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UCSBUCSBUCSBUCSB
U.S. Department of Defense
AlGaN/GaN HEMTs and AlGaN/GaN HEMTs and HBTsHBTs
Umesh K. Mishra
UCSBUCSBUCSBUCSB
U.S. Department of Defense
PART IPART I
AlGaN/GaN HEMTsAlGaN/GaN HEMTs
UCSBUCSBUCSBUCSB
U.S. Department of Defense
1500
260
5000
1300
µµµµ TmaxJFMRatio
BFOMRatio
EgεεεεMaterial
9.5
9.7
13.1
11.4
700 C8024.63.4GaN
600 C603.12.9SiC
300 C3.59.61.4GaAs
300 C1.01.01.1Si
BFOM = Baliga’s figure of merit for power transistor performance [K*µ*Ec3] JFM = Johnson’s figure of merit for power transistor performance
(Breakdown, electron velocity product) [Eb*Vbr/2π]
Materials Properties Comparison
UCSBUCSBUCSBUCSB
U.S. Department of Defense
NeedNeed Enabling FeatureEnabling Feature Performance AdvantagePerformance Advantage
High LinearityHigh Frequency
Technology Leverage
High TemperatureOperation
HEMT Topology
High Electron Velocity
Wide Bandgap
Direct Bandgap: Enabler for Lighting
High Power/Unit Width Wide Bandgap, High Field Compact, Ease of Matching
High Efficiency High Operating Voltage Power Saving, Reduced Cooling
Optimum Band Allocation
Bandwidth, µ-Wave/mm-Wave
Rugged, Reliable, Reduced Cooling
Driving Force for Technology:Low Cost
High Voltage Operation High Breakdown Field Eliminate/Reduce Step Down
Low Noise High gain, high velocity High dynamic range receivers
Thermal Management
SiC Substrate High power devices with reduced cooling needs
NeedNeed Enabling FeatureEnabling Feature Performance AdvantagePerformance Advantage
High LinearityHigh Frequency
Technology Leverage
High TemperatureOperation
HEMT Topology
High Electron Velocity
Wide Bandgap
Direct Bandgap: Enabler for Lighting
High Power/Unit Width Wide Bandgap, High Field Compact, Ease of Matching
High Efficiency High Operating Voltage Power Saving, Reduced Cooling
Optimum Band Allocation
Bandwidth, µ-Wave/mm-Wave
Rugged, Reliable, Reduced Cooling
Driving Force for Technology:Low Cost
High Voltage Operation High Breakdown Field Eliminate/Reduce Step Down
Low Noise High gain, high velocity High dynamic range receivers
Thermal Management
SiC Substrate High power devices with reduced cooling needs
Advantages of WBG Devices
UCSBUCSBUCSBUCSB
U.S. Department of Defense
GaAsGaAs High PowerAmplifier Module
EquivalentHigh Power
GaNGaN AmplifierModule
IN
OUT
IN
OUT
• 10-x power density ( > 10 W/mm)
• 10-x reduction in power-combining
• Improved efficiency (> 60 %)
• Improved reliability
• Compact size
• Superior Performance at reduced costGaAsGaAs High PowerAmplifier Module
EquivalentHigh Power
GaNGaN AmplifierModule
IN
OUT
IN
OUT
IN
OUT
IN
OUT
• 10-x power density ( > 10 W/mm)
• 10-x reduction in power-combining
