Present Status and Future Prospect of the Power ...

Beijing University SeminarBeijing, China, 2007.11.22 National Institute of Advanced Industrial Science and Technology

Beijing University Semimar

Present Status and Future Prospect ofthe Power Electronics Based on

Widegap Semiconductors

National Institute of Advanced Industrial Science & Industry

Power Electronics Research Center

Hajime Okumura

2007.11.22


o MITI(Ministry of International Trade and Industry)

AIST(Agency of Industrial Science and Technology)

…

…

Electrotechnical laboratory(ETL)

…

…

Establishment of New AIST(Re-organization of Japanese National Institutes)

(15 Institutes)(56 Research Units)

o METI(Ministry of Economy, Trade and Industry)

AIST(National institute of Advanced Industrial Scienceand Technology)

…

Power Electronics Research Center(PERC)Widegap Semiconductor Electronics

…

9 research bases and 2 headquarters

(3200 permanent staffs)

http://www.aist.go.jp


Organization of new AIST

Research Facilities Dept.

Financial Affairs Dept.

Human Resource Dept.

General Administration Dept.

Int. Affairs Dept.

Public Relation Dept.

AIST Innovations (TLO)

Collaboration Dept.

Technology Information Dept.

AIST Innovation Center for Start-ups

Int. Patent Organism Deposaitry

Tsukuba Advanced Computing Center (TACC)

2 Special Divisions (AIST Kansai) ........ ........

12 Research Initiatives ........ ........ ........

27 Research Centers ........ ........ Power Electronics Research Center (PERC) ........ ........

21 Research Institutes ........ ........ ........ ........

Research Coordinator

Superintendent

Planning Headquarters

General AdministrationHeadquarters

Evaluation Dept.

Safety and EnvironmentalProtection Dept.

AIST Advisory board

AIST fellow

Auditor

PresidentVice-PresidentTrustee

9 Research Bases AIST Hokkaido AIST Tohoku AIST Tsukuba AIST Tokyo Waterfront AIST Chubu AIST Kansai AIST Chugoku AIST Shikoku AIST Kyushu

2003.3


Mission and Activity Area

Mission(a) Industrial infrastructure technology, including measurement standards, geological

surveys, and the development of base technologies necessary for the maintenance of thetechno-infrastructure of Japan.

(b) Energy and environmental technology, which because of long lead times and highrisk require the government to search for solutions.

(c) Interdisciplinary and broad-spectrum research activities to promote innovation andreinforce the international competitive strength of Japanese industry and encourage thecreation of new industries.

Activity (Research Fields)(1) Life Science and technology

(2) Information Technology

(3) Environment and Energy

(4) Nanotechnology, Materials and Manufacturing

(5) Geological Survey and Geoscience, Marine Science and Technology

(6) Standards and Measurement Science and Technology


Research Scheme and FundSubsidy from METI,

Entrustment from METI,

Entrustment from other ministries,

Subsidy or entrustment from public research funding

organizations such as NEDO, JST

Entrustment from or collaboration with private

companies

Trend of recent governmental fund• Industrialization,• Collaboration of Univ., National Inst. and

Private Sector


Mission of PERC

Development of the electronics based on widegap

semiconductor materials and science,

Application of the related technology to actual information

and energy networks in the human society, in order to

contribute to the innovation of life line and energy saving

Teams of PERC1. Wafer & Characterization Team SiC bulk & epitaxial growth, wafer characterization2. SiC Power Device Team SiC device technology3. GaN Power Device Team III-Nitride device technology4. Power-Unit Super-Design Team Design & simulation of power devices and modules5. Super-Node Network Team Networking technology using low-loss power devices6. Advanced power electronics promotion team Industrialization of power device technology


Contents

1. Importance of wireless communication,power electronics in the 21th century

2. Requirements from system application tohigh-power electron device

3. Characteristics of widegap semiconductors

4. High-power operation by widegap semiconductor devices

5. Present R&D status of high-power electron devices

6. Problems and future prospectSiC devices or GaN devices ?

Electron devices (high-power)

•High-frequency device (analog appl.)

•Switching device (digital appl.)

