerbare Energien) evenannounced a scenario inJanuary 2009 with a share of47% of renewable electricalenergy for Germany by 2020.One important and increasingpart of this renewable sharewill be photovoltaic (PV)power. PV has some specialcharacteristics, whichpredestinate it to play a majorrole in the concert of differentrenewable energy sources.This issue was evaluated –using Germany as reference -by the study ‘The Role of SolarPower Generation in FutureEnergy Provision Structures –What Value has Solar Power?’.The study provides evidencethat wind and solar energycomplement one another in anideal way, because the solarpeak production in summercorrelates with the maximumwind power production inwinter. This demonstrates theneed of increasing solarenergy capacity with more andmore wind power plants beinginstalled.Solar energy will be one

pillar of the energy supply ofthe future. Grid-connectedphotovoltaic systems will thus– according to EPIA’s figures -generate more than 12% of theelectrical energy by 2020. Themaximum yield of a PV plantcoincides with the peak ofdaily energy demand.Therefore, and due to thedecentralised nature of PV

energy, a large amount of PVpower can be easily integratedinto the grid without expensivemeasures such as additionaltransmission grid lines.However, in future powersupply networks, renewableenergy sources must beintegrated into grid control.Consequently, solar inverterswill have to be able tocontribute to stabilising andsupporting grid operation.Active power has to be limitedwhen necessary, reactivepower has to be provided ondemand, and systems must notdisconnect under failureconditions such as voltagedips.

Best paper on SiC and PVThe paper covering SiC and

PV received the Best Paper

Award. Power ElectronicsEurope has sponsored andhanded over for the secondtime the Best Paper Award atthe PCIM 2009 openingceremony. The awardee willparticipate at PCIM China 2010including flight andaccomodation. The best paperhas been selected by the PCIMConference directors and thewinner is Benjamin Sahan fromUniversity of Kassel (Germany)with the paper ‘Photovoltaicconverter topologies suitablefor SiC-JFETs’. Silicon Carbide (SiC) is

characterised by electrical fieldstrength almost nine timeshigher than normal Si, allowingthe design of semiconductordevices with very thin driftlayers and, as a consequence,low on-state resistance and

reduced switching losses. Inother words, suchcharacteristics can be translatedinto the possibility of operatingat higher blocking voltages withreduced losses. Increasedreliability due to its robustness,especially against temperatureand cosmic radiation-inducedfailure are additional highlightsof this new technology.These characteristics are

especially interesting whenapplied in photovoltaicconverters. There, efficiency isstill one of the main marketdrivers in the industry. Today,enhancing the PV inverterefficiency by 1% could yield upto 45€/kWp…97€/kWpadditional profit after 10 yearsof operation. For this reason, PVinverter technology rapidlyimproved during the last

Higher Efficiency throughInnovative Power Semiconductors

Benjamin Sahan (middle) from University of Kassel (Germany) was awarded with the BPA handed over by PCIM organiser Udo Weller(left) and PEE editor Achim Scharf

Renewable energies and transportation can benefit heavily from the application of innovative powersemiconductors, as the keynotes and the best paper of PCIM 2009 have illustrated.

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PCIM 2009 REVIEW 15


decade, and a peak efficiency of99% will soon be achieved.From that point on, furtherincrease of efficiency is nolonger cost-effective. “As afuture trend, SiC offers thepossibility of operating at higherswitching frequencies withoutsignificant prejudice on theefficiency, which leads to thepossibility of reducing the sizeof passive components andconsequently the cost andvolume of the circuit”, Sahanstated.

SiC for PVIn order to achieve the 12%

share of electrical energy, thecost of PV energy mustdecrease significantly withinthe next decade. One approachfor price reduction of PVgenerated electrical energy is tomaximise the PV systemefficiency. In the last 20 years,the efficiency of PV invertersincreased from 91% up to 98%today, which reduces thenecessary generatordimensions for the same ACoutput power by 7.5%. “Withnew semiconductors (forexample SiC), the efficiencymay be increased to 99%. Thiswill reduce not only the PVgenerator dimensions for agiven rated power of the PVsystem, but also lead to ahigher power density and

therefore to less mechanicalequipment and thus reducecosts again. Therefore,increasing efficiency is aconstant challenge in the fieldof PV inverter development”,stated Andreas Falk from SMASolar Technologie AG/Germanyin his keynote ‘Efficiency andGrid Compatibility ofPhotovoltaic Inverters – Stateof the Art and Future Trends’.The progress from design to

efficiency and other trends in PVinverters was driven bysignificant improvements in thefield of semiconductors,magnetic components andcontroller hardware in the past,and will initiate furtherdevelopments in various fieldsof power electronics in thefuture. PV inverters cover asignificant market share of thepower electronics industry todayand will increase this share until2020.