• Improved efficiency (> 60 %)
• Improved reliability
• Compact size
• Superior Performance at reduced cost
Example of Advantage of WBG Devices
UCSBUCSBUCSBUCSB
U.S. Department of Defense
S C X Ku K Ka Q V W
0.1
1
10
100
1000
Frequency Band
MissileSeekers
ShipRadar
LMDS MVDS
DigitalRadio CAR
Airborne Radar
THAAD Navy Decoy
LONGBOW
BATP31
Satcom
VSAT
Base Station
SatcomWat
ts
Base StationDriverAmp
Shipboard Radar,DD21
MMDS,WLL3G
UNIIHiperlan
2 GHz 10 GHz 30 GHz 60 GHz
Military
Commercial
S C X Ku K Ka Q V W
0.1
1
10
100
1000
Frequency Band
MissileSeekers
ShipRadar
LMDS MVDS
DigitalRadio CAR
Airborne Radar
THAAD Decoy
BATP31
Satcom
VSAT
Base Station
SatcomWat
ts
Base StationDriverAmp
Shipboard RadarShipboard Radar
MMDS,WLL3G
UNIIHiperlan
2 GHz 10 GHz 30 GHz 60 GHz
Military
Commercial
S C X Ku K Ka Q V W
0.1
1
10
100
1000
Frequency Band
MissileSeekers
ShipRadar
LMDS MVDS
DigitalRadio CAR
Airborne Radar
THAAD Navy Decoy
LONGBOW
BATP31
Satcom
VSAT
Base Station
SatcomWat
ts
Base StationDriverAmp
Shipboard Radar,DD21Shipboard Radar,DD21
MMDS,WLL3G
UNIIHiperlan
2 GHz 10 GHz 30 GHz 60 GHz
Military
Commercial
S C X Ku K Ka Q V W
0.1
1
10
100
1000
Frequency Band
MissileSeekers
ShipRadar
LMDS MVDS
DigitalRadio CAR
Airborne Radar
THAAD Decoy
BATP31
Satcom
VSAT
Base Station
SatcomWat
ts
Base StationDriverAmp
Shipboard RadarShipboard Radar
MMDS,WLL3G
UNIIHiperlan
2 GHz 10 GHz 30 GHz 60 GHz
Military
Commercial
Shipboard RadarShipboard Radar
MMDS,WLL3G
UNIIHiperlan
2 GHz 10 GHz 30 GHz 60 GHz
Military
Commercial
Application Space
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Ball and Stick Diagram of the GaN Crystal
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Polarization World
UCSBUCSBUCSBUCSB
U.S. Department of Defense
AlxGa1-x N
GaN
+ + + + + + + + + + +
+ + + + + + + + + + +
,Q AlGaNπ−
,Q AlGaNπ
,Q GaNπ−
,Q GaNπ
⇒⇒⇒⇒ + + + + + + + + + + +( ) ( )( ) ,,x GaNAlGaNP Q Qπ π= + −
includes the contribution of spontaneous and piezo-electric contributionsQπ
How does the electron gas form in AlGaN/GaN structures? - A
UCSBUCSBUCSBUCSB
U.S. Department of Defense
GaNAlGaN
sp 2( )Q cmπ−
sn2( )Q cmπ−−
vE∆
,Eg GaN
l , Maximum Dipole MomentA GaN g GaN vV E E+ ∆ =!
How does the electron gas form in AlGaN/GaN structures? - C
UCSBUCSBUCSBUCSB
U.S. Department of Defense
EFGaNAlGaN
EDD
DDN + 2( )Q cmπ−
Qπ−sn
How does the electron gas form in AlGaN/GaN structures? - D
UCSBUCSBUCSBUCSB
U.S. Department of Defense
• High density 2DEG mobility limited by alloy scattering.
• Introduce thin AlN interlayer – remove alloy scattering by pushing 2DEG wavefunction out of the alloy region.
• Very useful result for attaining high conductivity 2DEG channels for HEMTs.
