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Semiconductor Materials and
Structures for Power Electronics
Mark Johnson
Department of Materials Science & Engineering,
NC State University
FREEDM Systems Center ERC
NC State UniversityNew: ARPA-E
(Feb 2010)
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Overview
I. Background on Emerging Systems for
Power Electronic Devices
II. Materials for Power Electronics
III. Epitaxial Dielectrics on GaN for FETs
IV. Summary
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Motivating System Needs For
Power Electronics• Greater Efficiency with reduced Size, Cost and Weight
• Applications Segmented By Voltage and Current Ratings
• Small Scale Power Supplies (man) to Vehicle Traction (air, sea or land) to Power Distribution Systems (grid)
• At Core: Systems Need Switches (transistors) and flyback Diodes (fast)
+
-
Inverter
3Φ (example)DARPA:
WBGS-HPE
Si ?
SiC ?
GaN ?
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System Efficiency Losses
60W inverter losses on a
1200W solar array is
equivalent to
19% vs 20% efficiency
Inverter Resistive and
Switching Losses:
Normally-off Transistor
Fast Recovery Diode
Energy for Cooling:
High Temperature Operation
Storage:
Round-trip Losses
© GaN Devices
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Today’s Power Grid
Problems:
• Not user friendly
• No plug-and-play interface
• Large-scale integration of Distributed Renewable Energy Resource (DRER) would cause system collapse due to:
• Lack of management system
• Lack of energy storage
Gas station
Power plantPower plant
Substation
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IEM: Intelligent Energy Management
IFM: Intelligent Fault Management
10 kVA100 kVA
1 MVA
ESD
User Interface
Distributed Grid Intelligence (DGI)FREEDM
Substation
12kV
120 V
Market &
Economics
69kV
IEM
AC
AC
IFM IFM
IFM
LOAD DRER DESD
IEM
AC
AC
LOAD DRER DESD
IEM
AC
AC
3Φ 480V
RSC
Legacy grid
Notional Distribution System
DRER: Distributed Renewable Energy Resource
DESD: Distributed Energy Storage Device
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7
Technology Path
Conventional Transformer
Solid State Transformer (SST)
12kV AC
480V AC[ 60Hz ]
[ 10-15 kHz ]
IEM
AC
AC
G
SS
G
SS
+
-
Inverter
3Φ (typ.)
Si ?
SiC ?
GaN ?
Leverage
SiC MOSFET
Technology
- John Palmour in Breakout -
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Technical Development Program Linkages
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Comparison of Power Densities
Hybrid Vehicle Inverter
20kW – 120 kW
< 0.1 m3
Silicon IGBT Based
Currently Water Cooled
Transformer Breaker
Power Distribution
20kVA – 120 kVA
~ 1 m3
All Passive – No Communication,
Control or Dispatch
Highly Efficient
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Why Anything but Silicon?
• Size: Limit to Current Rating Leads to Large Area Devices, Lower Frequency and Overall Weight
• Efficiency: Resistive and Switching Losses Potentially Less with SiC or GaN Devices
• Temperature: Larger Bandgap Energy Allows Higher-Temperature Operation Leading to System Efficiency
• Why Now?: Emergence of SiC and GaN Materials for Optoelectronic Applications Provides Unique Opportunity for Advancement in Power Electronics
• Gallium Nitride: Direct Wide Bandgap; Wurtzite (polar) Crystal Structure, AlGaN/GaN Heterostructures, good Electronic Transport Properties, …
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Periodic Table and
Wide Bandgap Semiconductors
Silicon
Microprocessors
Moore’s Law
Power Controllers
GaAs
Mobile Phones
Wireless
GaN & SiC
LEDs
Blue Lasers
Power Electronics
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Silicon Carbide and Gallium Nitride
Wide Bandgap Semiconductors
BA
ND
GA
P E
NE
RG
Y (
eV
)
6.56.05.55.04.54.03.53.02.5LATTICE CONSTANT (Å)
1
2
3
4
5
6II-VI's
III-V's
III-V Nitrides
IV's
AlN
ZnOGaN
InN
SiC(6H)
MgS
ZnS MgSe
ZnSe
CdS
ZnTe
CdSeInP
GaAs
GaP
GaN (Cubic)
SiGeIR
UV
Al2O3
(0001)
Most Wide Bandgap Semiconductors have a Hexagonal Structure
Most III-V Semiconductors have Zincblende Structure
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Comparison of Semiconductor Materials
Bandgap Energy (eV)
Dielectric Constant
Breakdown Field (MV/cm)
Electron Mobility (cm2/Vs)
Thermal Conductivity (W/mK)
Saturated Electron Velocity (cm/s)
Combined Figure of Merit
KthemevsEc2
Silicon
1.12
11.9
0.25
1500
150
1.0x107
1
4H-SiC
3.26
10.1
2.2
1000
490
2.0x107
286
GaN
(Epitaxial)
3.4
9
2.3
1250
130
2.2x107
102
Ex: L. Tolbert, et.al. “Power Electronics for Distributed Energy Systems (ORLN/TM – 2005/230)
Problems: 1) What is Device Meaning of the Combined Figure of Merit ?
