HIPER Lab
Harris Integrative Photonics and Electronics Research Laboratory
High Voltage GaN Devices for
Photovoltaics and High
Frequency Switched Power
Supplies
H. Rusty Harris
Texas A&M University
Depts. Of Physics and ECE
SLAC Advanced Instrumentation Seminars July 27, 2011
Outline
• Motivation
• GaN design for 5kV
• Additional device and integration needs
• Summary
http://www.tamu.edu/http://engineering.tamu.edu/
Students and Collaborators
• PhD Students
– Iman Rezanejhad
– Derek Johnson
– Mary Coan
– Jung Hwan Woo
• Collaborators
– Mark Holtz and Sergey Nikishin – Texas Tech
University
– Eddie Piner – Texas State University
http://www.tamu.edu/http://engineering.tamu.edu/
Outline
• Motivation
• GaN design for 5kV
• Additional device and integration needs
• Summary
http://www.tamu.edu/http://engineering.tamu.edu/
Area Needs for advanced HV
switches
Photovoltaics
High Voltage drives RF and microwave communications
http://www.tamu.edu/http://engineering.tamu.edu/
• We’re getting there
PV Progress
http://www.tamu.edu/http://engineering.tamu.edu/http://upload.wikimedia.org/wikipedia/commons/e/ed/PVeff(rev110408U).jpg
• Higher voltage in the
arrays will reduce
resistive losses
– Residential: 600V
– Commercial: 1000V
Switched power supplies in PV
http://www.tamu.edu/http://engineering.tamu.edu/
E
E
E
E
A
B
C
D
A
B
C
D
Higher Voltage requires more
components
Minority carriers in large drift regions
cause reverse current after switching
http://www.tamu.edu/http://engineering.tamu.edu/
• Clear Advantage in
terms of scaling for MV
applications
6 MW LV (690 V) full-power converter system for
wind turbine
6 MW MV (3300 V) full-power converter for wind turbine
Medium Voltage energy
advantage
Microwave Switches- RF Switches
http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/
Outline
• Motivation
• GaN design for 5kV
• Additional device and integration needs
• Summary
http://www.tamu.edu/http://engineering.tamu.edu/
0.1
1
10
100
100 1000 10000
On
-Sta
te R
esis
tan
ce (
milli O
hm
s.C
m^
2)
Breakdown Voltage (V)
GaN SiC
US
SankeMatsushit
Furukawa
Toshiba
Denso CELEGY
Siemens
Purdue
CREE SICE
D
Siemens Mitsubishi SICE
D
Kansai Power
Electronics & CREE Si-
Limit
SiC-
Limit
GaN-
Limit
Huang
[26]
[27] [28]
[29]
Current Status of Alternate
switching technology
http://www.tamu.edu/http://engineering.tamu.edu/
• AlGaN/GaN forms a high
density 2D Electron Gas
– Low on state resistance!
• We can grow high quality
GaN on a Si platform
– Up to 300mm possible!
Benefits of GaN HEMT in HV
applications
http://www.tamu.edu/http://engineering.tamu.edu/
Our Goal
• Use the HEMT architecture and all its benefits, but
still retain high voltage blocking characteristics
http://www.tamu.edu/http://engineering.tamu.edu/
Spontaneous and Piezoelectric
Polarizations
jijp
i eP
]0,0,0,,,[ 321
^
31
^
0
0
e
e
31
0
0
e 33
0
0
e 0
0
14e
0
0
15e
0
0
0
]2,0,0[. 333131
^^
eeeP p
)(2233
133331333131
c
cee
a
aaeePP
AlGaN
AlGaNGaNp
zPE
http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/
Degree of Relaxation
• Any strained epitaxial films
will have partial, spatially
dependant relaxation below
the critical thickness (Tc)
• Above Tc, full relaxation
occurs and the only
polarization left is PSP
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
0.2
0.4
0.6
0.8
1
Al Mole Fraction, x
De
gre
e o
f R
ela
xa
tio
n, r(
x)
r(x) = 3.5 (x - 0.35)
)()](1[233
133331
c
cee
a
aaxrP
AlGaN
AlGaNGaNPE
GaNrelaxedAlGaN
GaNstrainedAlGaN