SiC for railwaysSilicon Carbide will also

become the semiconductormaterial of choice, as MichaelFröhlich from BombardierTransportation in Mannheim/Germany in his keynote‘Technology trends in railwaytraction’ pointed out.Energy efficiency is a major

strategic asset of moderntraction and auxiliary

equipment. Increasing energyprices will lead to even higherfocus on overall energyefficiency of the railway. Keyelements of further energyefficiency improvement areenergy storage devices and theiroptimal use in the tractionsystem, permanent magnetmotors and, eventually, the useof medium frequencyconversion from catenaryvoltage to traction systemvoltage levels also. In addition,driving style managementsystems support the efficientuse of the trains during serviceoperation.The evolution from GTO to

IGBT technology was one ofthe major steps leading tolower costs, reduced weightand size. On thesemiconductor side, this trendis still ongoing. From a long-term perspective, theintroduction of SiC will lead toa huge step in higherintegration levels. The veryhigh junction temperatures ofSiC elements will lead to ahuge technology step in thecooling area. Considerablyhigher temperature differencesbetween heatsink and ambienttemperature will lead to muchhigher power densities. On the

other hand, the highertemperatures inside theconverter will requiretechnology changes on allinverter components. Forexample, Gate Drive Units willhave to operate reliably inmuch higher temperatures. Thesame goes for all convertercomponents such as capacitorsor busbars.Already today, IGBTs with

higher junction temperatures of150 to 175°C are in theintroduction phase. These IGBTswill already allow higher outputpower in actual converterdimensions.“Technology trends in

component basis such assemiconductors with SiliconCarbide in the long-term, orIGBTs with higher junctiontemperatures in the short-term,could be used for cost, size, andweight reduction. Size andweight reduction on tractionequipment enables new vehicleconcepts such as hybrid trains,which require heavycomponents such astransformers and diesel powerpacks on board one train”,Fröhlich said.

AS

www.pcim.de“With new semiconductors such as SiC, the efficiency of photovoltaic converters maybe increased to 99%”, stated Andreas Falk from SMA Solar Technologie

“Technology trends in component basis such as semiconductors with Silicon Carbide in thelong-term, or IGBTs with higher junction temperatures in the short-term, could be usedfor cost, size, and weight reduction”, pointed out Michael Fröhlich from BombardierTransportation

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16 POWER SEMICONDUCTORS www.uni-kassel.de


Photovoltaic Converter TopologiesSuitable for SiC-JFETsSiC semiconductors offer very interesting characteristics and can be considered as a future trend inphotovoltaic converter technology. The vertical JFET is an example of a very promising device, mainly due toits relative structural simplicity. Nevertheless, its inherent normally-on characteristic calls for specially tailoredtopologies. The following is a short version of PCIM’s best paper. Benjamin Sahan, Samuel V. Araújo,Thomas Kirstein, Lucas Menezes, Peter Zacharias, Kompetenzzentrum für DezentraleElektrische Energieversorgungstechnik (KDEE), University of Kassel, Germany

Silicon Carbide (SiC) is characterised byelectrical field strength almost nine timeshigher than normal Si, allowing the designof semiconductor devices with very thindrift layers and, as a consequence, low on-state resistance and reduced switchinglosses. In other words, such characteristicscan be translated into the possibility ofoperating at higher blocking voltages withreduced losses.

These characteristics are especiallyinteresting when applied in photovoltaic(PV) converters. There, efficiency is still oneof the main market drivers in the industry.For this reason, PV inverter technologyrapidly improved during the last decade, asa peak efficiency of 99% will soon beachieved (Figure 1).

As for SiC transistors, the vertical JFET is considered favorable because it

has a relatively simple structure.Nevertheless, specially tailored powerelectronic architectures are required forthis technology, as the device isinherently normally-on and has quitedifferent characteristics when compared with conventionalsemiconductors; namely pinch-offvoltage, gate drive units and transientcharacteristics. An alternative would bethe operation in cascade with a low-voltage MOSFET.

Properties of SiC VJFETThe junction field effect transistor

(JFET) is the most simply built-upunipolar transistor from the group of fieldeffect transistors and corresponds inconstruction to a modified diode. Typesemploying SiC are available only with thebase material doped with n chargecarriers. The corresponding JFETs consistof a n-type area surrounded by a p zone.To the n zone the connections are madefrom the drain and source, forming aconductive channel. The p zone formsthe gate electrode and, together with then channel, the referred pn diode. Themajority of JFETs are normally-ondevices.

As it is with MOSFETs, the highest

Figure 1: Benchmarkof commerciallyavailable PV inverters

Figure 2: Turn-on ofJFET, V = 400V

Figure 3: Turn-off ofJFET, V = 400V

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blocking voltage ratings at high currentsare achievable by a vertical structure(VJFET). When the gate-to-source voltageis zero (UGS = 0) the n channel behavesas a resistance. If the gate is connected toa negative voltage with respect to thesource, the conducting channel issqueezed by the extending blocking zone.With a maximum pinch of the channel, itpractically becomes non-conducting. Thistension is called pinch-off voltage (Up)and for SiC VJFETs is in the range of 16 to28V. At a certain gate to source voltage and

low UDS, the channels behaviour is ohmicwhile, above a so called knee-voltage, itbecomes close to a current source(current limiting characteristic). Anotheradvantage of SiC-JFETs is the possibility ofavoiding the use of gate oxide, which hadsome stability problems in the past. JFETs

also have promising perspectivesregarding manufacturing costs andruggedness.

