© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (b) 245, No. 7, 1223–1231 (2008) / DOI 10.1002/pssb.200844079

Expert Opinion

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Present status and future prospects for electronics in electric vehicles/hybrid electric vehicles and expectations for wide-bandgap semiconductor devices

Kimimori Hamada*

Toyota Motor Corporation, 543, Kirigahora, Nishihirose-cho, Toyota, Aichi 470-0309, Japan

Received 19 February 2008

Published online 12 June 2008

PACS 85.30.–z

* e-mail hamada@kimimori.tec.toyota.co.jp, Phone: +81 565 46 3352, Fax: +81 565 46 3382

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Issues surrounding automobiles We humans have achieved great cultural developments over the past thousand or more years. Figure 1 shows the changes in the atmospheric CO2 concentration over the past approxi-mately 1000 years. Together with the increase in the con-sumption of fossil fuels which began from the industrial revolution in the 18th and 19th centuries, we can also see a sudden rapid increase in the CO2 concentration. When we look at the ‘CO2 emissions by sector’, we see that, in fact, one-fourth of all CO2 emissions are due to transport. To-gether with the rapid increase in atmospheric CO2 concen-tration, global average temperatures are also rising. The documentary film An Inconvenient Truth described the way that a broad range of large-scale climate changes are occurring, such as the dramatic reduction in Greenland ice over recent years, as also shown in Fig. 2 [1, 2]. As all are aware, global warming caused by CO2 and other factors will not only raise sea levels due to the melting of the ice: there are also frightening warnings of large-scale climate

change such as increasing numbers of destructive storms resulting from changes in atmospheric circulation. Toyota understands that the rising concentration of CO2 in the at-mosphere is a serious problem. When evaluating the impact of CO2 generated by automotive fuels and power trains, it is important to evalu-ate not only the CO2 generated by consuming fuel, but also the total amount of CO2 generated from production to con-sumption – in other words the well-to-wheel CO2. Figure 3 shows a comparison of well-to-wheel CO2, using the well-to-wheel CO2 of a gasoline-powered automobile, shown at the top of the bar graph, as ‘1’. We can see that compared to this, the CO2 generated by a diesel automobile is 0.75, and the CO2 generated by a gasoline hybrid is only 0.45. Other examples of substitute fuels, such as bio-fuels, syn-thetic fuels, hydrogen, and electricity, are also shown. These show the different levels of well-to-wheel CO2 which result from different materials and production meth-ods. The amount of well-to-wheel CO2 that is generated by

Toyota refers to the ability of users to continuously enjoy the

convenience provided by automobiles as ‘sustainable mobil-

ity’. In order to achieve this, we are carrying out the endless

challenge of minimizing the negative aspects of automobiles,

such as CO2 emissions, air pollution, traffic fatalities, and

congestion, while maximizing automobile comfort, enjoy-

ment, and excitement. In the area of the environment, we be-

lieve that we can come closer to creating the ‘ultimate eco-

vehicle’ by increasing the environmental performance of the

power train, utilizing new fuels and electrical energy, and in-

tegrating hybrid technology into all of the results. In the To-

yota Group, we think that power electronics is a key technol-

ogy for the automotive technology of the future. We have de-

fined SiC and GaN as core items for breakthroughs in power

electronics technologies for the future, and we are energeti-

cally pursuing research and development in those areas.

1224 K. Hamada: Electronics in electric vehicles/hybrid electric vehicles

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

ph

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solid

i b

Source: IPCC 95

380

360

340

320

300

280

260800 1000 1200 1400 1600 1800 2000

year

CO

2co

nc. (

ppm

v)

Hawaii Mauna Loa Observatory data

D47D47D57D57

SipleSiple

South PoleSouth Pole

Industry19%

Electricity generation

43%

Source: IEA/WEO 20042002 data

CO２Emissions by SectorResidential

& commercial

15%

Transport23%

Transport23%

Source: IPCC 95

380

360

340

320

300

280

260800 1000 1200 1400 1600 1800 2000

year

CO

2co

nc. (

ppm

v)

Hawaii Mauna Loa Observatory data

Hawaii Mauna Loa Observatory data

D47D47D57D57

SipleSiple

South PoleSouth Pole

Industry19%

Electricity generation

43%

Source: IEA/WEO 20042002 data

CO２Emissions by SectorResidential

& commercial

15%

Transport23%

Transport23%

Figure 1 (online colour at: www.pss-b.com) Atmospheric CO2

concentration.

automobile use is determined by both the type of fuel and the type of power train. It is important that we consider a broad range of issues, such as the fuel resource amount, cost, energy density, and the well-to-wheel CO2 emissions, and incorporate them into power train development. We would like to take a look at traffic accidents. The number of traffic fatalities in Japan, the USA, and Europe has decreased slightly over the past 30 years; however, the overall level remains high (Fig. 4). In China, which ranks second in the world in the number of automobiles sold, there were 100000 traffic fatalities in 2005, making this is-sue a serious problem. Automobile manufacturers recog-nize the need for continued efforts aimed at reducing traf-fic fatalities to zero. Toyota refers to the ability of users to continuously enjoy the convenience provided by automo-biles as ‘sustainable mobility’. In order to achieve this, we are carrying out research and development under the slo-gan of ‘Zeronize & Maximize’. This refers to taking on the endless challenge of minimizing the negative aspects of automobiles, such as CO2 emissions, air pollution, traffic fatalities, and congestion, while maximizing automobile comfort, enjoyment, and excitement.

