1 Present status and future prospects for electronics in ... · 6 1Present status and future prospects for electronics Surplusenergy Regenerative braking-+ Energy is reused A c c

1

Silicon Carbide, Vol. 2: Power Devices and Sensors

Edited by Peter Friedrichs, Tsunenobu Kimoto, Lothar Ley, and Gerhard PenslCopyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-40997-6

Kimimori Hamada

1.1

Issues surrounding automobiles

We humans have achieved great cultural developments over the past thousand

or more years. Figure 1.1 shows the changes in the atmospheric CO2 concen-

tration over the past approximately 1000 years. Together with the increase in

the consumption 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. Together with the

rapid increase in atmospheric CO2 concentration, 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. 1.2 [1, 2]. As all are aware, global warming caused by CO2 and other fac-

tors 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. Toy-

ota understands that the rising concentration of CO2 in the atmosphere is a se-

rious problem.

When evaluating the impact of CO2 generated by automotive fuels and

power trains, it is important to evaluate not only the CO2 generated by consum-

ing fuel, but also the total amount of CO2 generated from production to con-

sumption – in other words the well-to-wheel CO2. Figure 1.3 shows a compari-

son 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 com-

pared 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, synthetic fuels, hydrogen, and electricity, are also shown.

These show the different levels of well-to-wheel CO2 which result from differ-

ent materials and production methods. The amount of well-to-wheel CO2 that

is generated by automobile use is determined by both the type of fuel and the

1

Present status and future prospects for electronics

in electric vehicles/hybrid electric vehicles and

expectations for wide-bandgap semiconductor devices

2 1 Present status and future prospects for electronics

Source: IPCC 95

380

360

340

320

300

280

260800 1000 1200 1400 1600 1800 2000

year

CO2conc.(ppmv)

Hawaii Mauna Loa

Observatory data

D47D47

D57D57

SipleSiple

South PoleSouth Pole

Industry

19%

Electricity

generation

43%

Source: IEA/WEO 20042002 data

CO２Emissions by Sector

Residential&

commercial15%

Transport

23%

Transport

23%

Source: IPCC 95

380

360

340

320

300

280

260800 1000 1200 1400 1600 1800 2000

year

CO2conc.(ppmv)

Hawaii Mauna Loa

Observatory data

Hawaii Mauna Loa

Observatory data

D47D47

D57D57

SipleSiple

South PoleSouth Pole

Industry

19%

Electricity

generation

43%

Source: IEA/WEO 20042002 data

CO２Emissions by Sector

Residential&

commercial15%

Transport

23%

Transport

23%

Figure 1.1 Atmospheric CO2 concentration.

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 fa-

talities in Japan, the USA, and Europe has decreased slightly over the past

30 years; however, the overall level remains high (Fig. 1.4). In China, which

ranks second in the world in the number of automobiles sold, there were

100000 traffic fatalities in 2005, making this issue a serious problem. Auto-

Source:GISSSource:GISS

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

0.8degree

(1900to2000)

19921992 20022002 20052005

Red : Melting Area

Figure 1.2 Global temperature and melting ice in Greenland.

1.1 Issues surrounding automobiles 3

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


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 1.3 Well-to-wheel CO2 emissions.

mobile manufacturers recognize the need for continued efforts aimed at reduc-

ing traffic fatalities to zero. Toyota refers to the ability of users to continuously

enjoy the convenience provided by automobiles as ‘sustainable mobility’. In

order to achieve this, we are carrying out research and development under the

slogan of ‘Zeronize & Maximize’. This refers to taking on the endless chal-

lenge of minimizing the negative aspects of automobiles, such as CO2 emis-

sions, air pollution, traffic fatalities, and congestion, while maximizing auto-

mobile comfort, enjoyment, and excitement.

We believe there are three major directions for technological development:

the environment, safety, and comfort. For the purpose of ‘Zeronize & Maxi-

mize’ we identify the precise items which must be zeronized or maximized in

each category, and are making definite progress in technological innovations

aimed at the ultimate goals. The ultimate goals are an ultra-highly efficient en-

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

TrafficFatalities[Thousands]

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

TrafficFatalities[Thousands]

Year

Figure 1.4 Trends of traffic fatalities.