Application of AlGaN/GaN 2DEG transport studies
UCSBUCSBUCSBUCSB
U.S. Department of Defense
• MBE growth of AlN/GaN structures is performedon GaN “templates” – thick GaN layers grown by MOCVD on (0001) sapphire
• Templates of two types are employed:(a) unintentionally doped GaN
(dislocation density: ~ 5x108 – 109 cm-2) (b) semi-insulating GaN
(dislocation density: ~ 1010 cm-2)
AFM image of type (a)GaN template
(0001) Sapphire
MOCVD GaN~ 2-3 µµµµm
MBE GaN~ 0.3 µµµµm
AlN d: 24 –50 Å
• Ga-stable growth (III/V flux ratio > 1)
• TS ~ 740 oC
RF Plasma-Assisted Molecular Beam Epitaxy of AlN/GaN Heterostructures
ONR/MURIONR/MURIONR/MURIONR/MURIIMPACT CenterIMPACT CenterIMPACT CenterIMPACT Center
UCSBUCSBUCSBUCSB
U.S. Department of Defense
AlN/GaN Structures Grown on Semi-Insulating GaN Templates
1
1.5
2
2.5
3
3.5
4
20 25 30 35 40 45 50
300 K77 K
NS (1
0 13
cm-2
)
AlN thickness (A)
0
1000
2000
3000
4000
5000
20 25 30 35 40 45 50
300 K77 K
Mob
ility
µ (c
m2 /V
s)
AlN thickness (A)
• Both room-temperatureand 77 K electron mobility decrease drastically as AlNbarrier width increases
• 2DEG sheet density reaches the value of 3.65x1013 cm-2
in the AlN/GaN structure with a 49 Å barrier
ONR/MURIONR/MURIONR/MURIONR/MURIIMPACT CenterIMPACT CenterIMPACT CenterIMPACT Center
UCSBUCSBUCSBUCSB
U.S. Department of Defense
AlN/GaN Structures Grown onUnintentionally Doped GaN Templates
1
10
1000
104
10.00 100.0
10100
NS (1
013 c
m-2
) µ (cm2/Vs)
1000/T (K-1)
T (K)
d ~ 35 Ad ~ 45 A
AlN ~ 35 ŵ(300 K) = 1460 cm2/Vs
N2DEG ª NS(20 K) = 2.2x1013cm-2
AlN ~ 45 ŵ(300 K) = 1230 cm2/Vs
N2DEG ª NS(20 K) = 2.7x1013cm-2
AlN ~ 50 ŵ(300 K) = 330 cm2/Vs; NS(300 K) = 5.6x1013cm-2
µ(77 K) = 660 cm2/Vs; NS(77 K) = 3.6x1013cm-2
Pessimistic estimate of the2DEG sheet resistance at 300 K:
R < 200 Ω/
ONR/MURIONR/MURIONR/MURIONR/MURIIMPACT CenterIMPACT CenterIMPACT CenterIMPACT Center
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Atomic Force Microscopy of AlN/GaNHeterostructures
1 µµµµm
0 nm
3 nm
AlN: 24 Å
AlN: 37 Å
AlN: 49 Å
Cracks in AlN layer
Tensile strain relaxation processbegins at d ~ 49 Å
ONR/MURIONR/MURIONR/MURIONR/MURIIMPACT CenterIMPACT CenterIMPACT CenterIMPACT Center
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Polarization doping – concept and demonstration
• 3D electron slab by Polarization doping demonstrated.
• Carrier density verified by self –consistentSchrodinger – Poisson calculations.
• How do transport properties of the3DES compare to the donor doped and 2DEG counterparts ?
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Comparison of transport properties
UCSBUCSBUCSBUCSB
U.S. Department of Defense
DC Device Level Issues (HEMTs)
If it ain’t good @ DC it ain’t goin’ to be good @ RF
SG
D
• Maximize I ⇒ Maximize nS, ν
• Maximize nS ⇒ Maximize PSP, PPE⇒ Maximize Al mole fraction
without strain relaxation
• Mazimize ν ⇒ Minimize effective gate length
⇒ Minimize Lg and gate length extension
• Maximize µ ⇒ Minimize dislocations⇒ Smooth interface
- - - - - - - - - - - - -AlGaN PSP + PPE
+ + + + + + +
Lg
GaN
Vmax
Imax
V
- - - - - - - - - - - - - - - - - - - -