Source:GISSSource:GISS

<source:@2005 ACIA [an inconvenient truth (by Al Gore)] ><source:@2005 ACIA [an inconvenient truth (by Al Gore)] >

0.8

deg r

ee(1

900

to 2

0 00)19921992 20022002 20052005

Red : Melting Area

Figure 2 (online colour at: www.pss-b.com) Global temperature

and melting ice in Greenland.

Japanese 10-15 test cycle

Well-to-Tank CO2 (WTT)Tank-to-Wheel CO2 (TTW)

Source: Mizuho Information & Research Institute report

Relative CO2 emissions indexed to gasoline as 1.0

-1 -0.5 0 0.5 １ 1.5

Gasoline hybridGasoline hybrid

GasolineGasoline

FT synthetic diesel: coalFT synthetic diesel: coal

FT synthetic diesel: biomassFT synthetic diesel: biomass

Ethanol: sugarcaneEthanol: sugarcane

Diesel fuelDiesel fuel

Electricity: coalElectricity: coal

Ethanol: coneEthanol: cone

Hydrogen: CNGHydrogen: CNG

Electricity: nuclearElectricity: nuclear

Japanese 10-15 test cycle

Well-to-Tank CO2 (WTT)Tank-to-Wheel CO2 (TTW)

Source: Mizuho Information & Research Institute report

Relative CO2 emissions indexed to gasoline as 1.0

-1 -0.5 0 0.5 １ 1.5

Gasoline hybridGasoline hybrid

GasolineGasoline

FT synthetic diesel: coalFT synthetic diesel: coal

FT synthetic diesel: biomassFT synthetic diesel: biomass

Ethanol: sugarcaneEthanol: sugarcane

Diesel fuelDiesel fuel

Electricity: coalElectricity: coal

Ethanol: coneEthanol: cone

Hydrogen: CNGHydrogen: CNG

Electricity: nuclearElectricity: nuclear

Figure 3 (online colour at: www.pss-b.com) Well-to-wheel CO2

emissions.

We believe there are three major directions for techno-logical development: the environment, safety, and comfort. For the purpose of ‘Zeronize & Maximize’ we identify the precise items which must be zeronized or maximized in each category, and are making definite progress in techno-logical innovations aimed at the ultimate goals. The ulti-mate goals are an ultra-highly efficient energy society, a CO2-free society, a vehicle society in which everyone can move with security, and providing emotional satisfaction to customers. Specifically, this means the four ideal types of vehicles which have been imagined by President Wata-nabe. These are a ‘vehicle which makes the air cleaner when it runs longer’, a ‘vehicle which can run around the world with a single full refuelling’, a ‘vehicle which never makes a collision’, and a ‘vehicle which makes passengers healthier the more time they spend in it’. Of course, achieving this vision is not an easy task, and we do not yet know the specific technologies which will make this possi-ble. However in the area of the environment, we believe that we can come closer to creating the ‘ultimate eco-vehicle’ by increasing the environmental performance of the power train, utilizing new fuels and electrical energy, and integrating hybrid technology into all of the results

0

20

40

60

80

100

120

1975 1980 1985 1990 1995 2000 2005

Japan; National Police Agency data

US; Traffic Safety Facts 2005 NHTSA, U.S.DOT

EU; Statistics of Road Traffic Accidents in Europe and N.A., United Nations

China; http://www.gov.cn/xwfb

Europe

U.S.Japan

China

Traf

fic F

atal

ities

[Th

ousa

nds]

Year

0

20

40

60

80

100

120

1975 1980 1985 1990 1995 2000 2005

Japan; National Police Agency data

US; Traffic Safety Facts 2005 NHTSA, U.S.DOT

EU; Statistics of Road Traffic Accidents in Europe and N.A., United Nations

China; http://www.gov.cn/xwfb

Europe

U.S.Japan

Europe

U.S.Japan

Europe

U.S.Japan

ChinaChina

Traf

fic F

atal

ities

[Th

ousa

nds]

Year

Figure 4 (online colour at: www.pss-b.com) Trends of traffic fa-

talities.

phys. stat. sol. (b) 245, No. 7 (2008) 1225

www.pss-b.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Expert

Opinion

DieselDieselengineengine

Diesel DI

DPNR

Diesel HV

ElectricElectricvehiclevehicle

EV

FCHV

GasolineGasolineengineengine

VVTLean-burn

D-4

THS

Ultimate Eco-VehicleUltimate Eco-Vehicle

Gate 1 Emissions

EnergyDiversification

Gate 2

Gate 3 CO2

CNG

AlternativeAlternativeenergyenergy

GTL

BTL PHV

the Right Place the Right Timethe Right Car

Hybrid Technology

Figure 5 (online colour at: www.pss-b.com) Creating the ‘ulti-

mate eco-vehicle’ (CNG, compressed natural gas; GTL, gas to

liquids; BTL, biomass to liquids; DI, direct injection; DPNR, die-

sel particulate NOx reduction system; VVT, variable valve tim-

ing; THS, Toyota hybrid system; PHV, plug-in hybrid vehicle;

EV, electric vehicle; FCHV, fuel cell hybrid vehicle).