2) Evolution of Measured Material Properties with
Advancement in Materials Technology (Ec, Kth, me, …)
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Figures of Merit
Combined Figure of Merit (General Assessment)
kthemevsEc2
Keyes Figure of Merit (Power Density & Speed)
kth √[c vs / (4p es)]
Baliga Figure of Merit (Resistive Losses)
emeEc3
Baliga High Frequency Figure of Merit
meEc2 (Switching Losses)
R. W. Keyes, "Figure of Merit for Semiconductors for High Speed Switches,“
Proc. IEEE, vol. 60, pp. 225-232, 1972
B. J. Baliga, "Semiconductors for High-Voltage, Vertical Channel Field-Effect Transistors,"
J.Appl.Phys., vol. 53, no. 3, pp. 1759-1764, 1982
B.J. Baliga, “Power semiconductor device figure of merit for high – frequency applications,”
IEEE Electron Device Lett., vol. 10, pp. 455-457, 1989.
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Resistive Loss in Power Rectifiers
• Minimize Series
Resistance Loss at
Voltage Rating
• Assume Series
Resistance Dominated by
n- Drift Region (Low
Contact Resistance)
• From Device Model
Resistive Loss/
Joule Heating
Breakdown
Voltage
n+
n-
Ohmic
Schottky
3
24
cEe
BVRon
me
Mobility at Drift Region Doping Levels
NOT Values for Undoped Material
BFM
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GaN Laser Diodes:
Lateral Growth Reduces Crystal Defects
Davis, et.al.
MaskedUnMasked
Laser Diode Lifetimes > 1000 hrs with Low Defect GaN
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Ndis=2x107 cm-2 Ndis=4x108 cm-2Ndis=3x108 cm-2Ndis=2x108 cm-2
250-mm-thickfree-standing
HVPE GaN
10-mm-thickHVPE GaN
3-mm-thick 3-mm-thick
sapphire sapphire6H SiCbuffer buffer
G. Brandes, IS on Bulk Nitrides, Sept. 2005, Bremen, Germany
Micro-morphology and dislocation density
HVPE
Ammonothermal
High Pressure
HVPE MOCVD MOCVD
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Thermal Conductivity of Low Defect
Bulk GaN by 3-ω Method
0
50
100
150
200
250
104
105
106
107
108
109
1010
1101001000104
105
K [
W.K
-1m
-1]
ND [cm
-2]
t [um]
♦ NCSU/Kyma bulk measurements
Typical heteroepi dislocation densities
Kyma bulk SI-GaN
Solution growth GaN
0
50
100
150
200
250
104
105
106
107
108
109
1010
1101001000104
105
K [
W.K
-1m
-1]
ND [cm
-2]
t [um]
0
50
100
150
200
250
104
105
106
107
108
109
1010
1101001000104
105
K [
W.K
-1m
-1]
ND [cm
-2]
t [um]
♦ NCSU/Kyma bulk measurements
Typical heteroepi dislocation densities
Kyma bulk SI-GaN
Solution growth GaN
C. Mion, NC State University (2005)
CL Imaging of Defects
ρ= ~107 cm-2
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GaN for Power Electronics
GaN as material for high-speed and high-power applications
BFM – minimized resistive losses [ εμEc3 ]
BHFFM – minimized switching losses ( μEc2 ]
JFM – minimized switching delay [ (vsatEc)2 ]
Silicon 4H-SiC GaN
(epi)
GaN
(bulk)
Eg (eV) 1.12 3.26 3.4 3.4
Diel. Constant 11.9 10.1 9 9
Kth (W/mK) 150 490 130 230
Ec (MV/cm) 0.3 2.2 2 3.3
2.7 (exp)
vsat (x1E7 cm/s) 1 2 3 3
mobility (cm2/Vs) 1350 900 1150 1150
BFM (rel) 1 223 190 850
BHFFM (rel) 1 45 36 98
JFM (rel) 1 215 400 1090
Low
Resistive
Loss
Low
Switching
Loss
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Metamorphic Quasi-Bulk GaN
• HVPE for GaN Boule Synthesis: NH3, Ga, HCl
• Wafering by slicing and polishing
• Defect density reduction with increased thickness: as low as mid-105/cm2
