university of notre dame gan based heterojunction bipolar transistors john simon ee 666 april 7,...
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University of Notre Dame
GaN based Heterojunction Bipolar Transistors
John Simon
EE 666
April 7, 2005
University of Notre Dame EE666: Advance Solid State Devices
OUTLINE
• Introduction
• Why GaN ?
• First GaN HBT
• Polarization Doping
• Collector up Structure
• Emitter up Structure
• Future Alternatives
• Conclusions
University of Notre Dame EE666: Advance Solid State Devices
INTRODUCTION
2
2
iE
iB
pE
nB
B
E
n
n
D
D
GN
GN
Improved speeds can also be obtained with graded base technology.
Heterojunctions allow us to dope the base heavily reducing the base resistance and still maintaining a large gain (β).
kT
EE gBgE
e
University of Notre Dame EE666: Advance Solid State Devices
Why GaN?
Break down Fields ~150kV/cm
Saturation Velocities ~3.5x107cm/sec
University of Notre Dame EE666: Advance Solid State Devices
HBT Requirements
• High Gain: High Emitter Injection Efficiency (), provided by
Heterojunction(s) High Base Transport Factor (~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.
University of Notre Dame EE666: Advance Solid State Devices
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)
Sapphire Substraten+ GaN Subcollector
n- GaN Subcollector
Mg Doped Base
n+ Emitter
AlN Barrier
Regrown Base
Etched Surface
University of Notre Dame EE666: Advance Solid State Devices
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.
University of Notre Dame EE666: Advance Solid State Devices
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).
University of Notre Dame EE666: Advance Solid State Devices
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.
Sapphire Substraten+ GaN Subcollector
n- GaN Subcollector
Mg Doped Base
n+ EmitterAlxNy
University of Notre Dame EE666: Advance Solid State Devices
Regrown Emitter StructureBase 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).
University of Notre Dame EE666: Advance Solid State Devices
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).
University of Notre Dame EE666: Advance Solid State Devices
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.
* McCarthy L, Smorchkova Y, Fini P, Xing H, Rodwell M, Speck J, DenBaars S and Mishra U 2000 BT on LEOGaN Proc. 58th DRC: Device Research Conf. (Denver, CO, 2000)
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).
University of Notre Dame EE666: Advance Solid State Devices
Improved HBTCommon 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.
University of Notre Dame EE666: Advance Solid State Devices
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
P
P In [0001] direction:
σ = n·(P1-P2)
University of Notre Dame EE666: Advance Solid State Devices
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)
University of Notre Dame EE666: Advance Solid State Devices
Polarization in Nitrides
• Electrostatic attraction from polarization charges creates regions of mobile charges.
qΦb
ρ
σMET
σPOL
2-DEG x
University of Notre Dame EE666: Advance Solid State Devices
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
University of Notre Dame EE666: Advance Solid State Devices
Polarization Doping
• By grading the Metal composition we can create 3-D bulk doping.
GaN
AlxGa1-xN
Gra
ded
up
x
Polarization Charges
3-DEG
ρ
University of Notre Dame EE666: Advance Solid State Devices
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
AlxGa1-xN
GaN
Gra
ded
dow
n
xPolarization Charges
ρ
3-DHG
University of Notre Dame EE666: Advance Solid State Devices
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.
Sapphire Substrate
n+ AlGaN Emitter
AlGaN Graded Base
n+ Subcollector
Graded down
n- Collector
University of Notre Dame EE666: Advance Solid State Devices
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
University of Notre Dame EE666: Advance Solid State Devices
Collector up
P.M. Asbeck, E.T. Yu, S.S. Lau, W. Sun, X. Dang, C. Shi. Solid-State Electronics 44 (2000) 211±219
University of Notre Dame EE666: Advance Solid State Devices
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 Sapphire Substrate
n+ GaN Subcollector
n- GaN Subcollector
AlGaN Graded Base
n+ Emitter
Graded up
University of Notre Dame EE666: Advance Solid State Devices
Alternative InGaN
Sapphire Substraten+ GaN Subcollector
n- GaN Subcollector
InGaN Graded Base
n+ AlGaN Emitter
Increasing In
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.
University of Notre Dame EE666: Advance Solid State Devices
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.