(Fig. 5). We are confident that hybrid technology will truly be one of the core technologies of the 21st century. 2 Past, present, and future of Toyota hybrid vehicles We released the Prius passenger hybrid vehicle (HV) and a small-size bus HV in 1997, and subsequently expanded our lineup of vehicle models with a minivan HV, diesel truck HV, sports utility vehicle HV, medium-size sedan HV, and others (Fig. 6). In the future, we will con-tinue expanding the number of HV models, and intend to achieve yearly sales of one million HVs as early as possi-ble in the 2010s. Figure 7 shows the relationship between vehicle weight and fuel consumption. The bottom line shows vehicles with conventional engines, the centre line shows vehicles with direct-injection gasoline engines, and the top line shows HVs. From this graph, we can see that improvement is limited to approximately 20% when improvements are made to a normal engine; however, an improvement in fuel

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

201X

HV s

ales

HV s

ales

1,0001,000

800800

600600

400400

200200

YearYear

1997

1998

1999

2000

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2002

2003

2004

2005

2006

2007

2008

201X

HV s

ales

HV s

ales

(Tho

usan

ds)

1,0001,000

800800

600600

400400

200200

YearYear

Figure 6 (online colour at: www.pss-b.com) Trend of Toyota

HV sales.

Vehicle weight [kg]500 1000 1500 2000 2500

0

5

10

15

20

25

30

Fuel

con

sum

ptio

n (J

apan

ese

10-1

5 m

ode)

[km

/l]

Prius

HVDirect injection gasoline engine

Conventional gasoline engine

(Japanese AT vehicles)

Estima HV

New Prius

Alphard HV

++100%100%Conventional engine

Direct injection engineLean burn engine

HV

Figure 7 (online colour at: www.pss-b.com) Vehicle weight and

fuel consumption.

efficiency of nearly twice the normal engine is possible with the HV. Figure 8 explains the reason for the improved efficiency of the hybrids. When the vehicle is stopped, the engine stops idling and does not consume energy. During acceleration and low-speed driving, in ranges where gaso-line engine efficiency is poor, the high-efficiency electric motor is primarily used for driving. When accelerating, both the gasoline engine and the electric motor are used to gain sufficient acceleration. During normal driving, the en-gine runs in the high-efficiency range at all times. The en-ergy is supplied from the battery when the vehicle has a shortage of energy, whereas the energy is restored to the battery when the vehicle has a surplus. When decelerating, mechanical energy is converted to electrical energy and re-covered by the battery. In this way, fuel efficiency is dra-matically improved by operating the engine only in high-efficiency ranges, and by recovering the energy during de-celeration which was previously wasted as heat. So in this system, the key to fuel economy improvement is energy management which switches between the gasoline engine and the electric motor at optimal times according to the driving conditions.

Surplus energy Regenerativebraking

--

++

Energy is reused

Acceleration

Engine outputDeceleration

Time0

Ene

rgy

Battery

Figure 8 (online colour at: www.pss-b.com) Hybrid technology

energy management.

1226 K. Hamada: Electronics in electric vehicles/hybrid electric vehicles

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

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Figure 9 shows the structure of Toyota’s present hybrid system named Toyota Hybrid System II (THS-II). The en-gine, generator, and motor are connected mechanically by means of a power split device, while motor, generator, and battery are connected electrically via an inverter. This sys-tem adds a boost converter between the battery and in-verter, in order to obtain high motor voltage and deliver higher output without increasing the number of battery cells – in other words, without increasing the cost so much. A hybrid car has a power electronics circuit called an in-verter that provides tens of kilowatts of power to drive the motor by converting direct current to alternating current. Figure 10 shows the electrical circuit. It is composed of the inverters for the boost converter, motor, and generator, as well as capacitor, inductor, control circuit, and other parts. These parts are contained within the power control unit (PCU). Figure 11 shows the structure of the PCU that is used in the GS450h. The optimal design of the inverter, converter, smoothing capacitor, and water-cooled heat sink allows the PCU to be kept to approximately 11 litres, or about the size of a battery. The power semiconductors that are used to control the current are therefore critical key de-vices for hybrid technology. For example, the Prius con-tains 18 insulated gate bipolar transistors (IGBTs) and 18 free wheeling diodes (FWDs) as power semiconductors that are used for driving (Fig. 12). The IGBT is approxi-mately 1 cm2 in size, and each IGBT can control a maxi-mum current of nearly 200 A. For failsafe operation, the current sensor and temperature sensor are built into the chip. At Toyota Motor Corporation, the in-house develop-ment of IGBTs and FWDs has made a major contribution to strengthening our capacity for hybrid system develop-ment, specifically the development of more compact, higher performance, and lower cost hybrid systems in a short period of time [3–5]. The vehicle that Toyota is researching in order to util-ize electrical energy in an ordinary automobile without restrictions on the cruising distance is the plug-in hybrid vehicle (PHV). We have positioned the PHV as a key technology for sustainable mobility in the near future, and are now carrying out verification trials on public roads

Power split

Generator

Mechanical power path

Electrical power pathHybrid transmission

Engine

Power control unit

device

Motor

Battery

Inverter Boost converter

Figure 9 (online colour at: www.pss-b.com) Toyota Hybrid Sys-

tem II.

BatteryMotor

IGBT module

Power control unitPower control unit

Boostersystem Generator

To HV-ECU

controlboard

Figure 10 (online colour at: www.pss-b.com) Electrical circuit

of the power control unit.

Reactor

Water-cooled heat sink

Smoothing capacitor

Boost power module

Inverter

& filter capacitor

Inverter power module

Converter

Figure 11 (online colour at: www.pss-b.com) GS450h power

control unit.

