Transcript JSimon_HBT - University of Notre Dame

GaN based Heterojunction Bipolar
Transistors
John Simon
EE 666
April 7, 2005
University of Notre Dame
OUTLINE
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Introduction
Why GaN ?
First GaN HBT
Polarization Doping
Collector up Structure
Emitter up Structure
Future Alternatives
Conclusions
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INTRODUCTION
Heterojunctions allow us to dope the base heavily reducing the
base resistance and still maintaining a large gain (β).
2
iB
2
iE
GN E DnB n

GN B D pE n
e
E gE  E gB
kT
Improved speeds can also be obtained with graded base
technology.
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Why GaN?
Break down Fields
~150kV/cm
Saturation Velocities
~3.5x107cm/sec
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HBT Requirements
• High Gain:
 High Emitter Injection Efficiency (g), provided by
Heterojunction(s)
 High Base Transport Factor (a~1), requiring a good quality p-type
base region (in npn structure), high minority lifetime in base,
proper base design.
• High Breakdown Voltage:
 Low doping in collector.
• Good RF Performance:
 Low base resistance, given by high base conductivity.
 Good ohmic contacts to base.
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First GaN HBT
AlN Barrier
• First GaN HBT grown by
MOCVD at UCSB in 1998.
Regrown Base
n+ Emitter
• Current gain of only 3.
• High Acceptor Activation
energies in GaN give poor pMg Doped Base
type lager.
• Thick base (200nm) needed
n- GaN Subcollector
for low base resistance.
• Base doping of 4x1019cm-3
n+ GaN Subcollector
resulting in a hole
Sapphire Substrate
concentration of 1x1018cm-3
McCarthy L S, Kozodoy P, Rodwell M, DenBaars S and Mishra U K 1999 First demonstration of an
AlGaN/GaN heterojunction bipolar transistor Proc. Int. Symp. on Compound Semiconductors (Nara, Japan)
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Etched
Surface
First GaN HBT
• Regrown Base was needed
to make ohmic contacts to
the base.
• Etch surface was shown to
have rectifying effects on
contacts.
• Nitrogen vacancies created
during RIE have donor like
characteristics.
McCarthy L S, Aluminum Gallium Nitride / Gallium Nitride Heterojunction Bipolar
Transistors, PhD Dissertation UCSB 2001.
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First GaN HBT
• Memory Effect
present in all
MOCVD grown
samples.
• Emitter-Base
junction placement
is erratic.
• No memory effect in
MBE grown
samples and no
annealing of p-type
layer is required.
H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari,
R Coffie, D S Green, G Parish, S Heikman, L Shen, N Zhang,
J J Xu, B P Keller, S P DenBaars and U K Mishra. J. Phys.: Condens. Matter 13
7139 (2001).
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Regrown Emitter Structure
• Regrown Emitter
structure developed.
• Eliminates memory
effects and etch
damage of base.
• Base was made
thinner (100nm) for
improved base transit
time.
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AlxNy
n+ Emitter
Mg Doped Base
n- GaN Subcollector
n+ GaN Subcollector
Sapphire Substrate
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Regrown Emitter Structure
Abrupt Emitter-Base Junction
Base Contact I-V
H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari,R Coffie, D S Green, G Parish, S Heikman, L Shen, N Zhang, J J Xu, B P
Keller, S P DenBaars and U K Mishra. J. Phys.: Condens. Matter 13 7139 (2001).
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RF Performance
• Current gains as large as 10
have achieved with this
structure.
• Early voltages as high as
400V are estimated.
• High Emitter-Collector
leakage attributed to donor
like dislocations in GaN.
• Dislocations are present in
both HBT structures.
H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari,
R Coffie, D S Green, G Parish, S Heikman, L Shen, N Zhang,
J J Xu, B P Keller, S P DenBaars and U K Mishra. J. Phys.: Condens. Matter 13
7139 (2001).
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LEO HBT
• GaN HBT’s were grown at
UCSB via Lateral Epitaxy
Overgrowth (LEO)*.
• Devices grown over windows
exhibited a much larger
leakage current than devices
grown on the LEO regions.
• Gain in both devices was
comparable.
• Threading Dislocations do
not contribute to minority
carrier recombination in the
base.
H Xing, S Keller, Y-FWu, L McCarthy, I P Smorchkova, D Buttari, R Coffie, D S
Green, G Parish, S Heikman, L Shen, N Zhang, J J Xu, B P Keller, S P DenBaars and U
K Mishra. J. Phys.: Condens. Matter 13 7139 (2001).