SiC JFET versus Si IGBTTo evaluate the performance of the SiC

VJFETs, a switching test with acommutation cell was performed. ATrench IGBT rated at 1200V, 25A fromInfineon and a new prototype SiC VJFETrated at 1200V with a nominal Rdson of0.13Ω (T0-220) were compared. A SiCfreewheeling diode C2D10120D wasemployed. The junction temperature was 125°C and the blocking voltage and gate resistance were 450V and 4.1Ωfor the IGBT and 400V and 5Ω for theJFET.The turn-on energy losses were

actually higher for the JFET: 213µWs forthe JFET at 10A and 180µWs for theIGBT at 15A. One explanation for theJFET’s slow dv/dt at turn-on is the highinternal gate resistance of the prototype.Further technological improvements willmost likely lead to an improved turn-onperformance. Nevertheless, the JFET hadmuch superior turn-off behaviour, withonly 30µWs at 10A in comparison withthe 783µWs at 15A from the IGBT. Sucha large difference is mainly explained bythe tail current during the blockingtransient of the IGBT. As a conclusion,the total specific switching losses of this JFET were approximately 60% lessthan the Trench IGBT (see Figures 2 to5).

Inverter example for SiC VJFETsThe normally-on characteristic of JFETs

promotes its usage in PWM CurrentSource Inverters (CSI), since itguarantees that there is always a pathfor the DC-link inductor current. ThePWM CSI has several advantages, suchas voltage boosting capability, but at thecost that each switch needs anadditional series diode to providereverse voltage blocking. Thissignificantly increases the conductionlosses so that the application is limitedto inverters with very small PV voltage

Figure 4: Turn-on ofIGBT, V = 450V

Figure 5: Turn-off ofIGBT, V = 450V

Figure 6: Single HFswitch IndirectCurrent SourceInverter

Figure 7: 1kW testboard of the IndirectCurrent SiC SourceInverter


http://www.uni-kassel.de

18 POWER SEMICONDUCTORS


range. One also needs to take into account that, in case all switches arenormally-on, the bridge works as a diode rectifier and reverse biases thePV generator. The topology in Figure 6 is derived from a classical powerfactor correction (PFC) circuit and can be regarded as an Indirect CurrentSource Inverter. Due to its simplicity, it is often used in small power PVapplications.The basic principle is to control the DC link current in L1 with an unipolar

buck converter, thus using only one HF switch. The buck converter provides arectified sinusoidal current which is inverted by the low-frequency switchesS2…S5. Finally, a relatively small AC capacitor smoothens the DC link current.HF common mode leakage currents are also minimised, since the potential ofthe PV generator is zero for one halfwave and equals the grid voltage for theother one.A 1kW laboratory prototype of this topology, as presented in Figure 7,

was implemented to further evaluate the performance of normally-onSiC JFETs. A first test was made with an ohmic load and the electricefficiency (without auxiliary power supply) reached 98.6% at full loadand 98.9% at half load. These results (Figure 8) show a very positivetrend for future applications, especially considering that only one HFswitch is needed.

ConclusionThe photovoltaic branch with its special requirements offers an

interesting application area for SiC JFETs. These can be operated at highvoltage and higher switching frequencies without significant prejudice onthe efficiency, which leads to the possibility of reducing the size of passivecomponents and consequently, the cost and volume of the circuit.However, since their characteristics are quite different from conventionalsemiconductors, new design strategies are required. Normally-on switchescan be employed by using specially tailored topologies. Some of themfeature an indirect series connection of fast switches (SiC) with possiblyhigh voltage stress and conventional (Si) switches with lower voltagestress or low switching frequency. This can provide a measure to avoidshort circuit paths in case of failure even when the HF switch is normally-on. A 1kW laboratory prototype inverter was constructed to evaluate theperformance of SiC JFETs. A fairly high efficiency using just one JFET wasmeasured which gives a positive outlook for its future application in PVsystems.

Figure 8: Experimental results at P = 1kW; U1 = 400VDC, UN = 230VAC

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Pros and Cons for Silicon CarbideMOSFETs, JFETs and BJTsThe most commonly pursued switches in SiC are compared in terms of device performance, reliability,and cost of manufacturing. The DMOSFET structure offers the most features, but it can be moreexpensive to manufacture. Normally-on JFETs can be manufactured at a lower cost and provideexcellent characteristics, but will have difficulty winning wide acceptance in the power electronics field.Normally-off JFET devices can also be produced at a lower cost, but sensitivity to materials andprocessing requirements may result in low yields, which can negate the lower fabrication cost, andprovide relatively poor performance compared to other structures. BJTs offer good performance andlower cost of manufacturing, but device stability is yet to be resolved. John W. Palmour, Sei-HyungRyu, Qingchun (Jon) Zhang, and Lin Cheng, Cree, Inc.; Durham, USA