Smoothing filtercapacitor

Inductor

IGBT is located under thecapacitor

IGBT chipDiode chip

The sensor isThe sensor isintegrated in theintegrated in thechip for failsafechip for failsafe

Figure 12 (online colour at: www.pss-b.com) Prius power con-

trol unit and power semiconductors.

(Fig. 13). For short trips the PHV uses electrical energy, while for longer trips it uses a hybrid mode that combines both electrical energy and gasoline. The PHV well-to-wheel CO2 emissions vary depending on the conditions of electricity in each country. However, using Japan as an ex-ample, we see that the emissions are approximately one-third of a conventional gasoline or diesel vehicle, and ap-

phys. stat. sol. (b) 245, No. 7 (2008) 1227

www.pss-b.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Expert

Opinion

a hybrid ofa hybrid ofelectric vehicle and gasoline (diesel, fuel cell) vehicleelectric vehicle and gasoline (diesel, fuel cell) vehicle

a gasoline (diesel, fuel cell) hybrid vehiclea gasoline (diesel, fuel cell) hybrid vehiclewith an external rechargerwith an external recharger

or

Household electrical energyGas station

Figure 13 (online colour at: www.pss-b.com) Definition of plug-

in hybrid vehicle. proximately half of a gasoline HV (Fig. 14). However, there remains a major issue which must be resolved in or-der to commercialize PHVs. If we assume that the neces-sary driving distance using electrical energy, in other words using the battery, is 60 km, then we require a battery capacity that is approximately 12 times that of the Prius. In order to ensure the necessary space for passengers and lug-gage, a revolutionary new battery must be developed. Toyota has positioned the fuel cell hybrid vehicle (FCHV) – a hybrid with a fuel cell instead of an engine, using hydrogen as the fuel and emitting no CO2 – as the ul-timate eco-vehicle, and we are actively proceeding with its development (Fig. 15). Toyota has been aware of the future potential of this technology from an early stage, and in 2002 we introduced the world’s first fuel cell vehicle to the market. In 2005, at Expo 2005 in Aichi, Japan, eight FCHV buses were used as a means of transport between the expo sites. However, there are a large number of issues that must be resolved before full-scale use of FCHVs in the market is possible. For example, the cost of such vehicles must be reduced to approximately 1/100 of the current level, and the cruising distance also remains an issue. Other issues include the establishment of a method for producing hydrogen fuel that has a low level of well-to-tank CO2 emission, and the creation of a hydrogen supply infrastructure.

1.01.0

0.50.5

2.02.0

PriusPrius PHVPHVConventionalpower train vehicle

Japanese 10-15 test cycle

1.51.5

Gas

olin

e

Die

sel

Gas

olin

e H

V

Gas

olin

ePH

V

Figure 14 (online colour at: www.pss-b.com) Well-to-wheel

CO2 emissions in Japan.

Hybrid vehicle FCHV

Battery Battery

Engine

Motor Motor

Fuel cellPowercontrolunit

Powercontrolunit

Power Control Unit

Toyota FC Stack

Motor

Battery

High-pressureHydrogen Tank

Toyota FCHVSeats: 5 peopleMax speed: 155 km/hMax cruising range: 330 km

Figure 15 (online colour at: www.pss-b.com) Toyota’s fuel cell

hybrid vehicle (FCHV).

3 Newest hybrid vehicle Toyota Motor Corporation has announced a luxury four-door sedan HV, the LS600h. It uses a 5 litre V8 engine, a 165 kW high-output electric motor, and a nickel–metal hydride battery. Combined with the effects of the two-stage speed reduction mechanism, it delivers power equivalent to a 6 litre engine, and although it is an all wheel drive vehicle, it still achieves fuel econ-omy of 12.2 km per litre in the 10–15 fuel consumption mode, a level of fuel economy that is unusual in its class. The inverter output density is increased so as to boost the motor output with almost no change in the inverter ca-pacity [6]. To handle the higher output density, a new in-verter structure was adopted that cools the power semicon-ductors, which generate heat, on both sides (Fig. 16). The final size is extremely compact. A set of IGBT and FWD is placed in a moulded package called a power card that can be cooled on both sides (Fig. 17). The power card utilizes a double-sided cooling structure. Excellent cooling per-formance is achieved by stacking multiple power cards in-side a cooling unit. This makes it possible to efficiently cool the increased element heating that occurs with higher output.

Card stack structure

Coolant

Power card

Figure 16 (online colour at: www.pss-b.com) LS600h power

control unit.

1228 K. Hamada: Electronics in electric vehicles/hybrid electric vehicles

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

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Bat

tery

Volta

ge-

boos

ting

circ

uit

Heat spreader(Lead frame)

Power chips(IGBT,FWD)

∑ Internal structure

Heat spreader(Lead frame)

Conductivespacer

M

- Compact structureachieved by single-unitconfiguration (IGBT, FWD)

- Efficient transmission ofheat to cooling water

Heat spreader(on both sides)

Figure 17 (online colour at: www.pss-b.com) The structure of a

power card. The new hybrid system is not the only new technology in the new HV. Many other new technologies are also em-ployed. Figure 18 shows the advanced systems in the LS600h where wide-bandgap semiconductors are currently used, or may be used in the future. Expectations are high for the use of wide-bandgap semiconductors in the power devices for the PCU; in the high-frequency devices for the millimetre-wave radar; in the harsh environment devices for the igniter, injection, combustion pressure sensor, and emission gas sensor; and in light emitting diodes (LEDs) for the interior lights and headlamps. LED headlamps util-izing GaN LEDs have been commercialized for the first time in the LS600h [7]. Unlike conventional headlamps, these headlamps combine the light from a series of three small projectors and small reflectors to create the beam pattern. This not only improves driver visibility but also forms an attractive lamp design unlike any other. In order to prevent deterioration in LED performance caused by ris-ing temperatures, we have utilized highly heat-resistant GaN LEDs and an original cooling structure. These head-lamps illuminate quickly when turned on, to reliably en-sure the field of view. They have a long lifetime and fea-ture superior performance, including almost zero drop in brightness or change in chromaticity over their lifetime.

