Gallium Nitride (GaN)A Potential Candidate of
Future Electronics
Lecture10
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Overview Overview
Introduction
Structure & Physical Properties
GaN vs. other semiconductors GaN vs. other semiconductors
Current research,
Limiting factors
Applications
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Introduction Introduction
GaNGaN is the next important is the next important semiconductor after Sisemiconductor after Si.
Operate at High Temperatures.High Temperatures.
Key material for the next generation Key material for the next generation of high frequency high frequency and high power high power applicationsapplications.
Belongs Wide Band Gap (WBG) Belongs Wide Band Gap (WBG) semiconductor familysemiconductor family..
http://www.phy.mtu.edu/yap/images/galliumnitride.jpg
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WBG Semiconductor with WBG Semiconductor with Direct Direct Bandgap EnergyBandgap Energy
Useful as Optoelectronics Useful as Optoelectronics DevicesDevices
oo Blue & Blue/green light emittersBlue & Blue/green light emitters
Transistors withstand Transistors withstand extreme heatextreme heat andand
Why Why GaNGaN is important ?is important ?
Transistors withstand Transistors withstand extreme heatextreme heat andand
High High frequencies frequencies and and power levelspower levels
GaN based Amplifiers more efficient at the base GaN based Amplifiers more efficient at the base
stationsstations
oo SiSi--only 10% power used and 90% wasted as heatonly 10% power used and 90% wasted as heat
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GaNGaN Crystal StructureCrystal Structure
GaNGaN grown ingrown in
Wurtzite crystal structure
Zinc-blende crystal structure Zinc-blende crystal structure
The band gap, Eg, effected by crystal structure
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WurtziteWurtzite Crystal StructureCrystal Structure
Wurtzite crystal structure is a member of the hexagonal crystal system
Several other compounds can take the wurtzite structure, including Agl, ZnO, CdS, CdSe, and other semiconductors.
Energy gap: 3.4 eV
An ideal angle: 1090
Nearest neighbor: 19.5 nm
Energetically favorable
Hexagonal Diamond http://en.wikipedia.org/wiki/Image:Wurtzite-unit-cell-3D-balls.png
CdS, CdSe, and other semiconductors.
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ZincZinc--blendeblende Crystal StructureCrystal Structure
Energy gap 3.2 eV
An ideal angle: 109.470 An ideal angle: 109.470
Nearest neighbor: 19.5 nm
http://en.wikipedia.org/wiki/Image:Sphalerite-unit-cell-depth-fade-3D-balls.png7
Tetrahedral bonds
sp3 hybridization
Bonding angle: 109.47
Bond Length: 19.5 nm
GaNGaN Bonding PropertiesBonding Properties
Bond Length: 19.5 nm
Ga-N bonds significantly stronger than Ga-Gainteractions (based on distance)
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IonicityIonicity
GaN exhibits mixed ionic-covalent bondingIonicity of a bond is the fraction fi of ionic character compared to the fraction of fh of covalent character
By Paulings definition
Modern definition is the ionicity phase angle
1http://www.bcpl.net/~kdrews/bonding/bonding2.html 9
GaNGaN Bonding PropertiesBonding PropertiesBased on calculations using both methods, typical values are
Compound Pauling ionicity Modern ionicity2
AlN 0.430 0.449
AlP 0.086 0.307
AlAs 0.061 0.274
GaN 0.387 0.500
GaP 0.061 0.327GaP 0.061 0.327
GaAs 0.039 0.310
InN 0.345 0.578
InP 0.039 0.421
InAs 0.022 0.357
NaCl 0.668 > 0.9
C (Diamond) 0 0
2J.C. Phillips, Bonds and Bands in Semiconductors 1973
Bond Character dependent on electronegativity10
GaNGaN--Crystal GrowthCrystal GrowthGaNGaN--Crystal GrowthCrystal Growth
Substrates for Epigrowth
650nm
AlN
BN
MgO
3C-SiC
6H-SiC
AlN Sapphire5.5
6.5
ZnOHexagonalCubic
650nm650nm
650nm
650nm
AlN
BN
GaNGaN
InN
Lattice Constant /
Ban
d g
ap /
eV 4.5
3.5
2.5
1.52 2.5 3 3.5 4 4.5 5 5.5
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Thermal & lattice mismatch Thermal & lattice mismatch -- Strain and DefectsStrain and Defects
Why Why GaNGaN based HEMTs important? based HEMTs important?
High Voltage OperationHigh Voltage Operation High power densities 4 to 8 watts/mm at 28 and
50 volt operation respectively High Frequency Performance High Frequency Performance High Efficiency High Efficiency Low Quiescent CurrentLow Quiescent Current Low Quiescent CurrentLow Quiescent Current High Native LinearityHigh Native Linearity Low capacitance per peak wattLow capacitance per peak watt
12% of LDMOS and 21% of GaAs MESFET supports broad bandwidths
Almost constant CAlmost constant CDSDS as a function of Vas a function of VDSDS Enable new amplifier architecturesEnable new amplifier architectures
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GaNGaN HEMT HistoryHEMT History
1960 GaN small crystals was made.
1980 Takashi Takashi MinuraMinura, Fujitsu laboratories , Fujitsu laboratories designed the features of the first HEMTfirst HEMT.