aa
aaxr
,
,)(
]1),(5.3min[
0)(
1xxxr
1
0
1
1
xx
xx
http://www.tamu.edu/http://engineering.tamu.edu/
Parametric numerical modeling
of polarization
• Any field above Ecrit
will result in device
failure
• Collapse to universal
polarization above Tc
)/(3.342.8, CmMVxE AlGaNcrit
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Resulting Sheet Carrier Density
• Consistently greater
than 1012 cm-3 sheet
charge density
• We now have a
parametric design
template for
concentration in terms
of Al fraction and
thickness
0 0.1 0.2 0.3 0.4 0.5 0.6 0.71012
1013
Al Mole Fraction, x
2D
EG
Sh
ee
t C
arr
ier
Co
nce
ntr
atio
n (
1/C
m2
)
d = 6nm
d = 10nm
d = 20nm
d = 30nm
d = 40nm
d = 50nm
][)(2
0CFb
AlGaNs EEe
deen
GaNSPAlGaNPEAlGaNSP PPP ,,,
)(84.03.1 eVxe b
)(7.02.1 2 eVxxEC
s
AlGaN
sF n
m
n
m
eE
*
23/2
*
0
2
]88
9[
http://www.tamu.edu/http://engineering.tamu.edu/
And (almost) finally…2DEG
resistivity
• Now we can design
our device
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7300
500
700
900
1100
1500
1900
2300
2700
3500
4300
5000
Al Mole Fraction, x
2D
EG
Sh
ee
t R
esis
tivity (
Oh
ms)
d = 6nm
d = 10nm
d = 20nm
d = 30nm
d = 40nm
d = 50nm
ss
DEGen
1
2
http://www.tamu.edu/http://engineering.tamu.edu/
Device consideration
• Our goal is to use the
2DEG sheet resistance to
create a device that
uniformly drops 5kV
without any formation of
electric field above Ecrit
gdgdx LVE /
22
xPAlGaN EEE
EAlGaN = Ecrit,AlGaN
2222222
, )/()/( gdBRPgdgdPxPAlGaNcrit LVELVEEEE
22
,/ PAlGaNcritBRgd EEVL
http://www.tamu.edu/http://engineering.tamu.edu/
Breakdown in GaN
• Peizoelectric field is
negligible in GaN
• Smaller subset of
previous Lgd
parameters due to
substrate breakdown
2
0
,22'2'2 )()(GaN
GaNSP
gd
dg
SPxGaN
P
L
VEEE
2
0
,22
, )()(GaN
GaNSP
gd
BRGaNcrit
P
L
VE
http://www.tamu.edu/http://engineering.tamu.edu/
0 0.1 0.2 0.3 0.4 0.5 0.6 0.710
-1
100
101
102
103
104
Al Mole Fraction, xD
rain
Re
sis
tan
ce
(m
illi O
hm
s.C
m2
)
d = 6nm
d = 10nm
d = 20nm
d = 30nm
d = 40nm
d = 50nm
The result
• A full set of design
parameters that allow
HV design with ultra-
low on-state resistance
– AlGaN Thickness
– AlGaN concentration
– Gate-drain distance
• Need corroboration!
http://www.tamu.edu/http://engineering.tamu.edu/
Outline
• Motivation
• GaN design for 5kV
• Additional device and integration needs
• Summary
http://www.tamu.edu/http://engineering.tamu.edu/
Caveat Emptor…
• Other consideration that we are researching
– Enhancement mode operation
– GaN breakdown to substrate
– Alternate electric field smoothing techniques
– Thermal management
http://www.tamu.edu/http://engineering.tamu.edu/
Depletion HEMT to Enhancement MOSHFET
• HEMT has Negative Threshold
Voltage!
• Application of a dielectric and
high work function metal lifts
the Fermi level, removing the
equilibrium 2DEG
GaN AlyGa1-yN
ΔPInAlN-InGaN
2DEG
GaN AlyGa1-yN Metal
Φ~5.1eV
http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/
Alternate Field Smoothing
26 July 2011
Drain Field
Plate
Gate Gate
Dielectric
Gate Field
Plate (FP)
Passivation
Dielectric
(translucent)
Source
http://www.tamu.edu/http://engineering.tamu.edu/
Summary
• Pushing high voltage design of low Ron,sp GaN
HEMT devices opens up miniaturization,
integration and efficiency options
• A first-time complete design spec outlined for HV
GaN HEMTs
• Alternate means being developed to eliminate
additional device design constraints
http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/