Figure 18 (online colour at: www.pss-b.com) Advanced tech-

nologies in the LS600h.

4 Expectations for wide-bandgap semiconduc-tors in HV inverter applications The chart in Fig. 19 shows a comparison of the electronic properties between Si, SiC, and GaN semiconductor materials. The electronic properties of SiC are superior to those of Si in many cases. Because of the high breakdown electric field strength and the thermal conductivity, SiC is expected to be used in high-power devices. SiC has approximately ten times the breakdown field strength and approximately three times the thermal conductivity of Si and, in theory, has the po-tential for approximately 1/300 the standardized on-resist-ance of Si. Using SiC would also be expected to increase the power density further. Measures such as utilizing the high-temperature operating characteristics of SiC to sim-plify the cooling structure, as well as taking advantage of its high-speed switching characteristics to make the boost converter reactor more compact, also raise expectations for making the entire system more compact and less costly. SiC is also used as a substrate for GaN LEDs. Still, it must be understood that despite these superior material properties, SiC has little chance of being used unless it can be obtained at a cost that is the same as or lower than that of Si, which currently dominates nearly all semiconductor applications for rational economic reasons. This is an era where the potential of SiC is under study. At the point when the possibility of lower cost becomes ap-parent, that potential will be verified through testing. And at the point when the cost becomes almost the same as Si, small-scale use of SiC will begin, and will be followed by full-scale use when it becomes less expensive than Si. We believe this will be the scenario for the success of SiC de-vices in HV systems (Fig. 20). We expect this third phase to arrive during the 2010s. To achieve that success, we believe that development of a variety of new technologies will be necessary. First of all, the substrate technologies required are large-size high-quality wafers of 5-inch diameter or larger, and a technol-ogy that is capable of extending the length of the crystal in order to reduce cost. Required device technologies include normally-off vertical power elements with loss density that is at least an order of magnitude lower than Si IGBTs, and

Melting point (°C)

Breakdownelectric field (V/cm)

Thermal conductivity(W/cm °C)321

4 5

Si

SiC Radiation: x 3

High frequency: x 10

High temperature: x 3Endurable for radioactivity: x 3

High temperaturesensor for car

Low loss power modulefor car communication

1

Saturation electronvelocity (x 107 cm/s)

2

3

Energy gap (eV)1

23

105

106

3k

2k

1k

Inverter for HVMultiple numbers: SiC/Si

Substrate for blue LEDand blue laser

GaN

Low loss: x 100High voltage: x 10

Figure 19 (online colour at: www.pss-b.com) Characteristics and

applications of wide-bandgap semiconductors.

phys. stat. sol. (b) 245, No. 7 (2008) 1229

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Expert

Opinion

SiC

SiSiC Si

Rel

ativ

e co

st

Future

SiC Si

SiC

Si

Research Trial adoption Adoption in small amount Popularity

～ ～ ～ ～ ～ ～

- High current density Downsizing of IPM- High speed SW Downsizing of reactor- Simplification of cooling

[Cost reduction factors of other parts by SiC adoption]

201XNear futurePresent

...

SiC

SiSiC Si

Rel

ativ

e co

st

Future

SiC Si

SiC

Si

Research Trial adoption Adoption in small amount Popularity

～ ～～ ～ ～ ～～ ～ ～ ～～ ～

- High current density fi Downsizing of IPM- High speed SW fi Downsizing of reactor- Simplification of cooling

[Cost reduction factors of other parts by SiC adoption]

201XNear futurePresent

...

...

Figure 20 (online colour at: www.pss-b.com) Scenario for the

successful introduction of SiC in the HV market. a large current density of 1000 A/cm2 or higher to exploit the material properties. And finally, the required packaging technologies include high-temperature packaging technol-ogy, and high-efficiency cooling technology. We firmly believe these technologies will lead to major breakthroughs in HV systems. As for the properties of GaN, this is a wide-bandgap semiconductor as is SiC. SiC has been described in terms of expectations, but GaN is already being used in materials for light-emitting devices and in high-frequency circuits, and it is attracting attention as a material for power elec-tronics as well. We think it is a material with even more potential than SiC to play a leading role in the next genera-tion of power electronics. Figure 21 shows the trend of power density of PCUs for Toyota’s recent HVs. The power density has increased year by year, growing by a factor of approximately five during the three years from the 2004 Prius to the 2007 LS600h. We expect that improved power density will lead to more compact and lighter weight devices, and also to lower costs. We also expect it to help deliver greater driv-ing pleasure. We believe that wide-bandgap semiconduc-tors are an essential technology for achieving future im-provements in the power density.

2004MY 2005MY 2006MY 2007MY 201XMY

Pow

er D

ensi

ty

SiCGaN

Figure 21 (online colour at: www.pss-b.com) Trend of PCU

power density for Toyota’s recent HVs.