Silicon Carbide (SiC) power devices offertremendous potential over existing siliconpower devices due to the much higherbreakdown electric field in SiC. Several highvoltage structures have been developed, andthe advantages of using SiC power switchesare beginning to be demonstrated. The fourmost common switches demonstrated inSiC are the MOSFET, the lateral/vertical JFET,the pure vertical JFET, and the BJT. Structuresother than these four, which include IGBTsand GTO thyristors, also have beendeveloped. However, the effect of theforward biased junction on the forwardvoltage drop limits their use to applicationsrequiring 5kV or higher voltage ratings.Each of these four structures offers their

own benefits and drawbacks. Severalparameters have been used for the deviceperformance comparisons. Area normalisedon-resistance, or specific on-resistance(Ron,sp) is one of the most frequently usedparameters for device comparison. If thedevices are designed for a specified on-resistance (Ron), then the Ron,sp valuedetermines the corresponding chip size,which determines both the cost and thevalues of parasitic capacitances, whichinfluence the switching losses in return. Forcomparison in dynamic characteristics, thedrain capacitances, specifically the CGD(Gate to Drain capacitance), and saturationcurrents should be evaluated. The value ofCGD is important in determining the capacityof gate drive, and saturation current is anindicator of the capability of the switch todischarge parasitic capacitances.There is no direct method of comparing

the cost of the devices, because coststructures and technologies available aredifferent. However, the complexity of thefabrication process can be compared instead.In addition, a sensitivity analysis of device

parameters can be performed using 2Dsimulations, which can indicate the difficulty ofyielding devices with uniform characteristics.

DMOSFETs and lateral channel verticalJFETsFigure 1 shows a simplified cross-section

of a 4H-SiC power DMOSFET [1-3]. TheDMOSFET has been the MOSFET structureof choice in 4H-SiC, since it is easier toreproduce excellent results than with otherstructures. The MOS channels are formed onimplanted p-wells, and the channel length isdefined by the distance between the edgeof the p-well and the edge of the n+ sourceimplanted region. The fabrication process

includes multiple ion implantation steps andnon-self-aligned processes, making theprocess rather complex and expensive. Thelimitation on the DMOSFET structure is inthe MOS channel, which currently suffersfrom low effective channel mobility (µeff) [1].Because of the low µeff, a moderately highgate bias is needed to fully turn on thedevice, which can potentially lead toreliability issues at high temperatures.Figure 2 shows a half-cell cross-section

of a lateral channel vertical JFET(LCVJFET) in 4H-SiC [4]. The mainmotivation for the development of thisstructure is to overcome the limitations ofthe MOS system in 4H-SiC. The LCVJFET

Figure 1: Simplified cross-section of a1200V 4H-SiC DMOSFET

Figure 2: Simplified cross-section of a4H-SiC lateral channel VJFET

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structure is very close to that of theDMOSFET, except that an epitaxiallygrown n-channel replaces the inversionchannel in the DMOSFET, and anepitaxially grown p-layer replaces thepolysilicon gate. A normally-off (N-off)device has been demonstrated bycontrolling the doping concentration andthe thickness of the lateral channel layer[5]. However, emphasis has been placedon normally-on (N-on), depletion-modedevices, to be used in a cascodeconfiguration with low voltage siliconMOSFETs to form N-off switches [4].It should be noted that, although the

fabrication processes are more complexand expensive for DMOSFETs andLCVJFETs, both structures contain pn-junction body diodes. These diodes can beoptimised to serve as free-wheeling anti-parallel diodes or avalanche diodes. Thesefeatures can improve the robustness of thestructure and reduce the number ofcomponents required to build the system.Figure 3 shows a simplified cross-section

of a vertical channel vertical JFET (VCVJFET).Several groups are pursuing this structure

due to its simplicity and the low cost aspectof the structure [6-8]. The device turns offwhen the depletion regions of the pn-junctions pinch off the channel, and turns onwhen a less negative gate bias is applied,which reduces the depletion widths. Theprocessing steps of this structure have beenminimised by applying the concepts utilisedin Silicon trench MOSFETs, and substantialreduction in number of photolithographicsteps has been reported [7]. This structure does not have a built-in

anti-parallel or avalanche diode. In anavalanche situation, the avalanche currentmust be handled in the gate control loop,which places a tremendous burden on thegate drive circuitry. This problem can be

circumvented for N-on devices by using thecascade configuration with low voltagesilicon MOSFETs [4], which moves theavalanche path from the gate control loopinto the main power loop. However, N-offVCVJFETs do not have this option. Forapplications which do not require avalanchecapability, this is not an issue. However, forapplications that require avalanchecapabilities, an N-off VCVJFET must bepaired with an avalanche diode, or pairedwith a very robust gate drive circuit, whichcan negate the cost reduction achieved bysimplification of the fabrication processes.

The sensitivity of the devicecharacteristics to the process variation mustalso be addressed for N-off VCVJFETs.Because of the triode like behaviour,achieving a simple depletion of the channelregion at a VGS = 0 is not sufficient forsupporting the full blocking voltage [9]. Thedoping concentration and the width of thechannel region must be very wellcontrolled, and relatively long channellengths are needed to maintain the barrierheight in the channel through the operatingvoltage and temperature ranges. The designcan result in very resistive channels, since

the maximum JFET gate voltage, if it is toremain a voltage driven device, is limited toaround 2.8V at room temperature. Thisproblem was addressed by using a relativelyhighly doped channel region, and extremelytight cell pitch design [8,10]. It has beenshown by 2D device simulations [10] that avery small deviation in the channel mesawidth or doping concentration in thechannel region can result in a factor of 2 orgreater variation in the drain current,indicating that N-off VCVJFET designs needextremely tight control in channel epi-layerdoping and mesa width. Although it ispossible to realise a N-off VCVJFET,moderate to large volume production withhigh yield and uniform device characteristicscould be quite difficult, which can lead tohigher chip costs.

