GaN based Heterojunction Bipolar Transistors

1
GaN based Heterojunction Bipolar Transistors

• John Simon
• EE 666
• April 7, 2005

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OUTLINE

• 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 (ß).
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 (a1), 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

• First GaN HBT grown by MOCVD at UCSB in 1998.
• Current gain of only 3.
• High Acceptor Activation energies in GaN give
  poor p-type lager.
• Thick base (200nm) needed for low base
  resistance.
• Base doping of 4x1019cm-3 resulting in a hole
  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)
7
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.

n Emitter
AlxNy
Mg Doped Base
n- GaN Subcollector
n GaN Subcollector
Sapphire Substrate
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Regrown Emitter Structure
Base Contact I-V
Abrupt Emitter-Base Junction
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 HBTs 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.

Ga
N
In 0001 direction s n(P1-P2)
P
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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.

?
sPOL
x
2-DEG
sMET
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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) 211219
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Polarization Doping

• By grading the Metal composition we can create
  3-D bulk doping.

x
AlxGa1-xN
Polarization Charges
Graded up
3-DEG
GaN
?
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Polarization Doping

• Same techniques can be used for p-type doping.
• Two configurations of HBTs result from this
• Emitter up Configuration
• Collector up Configuration

x
Polarization Charges
GaN
Graded down
3-DHG
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 layer
  improving the base conductivity.

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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) 211219
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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) 211219
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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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Alternative InGaN

• Advantages
• Can keep Emitter up structure and still produce
  the polarization doped p-type base.
• 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.
• Higher base transit times.

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Conclusions

• GaN HBTs have tremendous potential for high
  power applications.
• p-type conductivity is the limiting factor for
  all GaN base devices today.
• Normally doped GaN HBTs 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.