•SiC

•III-nitrides

Widegap semiconductors


21th century

networking

freightEV/HEV

Nest-generation

transportation

InformationInternet

Wireless communication

Computer

Man-machine interface

EnergyCooperation between

large scale generators

and dispersed generators

Electronics

Innovative high-power device

networking

Infrastructure of the 21th century

Information Electronics Energy Electronics

Environment Energy utilization Economic Growth

Energy

saving

Industry Convenience


Fixed Wireless CommunicationWireless Terminal Access

FWA, LMDS, WLL)Mobile CommunicationMobile Telephone System

Communication betweenBase Stations

PrincipalBackbone System

Home NetworkWireless LAN

Satellite CommunicationSatellite BroadcastingGround Broadcasting

Intelligent Transportation System (ITS) -- VICS, ETC

Various applications ofwireless communication


2nd gen. (Digital cellular)

Quasi-Linear region

multi-carrier common amp.

200W/sector

Trend of Power FET Specificationfor Mobile Telephone Base Station

In Out

Common amplification

of multi carriers

3rd gen. (Total Digital Service)

Strict Linear region

W-CDMA IMT-2000

500W 1kW/sector

1st gen. (Analog cellular)

Saturated region is enough

Strict Linear

Ou

tpu

t P

ow

er

(d

Bm

)

Quasi- Linear

Saturated

Input power (dBm)

Po

wer

Ad

ded

eff

icie

ncy (

%)

Keeping Transmission distanceEnlarging band width (Channel number)Keeping Linearity Higher Power


Energy Consumption and Electricity Ratio

0

50000

100000

150000

200000

250000

300000

350000

400000

10

15

20

25

30

1965 1970 1975 1980 1985 1990 1995 2000

(%)


Energy Saving in Electric Power

Application of Electronics to the control of Electric Power (Energy Electronics, Power Electronics) High Voltage, Large Current

Urgent issue, considering Electricity Ratio

Application Field of Power Electronics

Power Conversion(AD/DA conversion

Inverter circuit

Power Device(Transistor)

Operating Frequency (kHz)

Pow

er C

onve

rsio

n C

apac

ity

(kV

A)

Application fields

of WGS FETs

Thyristor

Power ICs

GTO

MOSFET

HVDCMachines

Railways

Switching regulators

VTRs, Mobile phonesTelephone exchangers,PDP drivers

0.1 1 10 100 1000

EVs/HEVs

104

103

102

101

100

105

IGBT

Motors, InvertersAir conditioners

UPSsDistgributed power supplies

Bipolartransistor


Example of Power device usagein Electric Power Converter

Device chips with small

size and low resistance

Device modules with small

size and higher operation

frequency

Simple and small peripherals

Innovation of

Power Electronics

Key of Energy saving

technology

in Electric Power

What is an ideal switch ?on-state zero resistance

off-state resistance infinity

Motor

(20kW)

Battery

EB=350V

IGBT

IGBT

SBD

SBD

DC48V

100A

DC/AC inverter

DC/DC converter

FET

Electrical switch (Switching device)

Blocking voltage and conduction loss (onresistance) show a trade-off relation


(a): Switching Power Supply (b): Motor Drive Inveter

Analysis of Power Loss in TypicalElectric Power Converters

Components of loss

Power devices Passive elements 60% : 40%

Lidow, et.al, Proc. of IEEE, 89, 803 (2001)

Capacitor

30%

Inductor

10%

Synchronous rectifying FET: 23%

Control FET

36%

Other

%IGBT/FRD

46%

Rectifying diode : 16%

Coil

10%

Control

7%

SMPS

12%

Capacitor 9%


Device Specification Requirementsfrom Application Needs

1. High frequency: Enlarged frequency domain,

large-capacity high-speed communication,

broad band

2. High output power long-distance transmission

broad band, low distortion

3. High operation voltage high-efficiency, low-loss, small size

High-frequency devices for wireless communication

1. High blocking voltage applicability, reliability

2. Low on-resistance reduction of conduction loss

3. High switching speed small size

4. Low electrostatic capacity reduction of switching loss, high-speed switching

5. High tolerance reliability, safety

Switching devices for power electronics


What is WidegapSemiconductors ?