1985 HEMT was announced the lowest noise HEMT was announced the lowest noise 1985 HEMT was announced the lowest noise HEMT was announced the lowest noise device.device.
1994 KahnKahn demonstrated the first first AlGaNAlGaN//GaNGaNHEMT.HEMT.
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Available Substrates Substrate Cost
GaN HEMT Substrate GaN HEMT Substrate
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GaN HEMT on Silicon Substrate has major advantages of highpower, cost effectiveness and reliability.
1989-Started III-V nitride research.
1990- a new two-flow MOCVD equipment was developed forgrowth of high quality single crystal GaN layers.
1992-grow InGaN single crystal layers for the production ofheterostructures
1995-Developed high-brightness SQW structure blue/greenLEDs with a luminous intensity of 2 cd and 10 cd, anddeveloped a violet laser diode using III-V nitride materials for
GaNGaN LED History LED History (Nakamura)
developed a violet laser diode using III-V nitride materials forthe first time.
1996-The first current infection III-V nitride based LDs werefabricated.
1996-Announces the first CW blue GaN based injection laserat room temperature.
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Comparison of Comparison of GaNGaN with other with other Comparison of Comparison of GaNGaN with other with other semiconductors semiconductors
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GaN ComparisonGaN Comparison
Silicon cannot provide the power-bandwidth product for military applications 17
GaNGaN Comparison Comparison
1x100
1x105
SiC
-3)
The small intrinsic carrier concentration in GaN at room temperature enables the high power and temperature applications.
GaN epitaxialfor 1025.2(300) 3-10i cmn
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2.5 3.0 3.5 4.010
-20
10-15
1x10-10
1x10-5
GaN
ni(T)=1.98x10
16T
(3/2)exp(-20488/T)
SiC
Intr
insi
c co
ncen
trat
ion
(cm
-3
Temperature (1000/K)
Intrinsic carrier concentration in SiC and GaN as a function of temperature. Ref. R. Kolessar et al., 2001.
0.01
0.1
1
GaN Schottky rectifiers AlGaN-UF
AlGaN-UF
AlGaN-UF
GaN-UF
GaN-UFGaN-UF
GaN-Caltech
Si
Spe
cifi
c on
-sta
te r
esis
tanc
e (
-cm
2 )
GaNGaN comparison comparison
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102
103
104
1E-4
1E-3
0.01
GaN-UF
GaN-UF
GaN-UTGaN
6H-SiC
Spe
cifi
c on
-sta
te r
esis
tanc
e (
Reverse breakdown voltage (V)
GaNfor 104.2 5.212 BVRON
Breakdown VoltageBreakdown Voltage
3
104
Non-punchthrough theoretical limit
GaN punchthrough diodeTheoretical breakdown voltage50 m
30 m
20 m
10 m
5 m
Bre
akdo
wn
volt
age
(V)
0
2
2PTB
PTcPT
WqNWEBV
GaNGaN punchthroughpunchthrough diodediode
n- n n+
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The calculated reverse breakdown voltage of punch-through diode as a function of doping concentration and standoff
region thickness
where EC is critical electric field, WP drift region thicknessNA doping concentration,
and permittivity
1E15 1E16 1E17
102
103
3 m
1 m
Bre
akdo
wn
volt
age
(V)
Doping concentration (cm-3)
02
3 m GaN epi can give more than 900V of reverse breakdown
voltage with the doping concentration of 1016 cm-3
Current Research and Current Research and Limiting factors Limiting factors Limiting factors Limiting factors
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Current Research Current Research
Fundamental Physics
Improving crystal quality (still very poor)
Ultraviolet lasers
Lattice matching with quaternary alloys (AlGaInN)
Nitride heterostructures and accompanying applications
Nitride heterostructures and accompanying applications
Current Issues in III- V Nitrides
1. Inability to grow good quality crystals
2. Inability to grow p-type crystals
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Crystal QualityCrystal Quality
Problem: No lattice matched substrates, high growth Problem: No lattice matched substrates, high growth
temperature results in convection currentstemperature results in convection currents
Sapphire is closest but is 15% off.Sapphire is closest but is 15% off.
SiCSiC is too expensiveis too expensive
MOCVD growth too fast for good control (few MOCVD growth too fast for good control (few mmm/min)m/min)
Solution: Buffer layers, new growth system
First grow GaN or AlN buffer layer
Two-flow MOCVD system
Still many many dislocations in material (1010 cm-2) but
dislocations dont matter?
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Research QuestionsResearch Questions
Even undoped, carrier densities in AlGaN/GaNheterostructures is 10 to 100 times larger than those in similar (AlGaAs/GaAs) systems.
What is the source of these carriers?
Carrier mobilities in AlGaN/GaN heterostructuresare 10 to 100 times lower than in the AlGaAs/ GaAs system.
What are the principle mechanisms limiting the mobility?