DieselDiesel

engineengine

Diesel DI

DPNR

Diesel HV

ElectricElectric

vehiclevehicle

EV

FCHV

GasolineGasoline

engineengine

VVT

Lean-burn

D-4

THS

Ultimate Eco-VehicleUltimate Eco-Vehicle

Gate 1

Emissions

Energy

Diversification

Gate 2

Gate 3CO2

CNG

AlternativeAlternative

energyenergy

GTL

BTL PHV

the Right Place the Right Timethe Right Car

Hybrid Technology

Figure 1.5 Creating the ‘ultimate eco-vehicle’ (CNG,

compressed natural gas; GTL, gas to liquids; BTL,

biomass to liquids; DI, direct injection; DPNR, diesel

particulate NOx reduction system; VVT, variable valve

timing; THS, Toyota hybrid system; PHV, plug-in hybrid

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

vehicle).

ergy 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 Presi-

dent Watanabe. 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 refuel-

ling’, 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 possible. However in the area of the environment, we be-

lieve that we can come closer to creating the ‘ultimate eco-vehicle’ by increas-

ing the environmental performance of the power train, utilizing new fuels and

electrical energy, and integrating hybrid technology into all of the results

(Fig. 1.5). We are confident that hybrid technology will truly be one of the

core technologies of the 21st century.

1.2

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 mini-

van HV, diesel truck HV, sports utility vehicle HV, medium-size sedan HV,

and others (Fig. 1.6). In the future, we will continue expanding the number of

1.2 Past, present, and future of Toyota hybrid vehicles 5

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

201X

HVsales

HVsales

1,0001,000

800800

600600

400400

200200

YearYear

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

201X

HVsales

HVsales(Thousands)

1,0001,000

800800

600600

400400

200200

YearYear

Figure 1.6 Trend of Toyota HV sales.

HV models, and intend to achieve yearly sales of one million HVs as early as

possible in the 2010s.

Figure 1.7 shows the relationship between vehicle weight and fuel consump-

tion. 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 approxi-

mately 20% when improvements are made to a normal engine; however, an

improvement in fuel efficiency of nearly twice the normal engine is possible

with the HV. Figure 1.8 explains the reason for the improved efficiency of the

hybrids. When the vehicle is stopped, the engine stops idling and does not con-

sume energy. During acceleration and low-speed driving, in ranges where

gasoline engine efficiency is poor, the high-efficiency electric motor is primar-

Vehicle weight [kg]

500 1000 1500 2000 25000

5

10

15

20

25

30

Fuelconsumption(Japanese10-15mode)[km/l]

1st gen.Prius

HVDirect injection gasoline engine

Conventional gasoline engine

(Japanese AT vehicles)

Estima HV

2nd gen. Prius

Alphard HV

++100%100%

Conventional engine

Direct injection engine

Lean burn engine

HV

Figure 1.7 Vehicle weight and fuel consumption.


Surplus energy Regenerativebraking

--

++

Energy is reused

Acceleration

Engine outputDeceleration

Time0

Energy

Battery

Figure 1.8 Hybrid technology energy management.

ily used for driving. When accelerating, both the gasoline engine and the elec-

tric motor are used to gain sufficient acceleration. During normal driving, the

engine runs in the high-efficiency range at all times. The energy 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 recovered by the bat-

tery. In this way, fuel efficiency is dramatically improved by operating the en-

gine 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 driv-

ing conditions.

Figure 1.9 shows the structure of Toyota’s present hybrid system named

Toyota Hybrid System II (THS-II). The engine, generator, and motor are con-

nected mechanically by means of a power split device, while motor, generator,

and battery are connected electrically via an inverter. This system adds a boost

Power split

Generator

Mechanical power path

Electrical power pathHybrid

transmission

Engine

Power control unit

device

Motor

Battery

Inverter Boost converter

Figure 1.9 Toyota Hybrid System II.


Battery

Motor

IGBT module

Power control unitPower control unit

Booster

system Generator

To

HV-ECU

controlboard

Figure 1.10 Electrical circuit of the power control unit.

converter between the battery and inverter, in order to obtain high motor volt-

age 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 inverter that provides tens of kilowatts of power to

drive the motor by converting direct current to alternating current. Figure 1.10

shows the electrical circuit. It is composed of the inverters for the boost con-

verter, motor, and generator, as well as capacitor, inductor, control circuit, and

other parts. These parts are contained within the power control unit (PCU).

Figure 1.11 shows the structure of the PCU that is used in the GS450h. The op-

timal 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 devices for hybrid technology. For example, the Prius

contains 18 insulated gate bipolar transistors (IGBTs) and 18 free wheeling di-

odes (FWDs) as power semiconductors that are used for driving (Fig. 1.12).

The IGBT is approximately 1 cm2 in size, and each IGBT can control a maxi-

mum current of nearly 200 A. For failsafe operation, the current sensor and

Reactor

Water-cooled heat sink

Smoothing capacitor

Boost power module

Inverter

& filter capacitor

Inverter power module

Converter

Figure 1.11 GS450h power control unit.