C : Carbides IV-IV group IV-IV group polytypespolytypes MOS structureMOS structure Indirect band structure Indirect band structure

N : Nitrides III-V group III-V group Hetero Hetero sturucturesturucture AlloysAlloys Direct band structure Direct band structure

O : Oxides II-VI group II-VI group Insulator to Superconductors Insulator to Superconductors

Light Element SemiconductorsLarge binding energyHigh melting pointSmall lattice constant

LargeBandgapThermal conductivityBreakdown voltageSaturation drift velocitythermal and chemical stability

Hard ElectronicsHigh temperature devicesHigh power devicesHigh frequency devices Short wavelength devices

Si

Ge

Sn

Al

Ga

In

P

As

Sb

S

Se

Te

Mg

Zn

Cd

2

3

4

5

B C N O

III IV V VI


Lattice Constants and Bandgap

c-BN

h-BN

��

AlN

3C-SiC

6H-SiC4H-SiC

15R-SiC

BP

GaN

ZnO

BAs

InN

Si

AlP

c-ZnS

GaP

h-ZnS

Ge

AlAs

GaAs

ZnSe HgSCdS

InP

HgSe

CdSeZnTe

InAs

GaSb

SnInSb

CdTe

HgTe

2H-SiC

MgS

MgSe

0

2

4

6

8

1.5 2.0 2.5 3.0

Ban

dgap

(eV

) -

R.T

.

Diamond

Bond Length (Å)


Physical Properties ofSemiconductors

Material Eg μ Ec sat band typeeV cm2/Vs 106V/cm 107cm/s W/cmK

Si 1.1 11.8 1350 0.3 1.0 1.5 IGaAs 1.4 12.8 8500 0.4 2.0 0.5 D

c-GaN 3.27 9.9 1000 1 2.5 1.3* Dh-GaN 3.39 9.0 900 3.3 2.5 1.3 D3C-SiC 2.2 9.6 900 1.2 2.0 4.5 I6H-SiC 3.0 9.7 370a, 50c 2.4 2.0 4.5 I4H-SiC 3.26 10 720a, 650c 2.0 2.0 4.5 I

AlN 6.1 8.7 1100 11.7 1.8 2.5 DDiamond 5.45 5.5 1900 5.6 2.7 20 I

a: along a-axis, c: along c-axis, *: estimate


Figures of Merits of Several Semiconductorsand their Hetrostuructures

Material Johnson’s FM Keyes’s FM Shenai’s FM(QF1) Shenai’s FM(QF2) Baliga’s FM Baliga’s HFM

(Ec sat/ )2 ( sat/ )1/2A AEc μEc

3 μEc2

Si 1 1 1 1 1 1GaAs 7.1 0.45 5.2 6.9 15.6 10.8

c-GaN 685 1.5 20 67 23 8.2h-GaN 760 1.6 560 6220 650 77.83C-SiC 65 1.6 100 400 33.4 10.36H-SiC 260 4.68 330 2670 110 16.94H-SiC 180 4.61 390 2580 130 22.9

AlN 5120 21 52890 2059000 31700 1100Diamond 2540 32.1 54860 1024000 4110 470

A=Shenai’s FM(QF3)=_ μEc3 T.P. Cho, Materials Science Forum, Vols. 338-342 (2000) 1155.

SH-HEMT on GaAs DH-HEMT on GaAs P-HEMT on InP GaN-HEMT on sapphire

μ (cm2/Vs) 5000 � 6500 5000 � 6500 9500 � 12000 800 � 1700

ns (1012/cm3) 1.5 � 2.5 2.0 � 3.0 3.0 � 4.0 15 � 20

ns μ (1015/Vs) 7 � 16 10 � 20 30 � 50 12 � 34

Rch ( /sq) 400 � 600 300 � 500 150 � 250 200 � 520

By H. Kawai


Saturation Drift Velocity &Breakdown Voltage vs. Electric Field

High Breakdown Voltage

High Drift Velocity

Saturation at High Field

50 100 150 200 250 3000Electric Field (kV/cm)

Vel

ocit

y (1

07cm

/s)

1

2

3

0

GaN

InP

Si

GaAs

Characteristics of GaN

Dri

ft v

eloc

ity

(10

cm

/s)

7

High Voltage operation ispossible even aftermicrointegratioin

Suitable for high-powerhigh-frequency operation


Properties of Wide BandgapSemiconductor Devices

Breakdown Voltage (V)100 1000 10000

100

1

0.01

0.0001On

Res

ista

nce

( c

m2 )

Si6H-SiC

4H-SiCGaN

Diamond

On resistance andBreakdown Voltage

Temperature dependence ofp-n junction leak current

100

10-2

10-4

10-6

1.0 1.5 2.0 2.5

Si

GaAs

6H-SiC

GaN

Diamond

1000/Temperature (1/K)

