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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
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 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.
4 1 Present status and future prospects for electronics
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.
6 1 Present status and future prospects for electronics
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.
1.2 Past, present, and future of Toyota hybrid vehicles 7
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.
8 1 Present status and future prospects for electronics
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.
1.2 Past, present, and future of Toyota hybrid vehicles 9
1.01.0
0.50.5
2.02.0
PriusPrius PHVPHVConventional
power train vehicle
Japanese 10-15 test cycle
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).
10 1 Present status and future prospects for electronics
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.
12 1 Present status and future prospects for electronics
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.
14 1 Present status and future prospects for electronics
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
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ÿ-ÿ
電総研
Okmetic
シクスオン
松下寿Semisouth
日本電気
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
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
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.
16 1 Present status and future prospects for electronics
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.
18 1 Present status and future prospects for electronics
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).
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22 1 Present status and future prospects for electronics
top related