SiC deviceSiC waferSiC wafer SiC device

Module

(1) Substrate & epitaxial technology- Sublimation method

(RAF method)- Ge-doped epitaxy

(2) Power devicetechnology

- SiC diode- SiC-MOSFET- GaN FET

- Control technology- Circuit design

(4) Inverter system technology

(3) High temperature bonding technology

- Bonding materials- Cooling design

Inverter systemInverter system

SiC deviceSiC waferSiC wafer SiC device

Module

(1) Substrate & epitaxial technology- Sublimation method

(RAF method)- Ge-doped epitaxy

(2) Power devicetechnology

- SiC diode- SiC-MOSFET- GaN FET

- Control technology- Circuit design

(4) Inverter system technology

(3) High temperature bonding technology

- Bonding materials- Cooling design

Inverter systemInverter system

Figure 22 (online colour at: www.pss-b.com) Research and de-

velopment on wide-bandgap semiconductors in Toyota Group.

5 Toyota Group research and development on wide-bandgap semiconductor devices In the Toyota Group, we think that power electronics is a key technology for the automotive technology of the future, and we have been doing research and development in the field for many years. We have developed power electronics systems, cir-cuit designs, and packaging technologies such as modules and the like from the very beginning, and we have now broadened our efforts to semiconductor devices that sig-nificantly affect performance and to the materials used to form their crystalline substrates. In particular, since the HV was first commercialized ten years ago, we have raised our expectations for the development of power electronics technologies even higher. We have defined SiC and GaN as core materials for breakthroughs in power electronics technologies for the future, and we are energetically pursu-ing research and development in those areas (Fig. 22). Of course, many of these research projects are being con-ducted in partnerships with research institutions, manufac-turers, and universities around the world. Figure 23 summarizes the history of SiC wafer devel-opment, based on patent application data. In the Toyota Group, Toyota Central Research and Development Labora-tories (TCRDL) has conducted research into crystalline

1980 1990 2000シャープ

ﾉｰｽ ｶﾛﾗｲﾅ大＆CREE

昭和電工

Sicrystal

ブリヂストン�－�

電総研

Okmetic

シクスオン

松下寿Semisouth

日本電気

01020791Research start timing of SiC wafer (By patent application)

Sharp

NCSU & CREENSC

Showa Denko

SicrystalBridgestone

II - VI

ETL

Siemens

Okmetic

SiXON

Matsushita-Kotobuki Semisouth

NEC

Dow Corning (Sterling)

HOYA

01020791

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TCRDLDENSO

シャープ

ﾉｰｽ ｶﾛﾗｲﾅ大＆CREE

昭和電工

Sicrystal

ブリヂストン －

電総研

Okmetic

シクスオン

松下寿Semisouth

01020791Research start timing of SiC wafer (By patent application)

Sharp

NCSU & CREENSC

Showa Denko

SicrystalBridgestone

II - VI

ETL

Siemens

Okmetic

SiXON

Matsushita-Kotobuki Semisouth

NEC

Dow Corning (Sterling)

HOYA

01020791

SanyoAIST

TCRDLDENSO

Figure 23 (online colour at: www.pss-b.com) History of SiC wa-

fer development.

1230 K. Hamada: Electronics in electric vehicles/hybrid electric vehicles

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

ph

ysic

ap s sstat

us

solid

i b

• Micropipe free • EPD: 250 cm-2

dislocation density

b)Growth Direction (G.D.)

Step1

Step2

Step3

G.D.

G.D.

Step1

{0001}

{110

0}

{1120}

Growth dir

ectio

n a

-axis

seed

gro

wn c

ryst

alStep2

a-axis

a*-axis

c-ax

is

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ectio

n a

*-axis

a*-ax

is

a-axis

c-ax

is

seed

gro

wn c

ryst

al

10mm

1.0mm

• Micropipe free • EPD: 250 cm-2

dislocation density

b)Growth Direction (G.D.)

Step1

Step2

Step3

G.D.

G.D.

Step1

{0001}

{110

0}

{1120}

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ectio

n a

-axis

seed

gro

wn c

ryst

alStep2

a-axis

a*-axis

c-ax

is

a-axis

a*-axis

c-ax

is

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ectio

n a

*-axis

a*-ax

is

a-axis

c-ax

is

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is

a-axis

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is

seed

gro

wn c

ryst

al

10mm

1.0mm

a)

Figure 24 (online colour at: www.pss-b.com) Repeated a-face

(RAF) process.

substrates since the early 1990s. The repeated a-face (RAF) growth method (Fig. 24) that TCRDL announced jointly with Denso Corporation in the journal Nature in August 2004 has attracted attention from academia as a crystal growth method that, in principle, does not generate micropipes [8]. This technology can be used to grow ultrahigh-quality SiC single crystals. The first step is growth of the first a-face. At this time, there is a high density of dislocations that are inherited from the crude seed crystal. Next, a sec-ond a-face is grown, perpendicular to the first a-face. At this time, there is a lower density of dislocations that are inherited from the seed crystal because most of the disloca-tions are parallel to the seed surface. By repeating this process, it is possible to reduce the density of dislocations that are inherited from the seed crystal. Finally, the c-face growth is performed with an offset angle of several de- grees. This eliminates stacking faults because the faults propagate only along the c-plane, making it possible to re-duce the dislocation density by 2 or 3 orders of magnitude. This process makes it possible to produce ultrahigh-quality SiC single crystals. The image on the right-hand side of Fig. 24 shows a 2-inch RAF substrate which is micropipe

1980 1990 2000

ローム日産

松下電器

関西電力

産総研

GE

Siemens(Infenion)