Lea

k C

urre

nt

Den

sity

(/c

m2 )


Applied electric field

Structures of High-Power Electron Device

n+ substrate

n- epilayer

n+

p+p+

drain

sourcegate

n+ substrate

n- epilayer

n+ n+

p+ p+

drain

gatesourceOxide

JFETJunction field Effect

Transistor

MOSFETMetal Oxide Semiconductor

Field Effect Transistor

p+ substrate

n- epilayer

n+ n+

p+ p+

drain

gatesourceOxide

IGBTInsulated Gate

Bipolar Transistor

Unipolar deviceBipolar device

drain

Semi-insulating substrate

Highly resistive buffer

Barrier

source gate

n- epilayer

- - - -

n+n+

HFETHeterojunction

Field Effect Transistor

Vertical deviceLateral device

2DEG channel


Under Class A amplification,Pout = (1/8) (VB-Vknee) Imax

Imax ns μ

VB EC

Under Imax=1A/mm, VB=80V (Vsd=40V), and Wg=20mm,

Pout=10W/mm and Ptotal=200W are obtained.

Vd

IdImax

VBVknee

Load line

Vg

Input signal

High-power operation of HF device


10-1

100

101

102

103

1G 10G 100GFrequency (Hz)

Thermal LimitMaterial Parameter Limit

Current Gain Limit

P fT vs Ec1 2

3P fT2 Constant

Ave

rage

Pow

er (

W)

Operation limit ofHigh-Frequency Devices


Out

put

Pow

er D

ensi

ty (

W/m

m) 100

10

1

0.1

0.011 2 3 5 7 10 20 30 50 70 100

Drain Voltage (V)

GaN HFET / SiC

GaN HFET / sapphire

GaAs MESFET

4H-SiC MESFET

6H-SiC MESFET

�

�

�

��

��

�

��

�

��

��

��

�: 4H SiC MESFET

�: GaN HFET / sapphire

�: Si-LDMOSFET�: GaAs MESFET

�: SiC-SIT.

: GaN HFET / SiC.

�: 6H-SiC MESFET

�

��

�

Operation Voltage and Power Densityof High-Power HF devices


0.1

1

10

100

1000

4åé/1 4åé/1 4åé/1 4åé/1 4åé/1 4åé/1 4åé/1

Tol

tal

Out

put P

ower

(W)

Calendar yearApril, '00 April, '02 April, '04 April, '06

2GHz band>20GHz

0.1

1

10

100

1000

1 10 100

Out

put

Pow

er /

Chi

p (

W)

Frequency (GHz)

GaN-related devicesGaAs-related devicesInP-related devices

(a) (b)

R&D Status of high-power HF devices


Comparison of Depletion Layer Expansionand Electric Field in a Switching Device

On-Resistance

RDS = WD (q nND)

= 4VB2 ( nEC

3)

Blocking Voltage

VB= (ECWD) 2

Carrier Density

ND = ( /q) (EC/WD)

= 2 EC2 qVB

Ele

ctri

c fi

eld

Position in drift layer

ND(Si) dE(x)/dx

ND(WGS) dE(x)/dx

EC(WGS)

0

EC(Si)

WD(Si)WD(WGS) x

V= E(x) dx

E(x)

p+

Depletion layerWGSVB

Junction edge

p+

n+n-

Drift layer

n- n+

VB

Si


Performance Indices of Power Switching Devices

10-2

10-1

100

101

102

101 102 103 104

On-

Res

ista

nce

Vlocking Voltage VB

RDS

for Si

RDS

for 4H-SiC RDS

for GaN

Ron

for 4H-SiClimited by R

ch and R

c

Rch

and Rc


R&D Status of Low-Loss Swtching Devices --- Fabrication Trials and Simulation ---

10-2

10-1

100

101

102

101 102 103 104

Spec

ific

On-

Res

ista

nce

(m c

m2 )

Blocking Voltage (V)

Si MOSFET 4H-SiC MOSFET 4H-SiC JFET/SIT GaN HFET GsN MISHFET

Si Unipolar Limit

GaN Unipolar Limit4H-SiC Unipolar Limit

GaN lateral FET

4H-SiC lateral FET

Diamond lateral FET

Limit by μch

=20cm2/ V s

Limit by μch

=200cm2/ V s

SiC IGBTSiC GTO

Si IGBT

Breakdown Voltage VB (V)