max max max18
S
P V II V n υ= ⋅= ⋅ ⋅
UCSBUCSBUCSBUCSB
U.S. Department of Defense
25 nm Al0.3Ga0.7N
1 nm AlN
UID GaN
SiC Substrate
Hall Data:
! Conventional Al0.3Ga0.7N/GaN HEMTns = 1.2 ×1013 cm-2
µ = 1200 cm2/V/s
! Al0.3Ga0.7N/AlN/GaN HEMTns = 1.65 ×1013 cm-2
µ = 1716 cm2/V/s
+ High charge
+ High mobility
AlGaN/AlN/GaN Heterostructure
UCSBUCSBUCSBUCSB
U.S. Department of Defense
250 Å Al0.3GaN/GaN HEMT 250 Å Al0.3GaN/ 10 Å AlN/GaN HEMT
Band Diagram of AlGaN/AlN/GaN HEMT
! ns = 1.35×1013 cm-2! Higher charge: ns = 1.56×1013 cm-2
due to higher effective ∆∆∆∆EC
! Higher mobility due to the removal of alloy disorder scattering
-50 0 50 100 150 200 250 300 350 400 450 500-1
0
1
2
3
Thickness (A)
Ener
gy (e
V)
-50 0 50 100 150 200 250 300 350 400 450 500-1
0
1
2
3
Thickness (A)
Ener
gy (e
V)
Thin AlN
Effective ∆∆∆∆EC
UCSBUCSBUCSBUCSB
U.S. Department of Defense
0.25 0.30 0.35 0.40 0.451.00E+013
1.50E+013
2.00E+013
2.50E+013
3.00E+013
2DEG
Den
sity
(cm
-2)
Al mole fraction x
Experiment Simulation
Sheet charge density vs. Al mole fraction
! Strongly dependent on the Al mole fraction
250Å AlxGa1-xN/10Å AlN/GaN
UCSBUCSBUCSBUCSB
U.S. Department of Defense
0 20 40 60 80 100 120 140 160 180 200 2200.00E+000
5.00E+012
1.00E+013
1.50E+013
2.00E+0132D
EG D
ensi
ty (c
m-2)
Thickness of AlGaN (Å)
Experiment of AlGaN/AlN/GaN Simulation of AlGaN/AlN/GaN Simulation of AlGaN/GaN
Al0.37Ga0.63N/10Å AlN/GaN
! Weak function of AlGaN thickness! Faster saturation than conventional
AlGaN/GaN HEMT
ConventionalAlGaN/GaN
Sheet charge density vs. AlGaN thickness
UCSBUCSBUCSBUCSB
U.S. Department of Defense
10 15 20 25 301.80E+013
2.00E+013
2.20E+013
2.40E+013
2DEG
Den
sity
(cm
-2)
Thickness of AlN (Å)
Simulation
250Å Al0.37Ga0.63N/ AlN /GaN
! 2DEG increases when AlN is thicker
Sheet charge density vs. AlN thickness
UCSBUCSBUCSBUCSB
U.S. Department of Defense
-2 0 2 4 6 8 10 12 14 16
0
200
400
600
800
1000
gm = 200 mS/mm
∆∆∆∆VG = 1 VVG = 2 V
I D (m
A/m
m)
VDS (V)0 5 10 15 20 25 30
0
5
10
15
20
25
30
35 8.47 W/mm
PAE
(%)
Pout Gain PAE
Pout
(dBm
), G
ain
(dB)
Pin (dBm)
0
5
10
15
20
25
30
35
40
! 8.47 W/mm with a PAE of 28% @ 8GHz
! Bias: class AB at 45 V × 160 mA/mm
! Gate dimension: 0.7×150 µm2
! Imax = 950 mA/mm
! gm = 200 mS/mm
Device Performance of AlGaN/AlN/GaN HEMT
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Issues With Mazimizing Al Mole Fraction in AlxGa1-xN
Eg
Lattice Constant
GaN
AlN
InN
AlxGa1-xN
G
Pseudomorphic Relaxed with Misfit Morphology MediatedBy Dislocaton
DISLOCATIONS LEAD TO PREMATURE RELAXATION OF AlGaN AND A POTENTIAL RELIABILITY PROBLEM BECAUSE OF THE METALLIZED PITS
xAl = 0.2 xAl = 0.4 xAl = 0.6
250 nm 250 nm 250 nm
xHC= 0.3
relaxed
GaN
2.5 3 3.5
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Minimizing Gate Length Extension
ELECTRONS IN SURFACE STATES AND/OR BUFFER TRAPS DEPLETE THE CHANNEL CAUSING GATE LENGTH EXTENSION
SEVERE CONSEQUENCE: DISPERSION BETWEEN SMALL SIGNAL AND LARGE SIGNAL BEHAVIOR BECAUSE OF THE LARGE TRAP TIME CONSTANTS
Vds (V)
I d(5
mA
)
DC
AC
Load line
Dispersion
WHY DO THESE TRAPS ARISE?