* McCarthy L, Smorchkova Y, Fini P, Xing H, Rodwell M, Speck J, DenBaars S and Mishra U 2000 BT on LEO
GaN Proc. 58th DRC: Device Research Conf. (Denver, CO, 2000)
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Improved HBT
Common Emitter Operation as high as 330V.
Huili Xing, Prashant M. Chavarkar, Stacia Keller, Steven P. DenBaars and Umesh K. Mishra. IEEE ELECTRON DEVICE LETTERS,
VOL. 24, NO. 3, MARCH 2003.
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Polarization in Nitrides
• Polarization fields present in
wurtzite structure of nitrides
allow for new novel devices.
• Polarization charges are
created by differences in
Polarization Fields.
   P
Ga
N
In [0001] direction:
σ = n·(P1-P2)
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Polarization in Nitrides
• Two types of Polarization in
Nitrides:
– Spontaneous Polarization
– Piezoelectric Polarization
• Gives us two degrees of
freedom to determine the
polarization charge:
– Semiconductor Composition
– Layer thickness
Debdeep Jena, Polarization induced electron populations in III-V nitride
semiconductors Transport, growth, and device applications. PhD Dissertation
UCSB (2003)
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Polarization in Nitrides
• Electrostatic attraction from polarization charges creates regions
of mobile charges.
qΦb
ρ
σPOL
σMET
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x
2-DEG
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GaN HEMT
• Polarization doping has
been used in High
Electron Mobility
Transistors (HEMT).
• Polarization doping can
increase the effective
AlGaN/Gate Barrier.
• No need to introduce
dopants.
• Higher gm at higher
voltages.
P.M. Asbeck, E.T. Yu, S.S. Lau, W. Sun, X. Dang, C. Shi. Solid-State
Electronics 44 (2000) 211±219
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Polarization Doping
• By grading the Metal composition we can create 3-D
bulk doping.
x
Graded up
AlxGa1-xN
Polarization
Charges
3-DEG
ρ
GaN
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Polarization Doping
• Same techniques can be used for p-type doping.
• Two configurations of HBT’s result from this:
– Emitter up Configuration
– Collector up Configuration
x
Polarization
Charges
Graded
down
GaN
3-DHG
ρ
AlxGa1-xN
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Collector up
• Using the Collector
up configuration
polarization doping
in base is produced.
• Base will produce a
dopant free p-type Graded down
layer improving the
base conductivity.
n+
Subcollector
n- Collector
AlGaN Graded Base
n+ AlGaN Emitter
Sapphire Substrate
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Collector up
• As Collector area scales down
so does collector current.
• Extrinsic emitter base current
becomes more dominant.
• Minority carriers injected into
the base contribute to base
current.
• Transistor gain is suppressed.
P.M. Asbeck, E.T. Yu, S.S. Lau, W. Sun, X. Dang, C. Shi. Solid-State
Electronics 44 (2000) 211±219
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Collector up
P.M. Asbeck, E.T. Yu, S.S. Lau, W. Sun, X. Dang, C. Shi. Solid-State Electronics 44 (2000) 211±219
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Emitter Up
• Switch crystal
orientation.
• N-face GaN gives
opposite
polarization charge
allowing p-type
doping of the base.
• Growth issues are
present with N-face
GaN
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n+ Emitter
Graded up
AlGaN Graded Base
n- GaN Subcollector
n+ GaN Subcollector
Sapphire Substrate
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Alternative InGaN
Advantages:
• Can keep Emitter up structure
and still produce the polarization
doped p-type base.
n+ AlGaN
Emitter
• InGaN smaller band gap, larger
band offset.
Disadvantages:
• Spontaneous polarization is
almost identical in InN and GaN
• Hard to produce polarization
charges.
• Difficult to grow In rich InGaN.
Increasing In
InGaN Graded Base
n- GaN Subcollector
n+ GaN Subcollector
Sapphire Substrate
• Higher base transit times.
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Conclusions
• GaN HBT’s have tremendous potential for high power
applications.
• p-type conductivity is the limiting factor for all GaN
base devices today.
• Normally doped GaN HBT’s have been
demonstrated, with operational voltages as high as
330V.
• Polarization doping gives a promising solution to the
p-type conductivity problem.
• Growth technique as well as device design must be
carefully chosen.
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