Bipolar junction transistorsFigure 4 shows a cross-sectional view of

a 4H-SiC bipolar junction transistor (BJT)[11]. BJTs in 4H-SiC provide negativetemperature coefficients in the commonemitter current gain, and normally-offbehaviors for a very wide range oftemperatures. Since a BJT is a minoritycarrier device, its electrical characteristicsdepend heavily on the quality of the epi-layers. Once a good quality NPN structurehas been grown, the remainder of thefabrication process is relatively simple,compared to those of DMOSFET andLCVJFETs. In addition, very low specific on-resistances can be achieved withouthaving to depend on a very tight cell pitchdesign, which makes 4H-SiC BJTs veryattractive for semiconductor devicemanufacturers [12]. The major drawbacks of a BJT from a

user’s point of view is that it is a currentdriven device which requires a high powergate drive capable of continuous currents.However, this is an old perception based onthe characteristics of low performing siliconBJTs. With the high current gains available in4H-SiC [13], the power and currentrequirements in the gate drive are muchsmaller. This substantially reduces the severity

20 POWER SEMICONDUCTORS www.cree.com


Figure 3: Simplifiedcross-section of avertical channelVJFET

Figure 4: Simplified cross-sectionof a 4H-SiC bipolar junctiontransistor


http://www.cree.com

of this issue, making the BJT a viable solutionfor high voltage switching applications.As with the VCVJFETs, the BJTs do not

have a built-in diode. Therefore, anexternal freewheeling diode is necessaryfor applications that require bi-directionalconducting switches. It should be notedthat the BJT can be turned off by eithershorting the base to the emitter, or just byopening the electrical connection to thebase. In the case of avalanche, theavalanche current flows from the collectorto emitter, without entering the gatecontrol loop, which limits the damage tothe gate drive circuit to a minimum.

Static characteristicsFigure 5 shows the I-V characteristics of

a 60A/1200V 4H-SiC DMOSFET [1]. TheRon,sp measured at room temperature (RT)with a VGS of 20V is approximately 8mΩ-cm² , which increases to 16mΩ-cm² at200°C [1]. Using a tight geometry design,it was shown that the Ron,sp of theDMOSFET structure can be reduced to4.6mΩ-cm² at RT [2]. The MOSFET Ron,sp can be further reduced

by using a trench structure [14]. The Ron,sp ofa 4H-SiC DMOSFET is made up of thechannel resistance (Rch) and the driftresistance (Rdrift). The Rdrift includes theresistances in the JFET region and the lightlydoped drift layer. With a VGS of 20V, Ron,spshows a monotonic increase withtemperature. However, the rate of increaseis not as high as expected based on thedecrease in µn,bulk. This is because the Rchdecreases with temperature as µeff increasesand the threshold voltage (VT) decreases,while Rdrift increases with temperature sincebulk electron mobility (µn,bulk) decreases withtemperature. This also suggests that thedrain saturation current, which is limited bythe µeff of the MOS channel, increasesmoderately with temperature.LCVJFETs showed comparable room

temperature performance. Ron,sp values of8mΩ-cm² and 12mΩ-cm² (VGS = 0) were

reported on 1200 and 1800V N-onLCVJFETs at room temperature,respectively [4]. The Ron,sp values of theDMOS-FETs and the LCVJFETs are quiteclose to each other, indicating that the Rchvalue of the DMOS-FET is low enough athigh gate biases, and the other Rdriftcomponents are comparable in theDMOSFETs and LCVJFETs due to thesimilarities in their structures. An Ron,sp of 4.4mΩ-cm was measured on

a 1200V BJT, with an active area of 9mm²[11]. This value is one of the lowest valuesreported in 1200V rated normally-offswitches in 4H-SiC. Unlike BJTs in silicon,the common emitter current gaindecreases with temperature, preventingthermal runaway of 4H-SiC BJTs.

Switching characteristicsGate resistance, CGD, and IDSS are the three

major factors affecting the switchingcharacteristics. In the turn-off transients, thevoltage rise time depends on the amount ofgate current (capability of gate drive)available to charge up the CGD. Hence, it isdesirable to minimise CGD and gateresistance to reduce switching losses. In aDMOSFET or a LCVJFET structure, the gate is

shielded from the drain by the p-wells,hence, CGD is relatively small. This is not thecase for VCVJFETs and BJTs. As shown inFigures 3 and 4, gate (or base) covers theentire active area. Therefore, CGD accounts formost, if not all, of the total drain capacitance.Capacitances in the edge terminationstructures are also added to the CGD values.This issue is alleviated for normally-onVCVJFETs when the cascode configuration isused, which converts CGD of the VCVJFETsinto CDS of the combined switch.

Robustness and reliabilityThe gate dielectric has been previously

predicted to be the weakest component inthe 4H-SiC DMOSFET structure due to thefact that the conduction band offsetbetween 4H-SiC and SiO2 is lower than thatof silicon and SiO2 [17]. This issue getsfurther complicated, since a relatively highgate oxide field is needed to fully turn onthe device due to low µeff values. However,very significant improvements have beenmade on the SiO2 quality, and currentTDDB measurements showed acceptableoxide lifetimes for oxide E-fields of lessthan 6MV/cm at temperatures up to 175°C[18]. In addition, due to the increase in µeff

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Figure 5: I-Vcharacteristics of a60A/1200V 4HSiCDMOSFET at 25 and150°C

Table 1: Comparison of 4H-SiC switches


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22 POWER SEMICONDUCTORS www.cree.com


at elevated temperatures, a significantlylower VGS can be used to turn on thedevice, which can reduce the oxide E-fieldand further improve high temperaturereliability of the 4H-SiC DMOSFETs.