UCSBUCSBUCSBUCSB
U.S. Department of Defense
AlGaN
2DEG
SOURCE DRAINGATE
GaN
NucleationLayer
GaN, AlGaN or AlNSubstrate: Typically Sapphire or SiC
SiN Passivation
Schematic of Device Structure
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Vds (V)
I d(5
mA
)DC
AC
Load line
Dispersion
Dispersion in AlGaN/Ga HEMTs
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Lg
AlGaN
GATE
Electrons inSurface States
GaN
Source Drain
Depletion of 2DEGcaused by occupied
surface states
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Performance of Passivated AlGaN/GaN HEMT on Sapphire
UCSBUCSBUCSBUCSB
U.S. Department of Defense
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20 25 300
20
40
60
80
100
120
140
160PoutGainPAEId
Pin (dB)
Pou
t(d
B),
Gai
n(dB
), PA
E (%
)
I d(m
A)
Pout = 10.3 W/mmPAE= 41.6%
f=8GHz, Tuned for Power
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20 25 300
20
40
60
80
100
120
140
160PoutGainPAEId
Pin (dB)
Pou
t(d
B),
Gai
n(dB
), PA
E (%
)
I d(m
A)
Pout = 10.3 W/mmPAE= 42%
f=8GHz, Tuned for Power
Performance of AlGaN/GaN HEMT on SiC (CLC)
UCSBUCSBUCSBUCSB
U.S. Department of Defense
0
1
2
3
4
5
6
7
8
9
10
-5 0 5 10 15 20 25-30
-20
-10
0
10
20
30
40
50
60
70
Pin (dB)
P out
(W/m
m)
f=8GHz
PAE
(%)
Increasing Vds
PAE; Pout
0
1
2
3
4
5
6
7
8
9
10
-5 0 5 10 15 20 25-30
-20
-10
0
10
20
30
40
50
60
70
Pin (dB)
P out
(W/m
m)
f=8GHz
PAE
(%)
Increasing Vds
PAE; Pout
Drain Bias Dependence of Rf Power (CLC)
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Flip-chip AlGaN/GaN HEMT for Thermal Management
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Vds (V/divsion)
I d(A
/div
sion)
Vg start: +2V, Step: -2 V
I-V Curves from 8mm-wide HEMT
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Low Flip-chip Wide Bandwidth Amplifier
UCSBUCSBUCSBUCSB
U.S. Department of Defense
0
5
10
15
20
25
30
35
40
18 22 26 30 34 38 4210
15
20
25
30
35
40
45
50GainDEPAEPout
Pin (dBm)
Gai
n (d
B),
DE
(%),
PAE
(%)
P out
(dBm
)
0
5
10
15
20
25
30
35
40
18 22 26 30 34 38 4210
15
20
25
30
35
40
45
50GainDEPAEPout
Pin (dBm)
Gai
n (d
B),
DE
(%),
PAE
(%)
P out
(dBm
)
Pulse Power Performance of mm-flipped Device
UCSBUCSBUCSBUCSB
U.S. Department of Defense
20-Watt Broadband SiC MESFET Amplifier
• 22 W at P1dB across a 400 MHz band• Advantage of wide bandgap transistors: power-bandwidth
product greatly exceeding Si LDMOS
Balanced amplifier with Cree’s 10Balanced amplifier with Cree’s 10--Watt commercial FETs, CRF22010Watt commercial FETs, CRF22010
2
4
6
8
10
12
14
16
1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4Frequency (GHz)
Gain
(dB)
30
32
34
36
38
40
42
44
P1dB
(dBm
)
GainP1dBmW-CDMA
3GPP Test Model 1 with 16 DPCH
CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.
UCSBUCSBUCSBUCSB
U.S. Department of Defense
75-Watt SiC MESFET Amplifier
• 75 W CW, 11 dB gain demonstrated from a single SiC MESFET
• Currently being optimized for a 60-Watt Class A MESFET product, targeted for release by the end of the year
28 30 32 34 36 38 4040
42
44
46
48
50
Freq. = 2.0 GHz
Power
Input Power (dBm)O
utpu
t Pow
er (d
Bm
)
0
10
20
30
40
50
PAE
PA
E (%
)
2 GHz test fixture for 60 2 GHz test fixture for 60 W MESFET W MESFET developmentdevelopment
CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.