100 1k 10k0.1

1

10

100

Sp

ecif

ic O

n-R

esis

tan

ce R

on (

mcm

2)

Si-limit 4H-SiC-limit

GaN-limit

4H-SiC-LFET

GaN-LFET

Diamond-LFET

By W. Saito (Toshiba)


Blocking Voltage and On-Resistance of GaN HFET

High blocking voltage HFET

0

0.005

0.01

0.015

0.02

0 400 800 1200 1600

Id-Vd

Dra

in C

urr

ent

[A]

Drain Voltage [V]

Vg = +2 V

Vg = 0 V

Vg = -2 V

Vg = -5 V

SiN : 5nm

Lg = 4 μm

Lsd = 34 μm

S. Yagi et al.: Solid-StateElectron. 50 (2006) 1057.

Lsd (μm)

Source-Drain distance dependence of on-resistance

M. Inada et al. : Proc. Int. Symp.Power Semicond. Devices & ICs,Naples, 2006, p.121.

Low on-resistance HFET

: 1.7kV

: 0.088m cm2


R&D trend of Current Capacityon SiC Devices

‘04‘02‘00‘98‘96‘940.1

1

10

100

1000C

urr

ent

Cap

aci

ty/C

hip

(A)

Cree

CreeSiCED

Cree & Auburn

Cree

SiCED

SiCED

Denso

‘06

Mitsubishi

Hitachi

Hitachi

SiCED

SiCED

SiCED

Toshiba

CRIEPI

Rutgers Univ.

CreeSiCED

SiCED

SiCED

SiCLAB

Cree

ABBCree

TDI(USA)

SiCLAB

Rockwell

Daimler

Cree

PiN

SBD

MOSFET

JFET(SIT)

Years


Recent Results of Device/Inverter R&D (1)

Mitsubishi Electric (2006.1.24)1. Module by 1200V, 10A MOSFETs (On-resistance :10m cm2

2. Inverter operation of a 3.7kW motor (SiC-MOSFET+SiC-SBD3. 54% reduction of a inveter loss (vs. Si-IGBT inverter)


Recent Results of Device/Inverter R&D (2)

Kansai Electric Power & Cree Inc. (2006.1.25)1. 4.5 kV, 100 A SiC Commutated Gate Turn-off Thyristor (SiCGT)

8x8mm2

2. 110kVA 3-phase inverter SiC-MOSFET+SiC-PiN Dwithout snubber circuit, operation at 300˚C

3. Reduction of inverter loss by more than 50% (vs. Si-IGBT inverter)


From Crystal to Application System

Device chips

MOSFET, SBD etc.

Ion Implantation

Dry etching

Metallization

RTA

Device modulesDicing

Bonding

mounting

Bulk waferSlice, wrapping

Circuit mounting

Epitaxial waferEpitaxial growth

0 1μm

2-inch

100μm

0 1μm

0

1nm

0.5nm

Gate

Source

Active area

200x200μm2

SiC powderIngot

sublimation

HP

Inverters

HP


Problems in Widegap Semiconductor Device Technology

III-Nitrides SiC

Self-heating, Thermal mounting

Channel mobility

Gate Leakage (degradation of VB, efficiency)

Current Collapse (Insulator, surface/interface control

Normally-off operation

Reliability (Oxide interface)

Tolerance (Avalanche/Short-circuit capability)

Wafer Quality

Surface Morphology

High Al content

Purification of channel layer

Highly resistive buffer layer

Reduction of off-angle

Polarity control

p-type control

Uniformity in a wafer (thickness, doping, content)

Dislocation, defects in epitaxial layers

Bulk wafer

Defect ControlDevice killer defects

Device structures

Module structures

Manufacturing TechnologyLarge size, multi-wafer

Uniformity, reproducibility

III-Nitrides SiC

Self-heating, Thermal mounting

Channel mobility

Gate Leakage (degradation of VB, efficiency)

Current Collapse (Insulator, surface/interface control

Normally-off operation

Reliability (Oxide interface)

Tolerance (Avalanche/Short-circuit capability)

Wafer Quality

Surface Morphology

High Al content

Purification of channel layer

Highly resistive buffer layer

Reduction of off-angle

Polarity control

p-type control

Uniformity in a wafer (thickness, doping, content)