Bipolar degradation can be an issue forthe 4H-SiC BJT. With improvements in thematerials and fabrication processes, thebipolar degradation has been substantiallyreduced [11]. However, furtherdevelopment is needed in this area.

LCVJFETs and VCVJFETs are unipolarstructures that do not depend on thindielectric layers. There are no reportedreliability issues on these devices to date.

ConclusionThe most common switches in silicon

carbide have been compared in terms ofperformance and cost. The summary isgiven in Table 1.

The DMOSFET structure in 4H-SiCprovides a normally-off switch with voltagecontrolled gates, avalanche capability, low on-resistance and reasonable IDSS. The N-onLCVJFET structure provides robustperformance and high operatingtemperatures. The N-on VCVJFETs have thesimplest fabrication process, hence thelowest cost of manufacturing. Extremely lowon-resistance and high IDSS have beendemonstrated on these devices, indicating

the chip size can be smaller than theDMOSFETs or LCVJFETs. The N-off VCVJFETscould also have a lower cost ofmanufacturing. However, the characteristicsof the device are very sensitive to variationsin doping concentration and channel mesawidth. The BJT does not require tight cellgeometry and has a relatively simple devicefabrication process. The cost of devices isexpected to be lower than DMOSFETs orLCVJFETs provided that the required epi-layers can be obtained at reasonable costs.The BJTs exhibit very low Ron,sp as well as anexcellent blocking capability. The drawbacksare that the BJT structure is a current-drivendevice, which has fallen out of favour in themainstream power electronics community,and the bipolar degradation phenomenonhas not been fully addressed yet. Withcontinuous improvement in current gainsand advances in materials and processingtechnologies, BJTs have the potential to be aviable low cost alternative to the DMOS-FETs.

Literature[1] B. Hull et al., presented at ECSCRM

2008, Barcelona, Spain, Sept. 7-11,2008.[2] K. Yamashita et al., Mat. Sci. Forum.

Vol. 600-603 (2009), p. 1115[3] N. Miura et al., Proc. ISPSD’06, p.

261

[4] http://siced.com/hp1016/Switches.htm[5] K. Asano et al., presented at

ISPSD’01, Osaka, Japan, June 4-7, 2001.[6] Y. Li et al., IEEE Trans. ED, Vol. 55,

No. 8, Aug 2008, p.1880.[7] V. Veliadis et al., Mat. Sci. Forum.

Vol. 600-603 (2009), p. 1047.[8] A. Ritenour et al., presented at

ECSCRM 2008, Barcelona, Spain, Sept.7-11, 2008.[9] R.K. Malhan et al., Mat. Sci. Forum.

Vol. 600-603 (2009), p. 1067.[10] J.H. Zhao et al., Mat. Sci. Forum.

Vol. 527-529 (2006), p. 1191.[11] Q. Zhang et al., to be presented

at ISPSD’09, Barcelona, Spain, June 14-17, 2009.[12] http://www.transic.com/[13] J. Zhang et al., Mat. Sci. Forum.

Vol. 600-603 (2009), p. 1159.[14] Y. Nakano et al., presented at

ECSCRM 2008, Barcelona, Spain, Sept.7-11, 2008.[15] L. Cheng et al., Mat. Sci. Forum.

Vol. 600-603 (2009), p. 1055.[16] I. Sankin et al., Proc. ISPSD’08, p.

260.[17] A. Agarwal et al., IEEE Electron

Dev. Letts. 18 (1997), p. 589.[18] S. Ryu et al., presented at MRS

Fall 2008 Meeting, Dec. 1-4, 2008,Boston. MA.

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GaN Based Power ConversionGaN based power devices such as HEMTs promise to deliver a figure-of-merit (FOM) performance that is atleast an order of magnitude better than state-of-the-art silicon MOSFETs. In addition to reviewing the distinctadvantages of a new GaN-on-Si technology platform, this article will also describe how DC/DC convertersbuilt using the new GaN technology platform will enable a new era in high frequency, high density, highlyefficiency power conversion solutions. Michael A. Briere, ACOO Enterprises LLC/InternationalRectifier, USA

Modern power electronics solutionsprovide an array of system levelenhancements such as communicationprotocols, load condition reporting, as wellas optimal balancing and coordination andprotection of power conversion subsystemsand loads. As important as these advanceshave been, it is the continued progress inthe performance of the power convertersub-systems themselves that have enabledincreasingly dense and efficient workingloads.

It can be argued that the intrinsic valueproposition of the power conversion sub-systems is density * efficiency/cost. Thisperformance/cost figure of merit (FOM) forpower processing is the equivalent drivingforce behind innovation as the logic unit/$FOM is to the well-known Moore’s law ofthe data processing industry. There havebeen significant advancements in bothFOMs over the past 40 years. It can beargued that the most significant advancesin energy conversion efficiency * density/cost have been achieved through requisiteimprovements in the power devices used.Generally, advances through improvedcircuit architectures, from linear to switchingregulation, hard to soft switching, passive tosynchronous rectification, etc., have allbeen accomplished by leveraging theinherent capabilities and avoiding theinherent limitation of the power switchcomponents used. It can therefore beexpected that radically improved powerswitch performance might well drive arevolution in power electronic architecturesand systems.