UCSBUCSBUCSBUCSB
U.S. Department of Defense
10-Watt Broadband GaN HEMT Amplifier
• 11 W at P1dB across the 1.8 - 2.2 GHz band• 17 dB gain with only ±0.15 dB ripple
30
32
34
36
38
40
42
44
1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4Frequency (GHz)
P1dB
(dBm
)10
12
14
16
18
20
22
24
Small Signal G
ain (dB)
P1dB (dbm)SS Gain (dB)
CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.
UCSBUCSBUCSBUCSB
U.S. Department of Defense
32 34 36 38 40 42 44
44
46
48
50 Pout Gain
Input Power (dBm)
Out
put P
ower
(dB
m)
0
5
10
15
20
25
30
35
40
Gain(dB
)
Demonstration of 100W CW GaN HEMT
•• Freq. = 2 GHzFreq. = 2 GHz•• VVDSDS = 52V= 52V•• Peak Drain Peak Drain EffEff. = 54%. = 54%•• 108 W achieved at P108 W achieved at P3dB3dB
103W @ 2.6 dB compression
CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.
Sponsored by Tom Jenkins, DUS&T/AFRL
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Summary of Technology Status
• Commercially-available SiC MESFETs
– 22 W broadband amplifier demonstrates power-bandwidth advantage of this technology
– 60 W Class A MESFET targeted for release by end of year
• GaN HEMTS on SiC Substrates
– excellent broadband and high power performance demonstrated
– emphasis of development is now on reproducibility and reliability
• SiC-based MMIC process developed for both technologies• 3-inch SI substrates will enable cost-competitive
manufacturing
CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.CREE, Inc.
UCSBUCSBUCSBUCSB
U.S. Department of Defense
••Includes Includes ALL LEADINGALL LEADING players in the fieldplayers in the field••CREE = CREE = Cree LightingCree Lighting + + CreeCree--DurhamDurham
1996 1998 2000 2002
Total power
Year
Cree
Tot
al p
ower
(W)
HRL
CREE
Cree
UCSBCornell0
10
20
30
40
50
60
Early
Players
SapphireSiC
0
1
2
3
4
5
6
7
8
9
10
1996 1998 2000 2002Year
Pow
er d
ensi
ty (W
/mm
)
CREE
Cree
Cree
CornellUCSB
HRL
Early
Players
SapphireSiC
11 Power densityCornell
UCSB
NEC
••Includes Includes ALL LEADINGALL LEADING players in the fieldplayers in the field••CREE = CREE = Cree LightingCree Lighting + + CreeCree--DurhamDurham
1996 1998 2000 2002
Total power
Year
Cree
Tot
al p
ower
(W)
HRL
CREE
Cree
UCSBCornell0
10
20
30
40
50
60
Early
Players
SapphireSiC
0
1
2
3
4
5
6
7
8
9
10
1996 1998 2000 2002Year
Pow
er d
ensi
ty (W
/mm
)
CREE
Cree
Cree
CornellUCSB
HRL
Early
Players
SapphireSiC
11 Power densityCornell
UCSB
NEC
Power Performance vs. YearCree108W,CW Cree
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Part II
High Voltage Operation (> 330 V) of AlGaN/GaN HBTs
UCSBUCSBUCSBUCSB
U.S. Department of Defense
Bipolar transistor key issues
• Injection
– γ"1– n " 1
[I=I0 exp (qv/nkt)]
Output ConductanceOutput Conductance– ∆∆∆∆IC/∆∆∆∆VCE """" 0
(∆∆∆∆WB/∆∆∆∆VCE """" 0)
CollectionCollection–Cbc """" 0– v"""" vsat [2 x 107 cm/s] (Kolnik et. al.)–Vbr """" Ecrit WC [Ecrit ~ 2 MV/cm] (Bhapkar and Shur.)