Dislocation, defects in epitaxial layers

Bulk wafer


Enlargement of SiC Wafer Size and Defects

25mm30mm

35mm

50mm

75mm

100mm

1992 1993 1994 1995 1996 1997 1998 1999 2000

Sublimation Method

X-Ray Topography Micro-pipesW

afer

Siz

e

Calendar Year


Comparison of SiC Single Crystal Wafer

C Corp. (2 inch)

MPD 15/cm2

C Corp. (3 inch)

MPD 5/cm2

B Corp. (2 inch)

MPD 5/cm2

A Corp. (2 inch)

MPD 30/cm2

MP Almost solved

Dislocation Killer defects

determining device performance

Threading Dislocation (Screw, Edge)

Basal Plane Dislocation

Distortion of crystal planes

(Thermal distortion)

EPD

4 inch


Shee

t R

esis

tanc

e (

/)

300

400

500

600

700

800

900

0.15 0.20 0.25 0.30 0.35

Al Content of AlGaN Barrier layer

Al Content and Sheet Resistance of an

AlGaN/GaN Heterostructure Wafer

GaN

AlxGaN1-xN


Surface morphology of high Al-contentAlGaN epitaxial layer

sub (RMS 0.2nm)

GaN epi-layer(RMS 0.3nm)

3nm

0nm

3nm

0nmquasi-Al0.5Ga0.5N(RMS 0.2nm)

3nm

0nm

Al0.5

Ga0.5

N(RMS 1.4nm)

5nm

0nm

AlGaN/GaN

(0.2<x<0.6)

GaN

AlxGa

1-xN

GaN(2 m)

Al2O3

2 (AlN/GaN)n/GaN

GaN

AlNGaN

GaN

GaN(2 m)

Al2O3


Al-content(equivalent) dependnceof Sheet resistance

Realization of

Lowest value

170 /

AlGaN alloy

quasi AlGaN

AlGaN alloy (ref.)

0

200

400

600

800

1000

0 10 20 30 40 50 60 70 80

(equivalent) Al composition [%]


•Recess gate structure

•Introduction of fixed charge

•MOS structure

•Utilization of non-polar surface

•pn-junction gate

•GaN Cap layer

•Asymmetry AlGaN/GaN/AlGaN channel

Normally-off operation ofGaN switching devices

Existing Gate drive circuit

incompatibility of control power supply, gate signal

Care for Power supply circuit (Safety)

Trials by various approach

Confirmation of necessity ?


Examples of Normally-off operation

Ron

A = 2.6m cm2, Idmax

= 200mA/mm

pn-junction gate

Y. Uemoto et al.,

IEDM2006, San Francisco,

USA (2006.12)

recess gate with

high-k insulator

H. Sazawa et al., IWN2006,

Kyoto, Japan (2006.10)


Important Technical Issuefor the Realization of WGS Inverters

• micropipe

• dislocation SD, ED, BPD etc.

• Grain boundary, oxide interface

• Channel mobility

• Blocking voltage, current leakage

• Reliability

(correlation between wafercharacteristics and device performance)

Characterization Techniques/Tools

Reflection X-ray topograph

image for a SiC SBD

threading screw 39 ( 3900 cm-2 )

threading edge 126 ( 12600 cm-2 )

basal plane 20 ( 200 cm-2 )

There remain many unknown factors in WGS Physics


Required specification for voltage and current,

relation with the density of device killer defects

2002 2010 202004 0806

1000

100

10

1

10

1

0.1

5.0kV,100A

3.3kV,150A

(100 mm2)2.5kV,100A

1.2kV,200A

(64 mm2)

600V,10A

(4 mm2)

1.2kV,70A

600V,100A

(36 mm2)

Co

nv

ersi

on

Cap

aci

ty (

kV

A)

(Volt

age

x C

urr

ent)

Year

Dev

ice

kil

ler

def

ect

den

sity

(cm

-2)


Summary

High-power electron devices are key components forwireless communication and power electronics, whichare necessary for the sustainable development in the21th century.

WGS are promising for high-power application, due totheir superior material characteristics.

Owing to the recent R&D, high-power electron deviceperformance by WGS has been well demonstrated,which much surpass those of conventional devices

There still remain technical issues to be solved, foractual system application.

Present Status and Future Prospect of the Power ...

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