The ability of power semiconductordevices to enhance the power electronicsperformance/cost FOM can be simplifiedby its own price/performance FOM, namelyswitching power loss * ohmic power loss *cost, where the switching power lossreflects the thermal limitation of density,most often achieved through increasingswitching frequency and subsequentreduction in output filter components. Forinverter circuits this can be referred to byQrr * Vceon * cost or more precisely Eoff * Vceon *cost, for silicon based IGBT switch/diode

pairs. For DC/DC converter circuits such ascommon buck regulators, the FOM is R(ds)on* Qsw * cost. Here, the specific-on-resistance,R(on), times cost, also serves to representthe generic price/performance FOM of apower switch as $/A.

Performance limitations of siliconbased power devices

Since the advent of commercially viableSi power FETs, introduced some 30 yearsago, enabled the widespread adoption ofswitch-mode power supplies, replacingthe linear regulator as the dominantpower architecture, the Si power FET hasbecome the dominant power device. TheSi IGBT, combining the ease of chargecontrol with the benefits of conductivitymodulated drift resistivity, has beenanother mainstay, especially in the lowerfrequency conversion systems, e.g. motordrive inverters. Of course, the sameminority carrier injection that provides forlower ohmic losses also increasesswitching losses through the effects ofsubsequent tail currents. Over the lastthree decades, significant engineeringefforts have driven the improvement inthe performance FOM of these devices bymore than an order of magnitude.

However, as this technology approachesmaturity, it becomes increasinglyexpensive to achieve even modestimprovements in the device FOM.

It is estimated that less than a factorof two improvement will beeconomically feasible to achieve for 30VMOSFETs (see Figure 1), with perhaps afactor of 5 possible for 600 to 1200VIGBTs [3]. If further advances in powerdevice performance are required byfuture electronic loads, as is currentlyapparent, then these advances must beachieved through the use of alternativematerials.

Pounding sand and beating siliconFirst, it should be noted that efforts to

displace existing technologies, especiallythose as entrenched as silicon basedtechnology platforms, have generally hadonly moderate success. A highly illustrativeexample which bears close resemblance tothe case currently in point is that of GaAstransistors. When Si transistors werelimited to about 1µm line widths, some 25years ago, it was suggested that GaAstransistors would command significantmarket share due to their faster switchingspeeds, especially using high electron

Figure 1: Comparison of Specific on Resistance versus Device Breakdown Voltage for Si, SiC and GaNbased power devices, showing that silicon power MOSFETs are near physical performance limits [2]

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mobility transistor (HEMT) structures. Ofcourse, the subsequent reduction in linewidth to deep sub-micron levels hasallowed Si transistors to deliver compellingprice/performance compared to bothGaAs and other compound semiconductoralternatives. This is due, in part, to the ballistic

nature of these fine line devices and thesuperior saturation velocity characteristicsof carrier transport in silicon over thealternatives, but more importantly, to theeconomics of the silicon device industry.With over 60 years of development andsupporting greater than $200 billion peryear of revenue, the silicon deviceindustry is a formidable, nearlyunbeatable behemoth. Just as opticalbased lithography has repeatedlysuccessfully defeated alternatives of x-ray,ion beam and e-beam lithography toremain the dominant printing methodwithin the industry, so too has siliconbased devices held back the forays by amyriad of proposed alternate materials.Even the recent push for self assemblednano-technology may be sidelined to arelative niche future by the dominantlithography based infrastructure based onthe well- established silicon deviceindustry.

Economics of proposed alternativesThe production of power devices

includes the costs of substrate, epitaxy,

device fabrication, packaging, supportelectronics and development. It is in thefirst of these costs that one alternativematerial for power devices falters,namely SiC.Typical device quality SiC substrates cost

more than $1000 for a 100mm diametersubstrate, about 50 times the cost of100mm diameter silicon wafers. Inaddition, the quality of the SiC substrates isfar inferior to that of the silicon substratesand defects from the substrate are knownto cause yield loss through any subsequentepitaxial layer.Commercially available SiC epitaxial

wafers, ready for viable devicemanufacture, cost some $5,000 for a100mm diameter wafer, or nearly$65/cm². This substantially exceeds theviable economical limit of about $3/cm²for substrate and epitaxy set by thepower device marketplace. Further, evenif 100mm substrates were consideredeconomically acceptable for large scaleproduction, the volume of substratesrequired to satisfy the power devicemarket (20 to1200V) of some 10 million150mm wafer equivalents, far outstrips

any plausible expansion of the SiCsubstrate supply chain. In fact, currently, substrate diameters of

at least 150mm are required to achievewidespread commercial viability for powerdevice fabrication. For these reasons, SiCis not considered a viable commercialalternative to silicon for power devicefabrication. In fact, only silicon itselfmeets the necessary economic, thermalperformance, crystalline perfection,supply scalability and size availability tomeet the broader power device marketneeds.Next to the cost of substrate and

epitaxial layers, device fabrication costsare the most critical. It has been typical incompound semiconductor devicefabrication to use specialised processessuch as e-beam and lift-off lithography, aswell as to utilise gold metallisation. Thesetechniques are understandable formilitary and RF applications, wheremarkets will support costs of more than$10,000 for finished 100mm wafers fordiscrete devices. The broad power devicemarket will not support this order ofmagnitude fabrication costs. In fact, to