NorthropGrumman

CREE

ABB

豊田中研＆デンソー

ROHMNissan

ETL

Matsushita

Kansai Electric Power

AIST

GE

Siemens (Infenion)

Northrop Grumman

Sanyo

CREEFuji Electric

HitachiToshiba

豊田中研＆デンソーTCRDL

DENSO

Research start timing of SiC device (By patent application)20101970

Mitsubishi

NASASharp

1980 1990 2000

ローム日産

松下電器

関西電力

産総研

GE

Siemens(Infenion)

NorthropGrumman

CREE

ABB

豊田中研＆デンソー

ROHMNissan

ETL

Matsushita

Kansai Electric Power

AIST

GE

Siemens (Infenion)

Northrop Grumman

Sanyo

CREEFuji Electric

HitachiToshiba

豊田中研＆デンソーTCRDL

DENSO

Research start timing of SiC device (By patent application)20101970

Mitsubishi

NASASharp

Figure 25 (online colour at: www.pss-b.com) History of SiC de-

vice development.

I-V characteristic of JBS

1.0

Forward Voltage [V]

–400–800–1200–1600–2000

-0.1

-0.2

-0.3

-0.4

-0.5

Lea

kage

Cur

rent

[m

A]

2.0 3.0 4.0 5.0

Reverse Voltage [V] Forw

ard

Cur

rent

[A

/cm

2 ]

300

600

40 A

1.0

Forward Voltage [V]

–400–800–1200–1600–2000

-0.1

-0.2

-0.3

-0.4

-0.5

Lea

kage

Cur

rent

[m

A]

2.0 3.0 4.0 5.0

Reverse Voltage [V] Forw

ard

Cur

rent

[A

/cm

2 ]

300

600

40 A

Low leakage current< 10 µA/cm2 @1200V

Picture of Mo-JBS.(Schottky contact area:11.9 mm2)

Resurf + GR

Schottky metal: Molybdenum

N-type epitaxial layer

4H-SiC substrate

JBS Structure

5 mm

Vb = 1660 V Ron= 7.5 mW cm2

VF= 2.5 V

Vb= 1660 V

Ron= 7.5 mWcm2

VF = 2.5 V

40A

F3.9mm

Figure 26 (online colour at: www.pss-b.com) High blocking

voltage, low-resistance JBS diode.

free, and which has an etch pit density (EPD) of approxi-mately 250 cm–2. This is a reduction of 1/100 to 1/1000 as compared with the EPD of a conventional substrate. Under the current conditions, this technology is applicable up to 3 inches. In the area of epitaxial growth technologies, Toyota is developing technologies for epitaxial growth with low dislocation density. We announced, at a Material Research Society (MRS) conference in 2006, the reduction of the dislocation density of the epitaxial layer by 50% by placing an approximately 10 nm thick Ge-doped buffer layer on the substrate under the epitaxial layer [9]. Figure 25 summarizes the history of SiC device develop-ment, based on patent application data. TCRDL began re-search in this area in the mid-1980s, and that research is actively continued by Denso today. Our work on SiC de-vices includes research on diodes and metal oxide semi-conductor field effect transistors (MOSFETs). Denso fab-ricated junction barrier Schottky (JBS) diodes with diam- eters of 3.9 mm (Fig. 26). The JBS diode has a large for-ward current of 40 A at 2.5 V forward bias and a high breakdown voltage of 1660 V [10, 11]. We have been in-vestigating techniques to improve the channel mobility of SiC MOSFETs. Denso found that a new wet annealing process on the (1120) a-face wafer is very effective for improving the channel mobility (Fig. 27). A MOSFET

0

50

100

150

200

250

300

0 5 10 15 20 25 30

Gate Voltage(V)Cha

nnel

Mob

ility(

cm2 /V

s)a-face substrate

n+substratep epitaxial layer

p+ n+n+

GateBase Source Drain

Lateral MOSFET on a-face wafer

Channel Mobility of (11-20) a-face

Si-face substrateWafer preparation

244cm2/Vs

Figure 27 (online colour at: www.pss-b.com) High channel mo-

bility of (1120) a-face MOSFET.

phys. stat. sol. (b) 245, No. 7 (2008) 1231

www.pss-b.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Expert

Opinion

ｐGaN:0.1um ｐGaN

ｎ-GaN:0.5um

PolySi

Freestanding GaNsubstrate

Buried-p– GaN

n– GaN

Drain

Gate

AlN

Undoped-GaN

AlGaN

t 100nm

t=300nm

Source

t = 3µmSi:1×1016/cm3

0

20

40

60

80

0 2 4 6 8 10

Drain voltage (V)

Dra

in c

urre

nt (A

/cm

2 )

Vg=10V

5V

0V-5V

Channel length=2µmAperture width=2µm

Source

Gate10?m

Source

Gate10µm

n–GaN

Mg:5×1019/cm3

Aperture

RON: 52mΩ cm2

ｐGaN:0.1um ｐGaN

ｎ-GaN:0.5um

PolySi

Freestanding GaNsubstrate

Buried-p– GaN

n– GaN

Drain

Gate

AlN

Undoped-GaN

AlGaN

t� 100nm

t=300nm

Source

t = 3µmSi:1×1016/cm3

0

20

40

60

80

0 2 4 6 8 10

Drain voltage (V)

Dra

in c

urre

nt (A

/cm

2 )