Smoothing filtercapacitor

Inductor

IGBT is located under thecapacitor

IGBT chipDiode chip

The sensor isThe sensor is

integrated in theintegrated in the

chip for failsafechip for failsafe

Figure 1.12 Prius power control unit and power semiconductors.

temperature sensor are built into the chip. At Toyota Motor Corporation, the

in-house development of IGBTs and FWDs has made a major contribution to

strengthening our capacity for hybrid system development, specifically the de-

velopment of more compact, higher performance, and lower cost hybrid sys-

tems in a short period of time [3–5].

The vehicle that Toyota is researching in order to utilize 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 technol-

ogy for sustainable mobility in the near future, and are now carrying out

verification trials on public roads (Fig. 1.13). 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, us-

ing Japan as an example, we see that the emissions are approximately one-third

a hybrid ofa hybrid of

electric 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 vehicle

with an external rechargerwith an external recharger

or

Household electrical energyGas station

Figure 1.13 Definition of plug-in hybrid vehicle.


1.01.0

0.50.5

2.02.0

PriusPrius PHVPHVConventional

power train vehicle


1.51.5

Gasoline

Diesel

GasolineHV

Gasoline

PHV

Figure 1.14 Well-to-wheel CO2 emissions in Japan.

of a conventional gasoline or diesel vehicle, and approximately half of a

gasoline HV (Fig. 1.14). However, there remains a major issue which must be

resolved in order to commercialize PHVs. If we assume that the necessary

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 luggage, 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 ultimate eco-vehicle, and we are actively proceeding with its devel-

opment (Fig. 1.15). Toyota has been aware of the future potential of this tech-

nology 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

Hybrid vehicle FCHV

Battery Battery

Engine

Motor Motor

Fuel cell

Powercontrolunit

Powercontrolunit

Power Control Unit

Toyota FC Stack

Motor

Battery

High-pressureHydrogen Tank

Toyota FCHV

Seats: 5 people

Max speed: 155 km/h

Max cruising range: 330 km

Figure 1.15 Toyota’s fuel cell hybrid vehicle (FCHV).


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 dis-

tance 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.

1.3

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 economy 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 capacity [6]. To handle the higher output den-

sity, a new inverter structure was adopted that cools the power semiconductors,

which generate heat, on both sides (Fig. 1.16). The final size is extremely com-

pact. A set of IGBT and FWD is placed in a moulded package called a power

card that can be cooled on both sides (Fig. 1.17). The power card utilizes

a double-sided cooling structure. Excellent cooling performance is achieved

by stacking multiple power cards inside a cooling unit. This makes it pos-

sible to efficiently cool the increased element heating that occurs with higher

output.

Card stack structure

Coolant

Power card

Figure 1.16 LS600h power control unit.

1.3 Newest hybrid vehicle 11Battery

Voltage-

boostingcircuit

Heat spreader

(Lead frame)

Power chips

(IGBT,FWD)

∑ Internal structure

Heat spreader(Lead frame)

Conductive

spacer

M

- Compact structure

achieved by single-unit

configuration (IGBT, FWD)

- Efficient transmission of

heat to cooling water

Heat spreader

(on both sides)

Figure 1.17 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 employed. Figure 1.18 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 de-

vices for the igniter, injection, combustion pressure sensor, and emission gas

sensor; and in light emitting diodes (LEDs) for the interior lights and head-

lamps. LED headlamps utilizing 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 rising temperatures, we have utilized highly heat-

resistant GaN LEDs and an original cooling structure. These headlamps illu-

Figure 1.18 Advanced technologies in the LS600h.


minate quickly when turned on, to reliably ensure the field of view. They have

a long lifetime and feature superior performance, including almost zero drop in

brightness or change in chromaticity over their lifetime.

1.4

Expectations for wide-bandgap semiconductors

in HV inverter applications

The chart in Fig. 1.19 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 elec-

tric 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 potential 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 characteris-

tics of SiC to simplify 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 semi-

conductor 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 apparent, 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

Melting point (°C)

Breakdownelectric field (V/cm)

Thermal conductivity

(W/cm °C)3

21

4 5

Si

SiC Radiation: x 3

High frequency: x 10

High temperature: x 3

Endurable for radioactivity: x 3

High temperature

sensor for car

Low loss power modulefor car communication

1

Saturation electron

velocity (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 100

High voltage: x 10

Figure 1.19 Characteristics and applications of wide-bandgap semiconductors.