Subcollector
Base
Collector
EmitterTransportTransport
-- αααα """" 1
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Hurdles with GaN bipolar transistors
• Lack of low damage etch to reveal base
– Leaky E/B junction– Bad base contact– No etch stop
• High RB
– Poor p-GaN base contact– Low p-GaN base
conductivity• Deep acceptor (~160 meV)
Subcollector
Base
Collector
Emitter
Surface leakage Surface leakage due to etch damagedue to etch damage
Dislocation causes Dislocation causes leakageleakage
•• Hard to control junction placement in Hard to control junction placement in MOCVD due to memory effect of pMOCVD due to memory effect of p--dopantdopant MgMg
Low minority carrier Low minority carrier lifetimelifetime
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Demonstration of dislocation enhanced leakage
1 10-15
100 10-15
10 10-12
1 10-9
100 10-9
10 10-6
-20 -15 -10 -5 0 5 10 15 20
Leak
age
Cur
rent
[A]
Applied Bias [V]
Leakage from Collector to Emitter, Wing vs Window
LEO used to investigate leakage of devices LEO used to investigate leakage of devices without dislocations. (Lee McCarthy et al.)without dislocations. (Lee McCarthy et al.)
Dislocated
LEO
AFM scan of wing vs Window on LEO GaN
Regrowth Mask Regrowth MaskStandard template
LEO GaN
Results: LEO device demonstratedReduction in LeakageStable operation past 20VGain unchangedDevices on dislocated material also functional
ExplanationThick substrate sufficiently reduces
dislocations to prevent C/E short in window region
Gain (ττττe) not currently limited by dislocation density
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Strategy: Thick Collector
• Decent dislocation density
– High quality MOCVD templates achieved Dislocation density ~ 5e8 cm-2
• Low background doping
– ND < 1e16 cm-3 (Assuming uniform doping ND and Ecritical = 2 MV/cm, requires 10 µm to achieve 1 KV breakdown voltage.)
Doping vs. Depth (010704GA, 8 µµµµm collector)
1.E+14
1.E+15
1.E+16
1.E+17
0 1 2 3
Depth (µµµµm)
Dop
ing
(cm
-3)
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Emitter Regrowth Process Flow
• Selectively grow MOCVD emitter on base-collector structures.
1. Pattern regrowth mask2. Regrow emitter layer by
MOCVD3. Remove mask and contact
base and etch to collector4. Contact collector, emitter
n+ Subcollector
Sapphire Substrate
n- Collector
p Base
Mask
n+ Subcollector
Sapphire Substrate
n- Collector
p Base
n+
Emitter
n+ Subcollector
Sapphire Substrate
n- Collector
p Base
n+
Emitter
n+ Subcollector
Sapphire Substrate
n- Collector
p Base
n+
Emitter
1. 2.
3. 4.Pd/Au
Al/Au
Al/AuRegrown emitter
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Device structure
• Utilization of uid GaN spacer and grading layer
- HBTs with high emitter injection coefficiency• Etch damage and current mask layout limits Vbr
n+ Subcollector
Sapphire Substrate
1000 Å p Base
n+
Emitter
Sapphire2 µm GaN:Si (1e18 cm-3) subcollector
8 µm uid GaN (4e15 cm-3) collector
100 nm GaN:Mg (2e19 cm-3) base8 nm uid GaN spacer
8 nm GaN->Al 0.05 GaN (?3e18 cm-3) grading
4 nm GaN:Si (1e18 cm-3) contact4 nm Al 0.05 GaN->GaN:Si (1e18 cm-3) grading
105 nm Al 0.05 GaN:Si (1e18 cm-3) emitter
8 µm n- collector
Effective collector thickness ~ 2-3 µm
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HBT with 8 mm GaN collector
• Current gain (β) > 20• Common emitter operation > 300 V• Non-passivated • Base thickness 1000 Å• Al0.05GaN emitter Vbr ~330 V
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Summary
• Conclusion
– In selective emitter regrowth, a sharp Mg profile, ~ 40 nm/decade, enables the precise junction placement
– Improvement of regrown-emitter/base diodes
– Demonstration of high Vbr (> 300 V) with high β (DC common emitter operation up to 35)