Vg=10V

5V

0V-5V

Channel length=2µmAperture width=2µm

Source

Gate10?m

Source

Gate10µm

n–GaN

Mg:5×1019/cm3Mg:5×1019/cm3

Aperture

RON: 52mΩ cm2

Figure 28 (online colour at: www.pss-b.com) Normally-off ver-

tical device of AlGaN/GaN HEMT.

with a high channel mobility of 244 cm2/(V s) on the a-face was obtained [12–14]. As for GaN devices, a nor-mally-off vertical device structure is considered essential for power semiconductors, and we are researching ways to create such a structure using GaN high electron mobility transistors (HEMTs). Toyota and TCRDL showed the world’s first ‘normally-off vertical AlGaN/GaN HEMTs’ (Fig. 28) [15]. Strictly speaking, it is not truly normally-off, and the performance is not yet satisfactory. However, we are proceeding with continued research concerning this technology for use in power elements for HVs. We are also conducting research on highly reliable high-temperature bonding technologies, which are essential for using these wide-bandgap semiconductors, and on a device model for accurately predicting the effects of using these technolo-gies for inverter circuits. TCRDL and Tohoku University have found that a new solder, in which CuAlMn has been added to Bi, yields reliability at −40 to 250 °C over 200 cycles, and no marked failures have been found on the bonding face after 2000 cycles of a thermal cycle test at −40 to 200 °C [16]. We understand that high-accuracy circuit simulation is essential for high-performance inverter design, and we are proceeding with research of both inverter circuit models and models of the elements that are used in them. Toyota is conducting research for creating a physical base model for SiC diodes and SiC MOSFETs in cooperation with War-wick University. Element modelling is nearly completed, and we are successfully obtaining results that have good consistency with the actual switching waveform [17].

6 Conclusions If we are to achieve the sustainable mobility society before global warming reaches the critical stage, we must develop and provide vehicles with the least environmental burden possible. The issue, in other words, is how to provide to society the current eco-vehicle, the HV, and the ultimate eco-vehicle, the FCHV, quickly and at low cost. We must also further evolve and widen the use

of the hybrid technology that is the core technology shared by both the HV and the FCHV. To do so, we absolutely must reduce the loss and lower the cost of power electron-ics parts, especially the inverter, while making them more compact as well. Toyota is actively pursuing research and development on wide-bandgap semiconductors, par-ticularly those using SiC and GaN, as key devices for achieving those goals. But this sort of grand-scale research and development cannot be done by just one company or group of companies. We hope that the professionals from around the world will share in our commitment, and share with us their wisdom and passion, by pursuing research and development to bring a wonderful future to human-kind.

Acknowledgements The author would like to thank Mr

Shoichi Onda, Mr Fusao Hirose, Dr Eiichi Okuno, Mr Takeshi

Endo, Mr Takeo Yamamoto (Denso Corporation), Mr Toyokazu

Ohnishi, Mr Hirokazu Fujiwara, and Mr Konishi Masaki (Toyota

Motor Corporation).

References [1] A. Gore, An Inconvenient Truth (Rodale Press, 2006),

pp. 194–195.

[2] ACIA, Arctic Climate Impact Assessment (Cambridge Uni-

versity Press, 2005), p. 205 (http://www.acia.uaf.edu/pages/

scientific.html).

[3] K. Hamada, T. Kushida, A. Kawahashi, and M. Ishiko,

Proc. ISPSD 2001, p. 449.

[4] A. Kawahashi, Proc. ISPSD 2004, p. 23.

[5] K. Hamada, T. Fukami, K. Hotta, T. Sugiyama, S. Kawaji,

and M. Ishiko, Proc. IPEC-Niigata, 2005, p. 321.

[6] H. Ishiyama et al., SAE World Congress & Exhibition,

2007, SAE 2007-01-0271.

[7] Koito Manufacturing Co. Ltd HP, http://www.koito.co.jp/

pdf/news/07/20070327.pdf.

[8] D. Nakamura, I. Gunjishima, S. Yamaguchi, T. Ito, A. Oka-

moto, H. Kondo, S. Onda, and K. Takatori, Nature 430,

1009 (2004).

[9] A. Seki, A. Manabe, Y. Ishikawa, and N. Shibata, Proc. Ma-

ter. Res. Soc. Symp. 911, B2–4 (2006).

[10] T. Yamamoto, T. Endo, N. Kato, H. Nakamura, and T. Sa-

kakibara, Mater. Sci. Forum 556/557, 857 (2007).

[11] T. Yamamoto, T. Endo, E. Okuno, T. Sakakibara, and

S. Onda, ICSCRM 2007, We-P-74.

[12] E. Okuno, T. Endo, H. Matsuki, T. Sakakibara, and H. Ta-

naka, Mater. Sci. Forum 483/485, 817 (2005).

[13] T. Endo, E. Okuno, T. Sakakibara, and S. Onda, ICSCRM

2007, We-P-50.

[14] E. Okuno, T. Endo, T. Sakakibara, and S. Onda, ICSCRM

2007, Th-3B-6.

[15] M. Sugimoto, H. Ueda, M. Kanechika, N. Soejima, T. Ue-

sugi, and T. Kachi, Proc. PCC-Nagoya, 2007, p. 368.

[16] Y. Yamada, Y. Takaku, Y. Yagi, Y. Nishibe, I. Ohnuma,

and K. Ishida, Microelectron. Reliab. 46, 1932 (2006).

[17] G. J. Roberts, A. T. Bryant, P.A. Mawby, T. Ueta, T. Nishi-

jima, and K. Hamada, EPE 2007, p. 0277.

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