1.4 Expectations for wide-bandgap semiconductor in HV inverter applications 13

SiC

Si

SiC Si

Relativecost

Future

SiC Si

SiC

Si

Research Trial adoptionAdoption insmall 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

Si

SiC Si

Relativecost

Future

SiC Si

SiC

Si

Research Trial adoptionAdoption insmall amount Popularity

～～～～～～～～～～～～

- High current densityfifi Downsizing of IPM- High speed SW fifi Downsizing of reactor

- Simplification of cooling

[Cost reduction factors of other parts

by SiC adoption]

201XNear futurePresent

.

.

.

.

.

.

Figure 1.20 Scenario for the successful introduction of SiC in the HV market.

than Si. We believe this will be the scenario for the success of SiC devices in

HV systems (Fig. 1.20). 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 ele-

ments with loss density that is at least an order of magnitude lower than Si

IGBTs, and a large current density of 1000 A/cm2 or higher to exploit the ma-

terial properties. And finally, the required packaging technologies include

high-temperature packaging technology, and high-efficiency cooling technol-

ogy. 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

22000044MMYY 22000055MMYY 22000066MMYY 22000077MMYY 220011XXMMYY

PowerDensity

SiCGaN

Figure 1.21 Trend of PCU power density for Toyota’s recent HVs.


used in materials for light-emitting devices and in high-frequency circuits, and

it is attracting attention as a material for power electronics as well. We think it

is a material with even more potential than SiC to play a leading role in the

next generation of power electronics.

Figure 1.21 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 ap-

proximately 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 driving pleasure. We believe that wide-bandgap semiconductors

are an essential technology for achieving future improvements in the power

density.

1.5

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, circuit designs, and packaging technologies such as modules and the

like from the very beginning, and we have now broadened our efforts to semi-

conductor devices that significantly 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 devel-

opment 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 pursuing research and development in

those areas (Fig. 1.22). Of course, many of these research projects are being

SiC deviceSiC waferSiC wafer SiC device

Module

(1) Substrate &

epitaxial technology- Sublimation method

(RAF method)

- Ge-doped epitaxy

(2) Power device

technology- 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 device

technology- 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 1.22 Research and development on wide-bandgap

semiconductors in Toyota Group.

1.5 Toyota group research and development on wide-bandgap semiconductor devices 15

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電総研

Okmetic

シクスオン

松下寿Semisouth

01020791

Research 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 1.23 History of SiC wafer development.

conducted in partnerships with research institutions, manufacturers, and uni-

versities around the world.

Figure 1.23 summarizes the history of SiC wafer development, based on pat-

ent application data. In the Toyota Group, Toyota Central Research and De-

velopment Laboratories (TCRDL) has conducted research into crystalline

substrates since the early 1990s. The repeated a-face (RAF) growth method

(Fig. 1.24) that TCRDL announced jointly with Denso Corporation in the jour-

nal 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 second

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

• Micropipe free

• EPD: 250 cm-2

dislocation density

b)Growth Direction (G.D.)

Step1

Step2

Step3

G.D.

G.D.

Step1

{0001}

{1100}

{1120}

Growthdirection

a-axis

seed

growncrystal

Step2

a-axis

a*-axis

c-axis

Growthdirection

a*-axis

a*-axis

a-axis

c-axis

seed

growncrystal

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}

{1100}

{1120}

Growthdirection

a-axis

seed

growncrystal

Step2

a-axis

a*-axis

c-axis

a-axis

a*-axis

c-axis

Growthdirection

a*-axis

a*-axis

a-axis

c-axis

a*-axis

a-axis

c-axis

seed

growncrystal

10mm

1.0mm

a)

Figure 1.24 Repeated a-face (RAF) process.


the dislocations 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

degrees. This eliminates stacking faults because the faults propagate only

along the c-plane, making it possible to reduce 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. 1.24

shows a 2-inch RAF substrate which is micropipe free, and which has an etch

pit density (EPD) of approximately 250 cm–2. This is a reduction of 1/100 to

1/1000 as compared with the EPD of a conventional substrate. Under the cur-

rent conditions, this technology is applicable up to 3 inches. In the area of epi-

taxial 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 1.25 summa-

rizes the history of SiC device development, based on patent application data.

TCRDL began research in this area in the mid-1980s, and that research is ac-

tively continued by Denso today. Our work on SiC devices includes research

on diodes and metal oxide semiconductor field effect transistors (MOSFETs).