• Orientation controlled by wafering direction
Substrate
Nucleation
Quasi-bulk
HVPE
Boule
Wafering
Boule
Polar Cut
Non – Polar
Wafering
[1120] or
[1-100][0001]
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Substrate Series Conductivity
• n-ohmic ~10-6 Ωcm2
• Drift <10-3 Ωcm2 @ 1kV
• substrate 100 μm
(target) 2x1018 cm-3 n-type
500 cm2/Vs mobility
6x10-5 Ωcm2
• Need: Thin, highly doped, highly conductive substrates
n+
n-
Ohmic
Schottkydrift
substrate
contact
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Nominal GaN MOS Power Transistor and
Materials Development Issues
ACCUMULATION-MODE
Vertical IG-HFET
N- DRIFT REGION
P- SHIELDING REGION
N+ SOURCE
P+
N+ SUBSTRATE
DRAIN
SOURCE
AlGaN Strain Dielectric
GATE
Low Defect GaN Substrate
Drift Region Power Limit
2DEG Fabrication & Epitaxy
Materials Focus / Problem
III-N MOS Interface
Structures Dielectric
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Lateral GaN MOS Power Transistor and
Materials Development Issues
Enhancement-mode Lateral
MOS-HFET
GaN BUFFER
LAYER
SUBSTRATE
SOURCE
AlGaN 2DEG
GATE
Buffer Layer Defects and Leakage
2DEG Fabrication & Epitaxy
Materials Focus / Problem
MOS Interface & Structures Low-k
Dielectric
DRAIN
NUCLEATION LAYERS
Gate Dielectric
Recess Etching
Silicon, Silicon Carbide, Sapphire
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GaN Dielectric Interface – Fab Process
-4
-2
0
2
4
6
8
GaN
La
2O
3
SiO
2
Al 2
O3
Y2O
3
Ta
2O
3
ZrO
2
HfO
2
Ban
d E
nerg
y (
eV
)
-4
-2
0
2
4
6
8
GaN
La
2O
3
SiO
2
Al 2
O3
Y2O
3
Ta
2O
3
ZrO
2
HfO
2
Ban
d E
nerg
y (
eV
)
0
1
2
3
4
5
6
7
8
9
0.28 0.3 0.32 0.34 0.36 0.38 0.4 0.42
Anion Sub-Lattice Tetrahedral Spacing (nm)
Ba
nd
gap
En
erg
y (
eV
)
GaN
AlN
InN
MgOCaO
SrO
CdO
BaO
HfO2
ZrO2
ZnO
Sih-Ga2O3
h-Gd2O3
GaAs
h-La2O3
(R30)
0
1
2
3
4
5
6
7
8
9
0.28 0.3 0.32 0.34 0.36 0.38 0.4 0.42
Anion Sub-Lattice Tetrahedral Spacing (nm)
Ba
nd
gap
En
erg
y (
eV
)
GaN
AlN
InN
MgOCaO
SrO
CdO
BaO
HfO2
ZrO2
ZnO
Sih-Ga2O3
h-Gd2O3
GaAs
h-La2O3
(R30)
La2O3 (MBE) on GaN
• Deposition of GaN MOS Dielectric
• Consideration: Structure, Electron
Energy and Thermal Stability
• Crystal Growth on III-Nitride Surface:
Ga2O3 Interlayer
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Summary• Wide bandgap semiconductors opportunity in Power Systems
- „Last mile‟ of Electric Power Systems
- High Voltage Transmission and Distribution
- Renewable Energy Generation
- Smarter Reactive & Resistive Loads
• Properties of WBGS advantageous in efficient power conversion
• Defect Density Key Issue in Wide Bandgap Semiconductors
• Gate Dielectrics for MOS applications
• Focus on GaN and SiC in Breakouts: John Palmour and Keith Evans
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Acknowledgements
• Thanks:
J. Matthews, J.Jur, J.A. Grenko, K-Y Lai, V. Wheeler,
N. Biswas, L. Ma, Y. Jin, Y. Saripalli, X-Q Liu, C. Vercellino, R. Fava, M. Veety, J-H Park, J. Muth, K. Evans, D. Hanser, E. Preble, T. Paskova, G. Mulholland, A. Huang, J. Baliga, S. Bhattacharya, H. Xu, Y. Wang, M. Park
• Support– NSF-ERC Program: FREEDM Systems Center
– DARPA: Young Faculty Award
– MDA, AFRL and ARO SBIR/STTR Programs
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Questions?
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