24 POWER SEMICONDUCTORS www.irf.com


Figure 2: 6in wafer with GaNpowIR devices

Figure 3: Power Conversion loss for 12 to 2V synchronous buck conversion power stages using 2006benchmark silicon devices and first generation GaN based power devices as a function of switching

Figure 4: Conversion efficiency at 700kHzswitching frequency for a 30A synchronous buckconverter stage using state-of-the-art silicon(lower 2 curves) and GaN based power devicesexpected over the next 5 years (upper 3 curves)


http://www.irf.com

gain broad adoption of alternativematerial based power devices, fabricationcosts must approach that of Si powerdevices. In addition, the aforementionedvolume necessary to support the broadpower device market requires scalabilityin device manufacture provided mostreadily by existing silicon devicefabrication facilities.It is for these reasons that

International Rectifier has developed itsGaNpowIR technology platform usingGaN-on-Si hetero-epitaxy and devicefabrication processing that can beperformed in a standard modern siliconCMOS manufacturing line with littlemodification to equipment or processdiscipline. It is this approach that allowsthis technology platform to providepower devices with compellinglysuperior performance/cost FOMscompared to silicon which will promotewidespread adoption (Figure 2).

GaNpowIR device performanceAs shown in Figure 1, GaN based

power devices already provide a factor of2 to 10 in specific on-resistanceimprovement over state-of-the-art siliconbased devices, especially for unipolar,non-compensated devices [2]. It isimportant to note that the GaN devicesrepresent very early stage prototypes andthat, as was the case in silicon powerdevice development over the last 30years. Even though the basic GaN HEMTtransistor was first invented some 15years ago by M. Asif Khan [4], significantdevelopment efforts on practical powerdevices using GaN-on-Si technology havebeen fairly recent, predominantly in thepast 5 to 7 years. GaN based powerdevices are expected to improve rapidlyover the next 10 to 20 years. In fact, it is expected that an order of

magnitude in improvement in the keydevice performance FOMs will be

achieved over the next 5 years. It is alsoinstructive to note that at lower voltageratings, the device performance isgenerally limited by extrinsic parasiticeffects (e.g. interconnects, contactresistance), causing the results to divergefrom the theoretical limits, calculated asideal (drift) resistor behaviour in thevarious semiconductor materials. As has been previously reported [5],

the initial GaNpowIR products will be lowvoltage (30V) DC/DC power stagemodules. This approach is different frommany commercial efforts which focus onthe obvious advantages of GaN basedpower devices at higher voltage ratingsabove 600 V [2]. Although, as seen inFigure 1, the distinct advantage of lowvoltage GaN-based HEMTs is not asobvious in terms of specific on-resistance,it is important to note that it is the R(on) *Qsw FOM which is critical to many of thelow voltage applications. In this regard,

the GaN HEMTs are expected to achievemore than an order of magnitudeimprovement over state of the silicondevices within the next five years.Quantitatively, this means a R(on) * Qgdevice performance of less than 4mΩ *nC compared to next generation siliconFOM of 45mΩ * nC.Figure 3 shows the relative power loss

in synchronous buck converters usingprevious generation silicon FETs ( RQ >100) and first generation GaN-basedHEMTs (RQ = 30). The relative powerlosses in such conversion circuits as thecommon synchronous buck regulator canbe well understood by such simplifiedswitch technology FOM. The lower RQFOM flattens the power loss curve overswitching frequency. A power conversionstage using such future GaN-based powerdevices with RQ FOM less than 4mΩ 4 nCwill enable more than a factor of 10 inswitching frequency at comparable

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Figure 5: Measured power conversion efficiency,including effects of output filter for prototypeGaN based power stage, 5MHz switchingfrequency(12Vin, 1.8Vout , 86.5% peak)

Figure 6: Expected improvements (simulations) in power conversion efficiency, including effects ofoutput filter for GaN based power stage, 5MHz switching frequency (12Vin, 1.8Vout) for first twogenerations of 30V GaNpowIR products: 2009 (87.5% peak) and 2010 (90% peak)


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conversion efficiency. Alternatively,increased conversion efficiency can beobtained at the same switchingfrequency, as shown in Figure 4. Here itcan be seen that the GaN-based powerdevices provide a 3% improvedconversion efficiency over state-of-the-artsilicon FETs.

First product prototypesThe ability to switch efficiently at high

frequencies enables significantimprovements in the power conversionFOM, efficiency * density/cost, as outputfilter components decrease in value (andsize) as the switching frequency isincreased. In particular, the requiredoutput capacitance rapidly vanishes,though many small passives are requiredto minimise L(series) for rapid currenttransients. Reduction in outputinductance is similarly limited by output

current ripple management. Prototypes of high-frequency

synchronous buck conversion stagesusing early prototype GaN based devices(30V GaNpowIR 1.0) have beencharacterised. The

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