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Limiting FactorsLimiting Factors
Scattering mechanisms
Coulomb fields
Phonons
Alloy Disorder
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Coulomb ScatteringCoulomb Scattering
Electrons are affected by the long-range Coulomb
fields of randomly distributed ionized donor
atoms.
Thicker barriers move ionized surface donors further Thicker barriers move ionized surface donors further
away from carriers.
Large 2-DEG densities screen the effect of these
Coulomb fields.
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Phonon scatteringPhonon scatteringPhonons are lattice vibrations in a crystal.Phonons are lattice vibrations in a crystal.
Acoustic phononsAcoustic phonons
Both types of atoms move in-phase
Low energy vibrations
Optical phononsOptical phonons
Atoms of different types move out-of-phase Atoms of different types move out-of-phase
High energy vibration
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Phonon ScatteringPhonon Scattering
Phonons scatter carriers by creating small Phonons scatter carriers by creating small fluctuating dipoles between atoms fluctuating dipoles between atoms (piezoelectric mode).(piezoelectric mode).
Phonons scatter carriers by disturbing the Phonons scatter carriers by disturbing the periodicity of the crystal lattice periodicity of the crystal lattice (deformation potential mode).(deformation potential mode).
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Alloy DisorderAlloy Disorder
Electron Electron wavefunctionwavefunctionpenetrates into penetrates into AlGaNAlGaNbarrier.barrier.
Al and Al and GaGa atoms are atoms are distributed randomly in distributed randomly in distributed randomly in distributed randomly in AlGaNAlGaN
Randomly varying Randomly varying potential scatters potential scatters electronselectrons.
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Overview of RFOverview of RF--Device TechnologiesDevice Technologies(Theoretical Thermal Limitations) (Theoretical Thermal Limitations)
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Advantage of GaN HEMT Device Advantage of GaN HEMT Device
Si vs GaN technology
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GaNGaN Applications Applications
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GaNGaN NanotubesNanotubes
Single Crystal Single Crystal NanotubesNanotubesfabricatedfabricated
Gallium Nitride Gallium Nitride nanotubesnanotubeshave diameter between 30 have diameter between 30 200 nm200 nm
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GaNGaN Laser DiodeLaser Diode
Normally emit ultraviolet
radiation
Indium doping allows variation in
band gap sizeband gap size
Band gap energies range from
0.7eV 3.4eV
Applications in: Blu-Ray technology Laser Printing
http://www.lbl.gov/Science-Articles/Archive/assets/images/2002/Dec-17-2002/indium_LED.jpg 34
BluBlu--ray Discray Disc
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GaNGaN Solar CellsSolar Cells
Indium doped (Indium doped (InGaNInGaN)) Conversion of many wavelengths for energy Theoretical 70% maximum conversion rate.
Multiple layers attain higher efficiency.Multiple layers attain higher efficiency. Need many layers to attain 70%Need many layers to attain 70%
Lattice matching not an issue
Advantages:Advantages:Advantages:Advantages: High heat capacity Resistant to effects of strong radiation High efficiency
DifficultiesDifficulties: Too many crystal layers create system damaging stress Too expensive
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Comparison Comparison -- bulbs bulbs vs.vs. LEDsLEDsComparison Comparison -- bulbs bulbs vs.vs. LEDsLEDs
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High Power RectifiersHigh Power Rectifiers
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SiC high power rectifier product
Current ratings of 1A to 20A at 600V, and 5A to 10A at 1200V
http://www.cree.com
The applications of IGBT modules
- UPS Power Supply,Servo Drive, Medical Power Supply, Motor Drives, Inverters
http://www.pwrx.com
ObjectiveObjectiveDevelop GaN-based rectifiers at power levels above 1MW
Military Applications Military Applications Electronic warfareElectronic warfare
Broadband and high-power microwave emission Interrupt and jam RF signals
AESA RadarAESA Radar Arrays of hundreds or thousands T/R modules
Phase shifted to form and steer the beamPhase shifted to form and steer the beam Phase shifted to form and steer the beamPhase shifted to form and steer the beam So many modules place a premium on size, weight, power
efficiency and high power performance
Tactical Radio CommunicationsTactical Radio Communications To operate over a frequency range of 30-3000 MHz at power levels up to 100W,
Accommodate portable, Accommodate portable, manpackmanpack and vehicle mount form and vehicle mount form
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GaN Device Manufacturers GaN Device Manufacturers
Cree is the biggest US GaN chipmakers
Raytheon is running project on X to Ku-band
(7-11.2 GHz) applications
TriQuint has independent program for S-band
(2-4GHz) and wideband (6-18GHz) MMICs
Northrop Grumman has MMIC project for Ku-
band (7-11GHz).
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RF RF Device Technology ComparisonDevice Technology Comparison
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Pricing of LDMOS has reached ~$0.20s/W. Does Pricing of LDMOS has reached ~$0.20s/W. Does GaNGaN HEMT HEMT compete this price? compete this price?
Thank you to participate in Thank you to participate in this course this course this course this course
Hope all of you enjoy this course and enhance Hope all of you enjoy this course and enhance knowledge as wellknowledge as well
Good Luck for exam Good Luck for exam
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