Denso fab-ricated junction barrier Schottky (JBS) diodes with diameters of

3.9 mm (Fig. 1.26). The JBS diode has a large forward current of 40 A at 2.5 V

forward bias and a high breakdown voltage of 1660 V [10, 11]. We have been

investigating 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. 1.27). A MOSFET

with a high channel mobility of 244 cm2/(V s) on the a-face was obtained

[12–14]. As for GaN devices, a normally-off vertical device structure is con-

sidered essential for power semiconductors, and we are researching ways to

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

ROHM

Nissan

ETL

Matsushita

Kansai Electric Power

AIST

GE

Siemens (Infenion)

Northrop Grumman

Sanyo

CREE

Fuji Electric

Hitachi

Toshiba

豊田中研＆デンソーTCRDL

DENSO

Research start timing of SiC device (By patent application)

20101970

Mitsubishi

NASA

Sharp

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

ROHM

Nissan

ETL

Matsushita

Kansai Electric Power

AIST

GE

Siemens (Infenion)

Northrop Grumman

Sanyo

CREE

Fuji Electric

Hitachi

Toshiba

豊田中研＆デンソーTCRDL

DENSO

Research start timing of SiC device (By patent application)

20101970

Mitsubishi

NASA

Sharp

Figure 1.25 History of SiC device development.

1.5 Toyota group research and development on wide-bandgap semiconductor devices 17

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

kag

eC

urr

ent

[mA

]

2.0 3.0 4.0 5.0

Reverse Voltage [V] Fo

rward

Cu

rren

t[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

kag

eC

urr

ent

[mA

]

2.0 3.0 4.0 5.0

Reverse Voltage [V] Fo

rward

Cu

rren

t[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 1.26 High blocking voltage, low-resistance JBS diode.

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. 1.28) [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 semi-

conductors, and on a device model for accurately predicting the effects of us-

ing these technologies 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 in-

verter circuit models and models of the elements that are used in them. Toyota

0

50

100

150

200

250

300

0 5 10 15 20 25 30

Gate Voltage(V)ChannelMobility(cm2/Vs)

a-face

substrate

n+substrate

p epitaxial layer

p+ n+n+

GateBase Source Drain

Lateral MOSFET

on a-face wafer

Channel Mobility of

(11-20) a-faceSi-face substrateWafer preparation

244cm2/Vs

Figure 1.27 High channel mobility of (1120) a-face MOSFET.


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 = 3µm

Si:1×1016/cm3

0

20

40

60

80

0 2 4 6 8 10

Drain voltage (V)

Draincurrent(A/cm2)

Vg=10V

5V

0V

-5V

Channel length=2µm

Aperture 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 = 3µm

Si:1×1016/cm3

0

20

40

60

80

0 2 4 6 8 10

Drain voltage (V)

Draincurrent(A/cm2)

Vg=10V

5V

0V

-5V

Channel length=2µm

Aperture width=2µm

Source

Gate10?m

Source

Gate10µm

n–GaN

Mg:5×1019/cm3Mg:5×1019/cm3

Aperture

RON: 52mΩ cm2

Figure 1.28 Normally-off vertical device of AlGaN/GaN HEMT.

is conducting research for creating a physical base model for SiC diodes and

SiC MOSFETs in cooperation with Warwick University. Element modelling is

nearly completed, and we are successfully obtaining results that have good

consistency with the actual switching waveform [17].

1.6

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 electronics parts, especially the inverter, while making them

more compact as well. Toyota is actively pursuing research and development

on wide-bandgap semiconductors, particularly 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 commit-

ment, and share with us their wisdom and passion, by pursuing research and

development to bring a wonderful future to humankind.

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

References 19

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 Assess-

ment (Cambridge University Press,

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

pages/ scientific.html).

3. K. Hamada, T. Kushida, A. Kawaha-

shi, 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,

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6. H. Ishiyama et al., SAE World Con-

gress & Exhibition, 2007, SAE 2007-

01-0271.

7. Koito Manufacturing Co. Ltd HP,

http://www.koito.co.jp/pdf/news/

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8. D. Nakamura, I. Gunjishima, S. Yama-

guchi, T. Ito, A. Okamoto, H. Kondo,

S. Onda, and K. Takatori, Nature 430,

1009 (2004).

9. A. Seki, A. Manabe, Y. Ishikawa, and

N. Shibata, Proc. Mater. Res. Soc.

Symp. 911, B2–4 (2006).

10. T. Yamamoto, T. Endo, N. Kato,

H. Nakamura, and T. Sakakibara, Ma-

ter. 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. Sa-

kakibara, and H. Tanaka, 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. Uesugi, 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. Nishijima, and

K. Hamada, EPE 2007, p. 0277.


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