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A Physics-Based Compact Model for Gate
Capacitance in AlGaN/GaN HEMT Devices
S. Khandelwal and T. A. Fjeldly
[email protected], [email protected]
Dept. of Electronics & Telecommunications
Norwegian University of Science & Technology, NTNU, Norway
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Outline
Motivation 2-DEG Charge Density Modeling
Model Description
Results and Highlights
Gate Capacitance Modeling
Model Description
Results and Highlights
Conclusions
References
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2-DEG Charge Density
2DEG formation is the heart of
HEMT device operation
Accurate model for 2DEG chargedensity is essential for complete
compact model
2-DEG formation takes place atheterojuction in quantum-wellRef. Fig. [1]
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2-DEG Charge Density Modeling
Basic device equations are transcendental in
nature
We divide variation ofn
s withV
g intoregions
todevelop fully analytical expression
Regional models are combined in one analytical
expression
No fitting parameters introduced
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Numerical Solution and Regions
Region I: Sub-Voff region
Region II:EfE0 strong 2-DEG region
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Regional models for ns
Sub-Voff model
Moderate 2-DEG region model
Strong 2-DEG region model
2/ 30
2/ 3
0
3
21
3
g go
go
g goIII
s
g go
th go
C VV
C V qn
q C VV V
q
,2 exp
off
go
s sub V th
th
Vn DV
V
2/ 3
0
2/ 3
0
1 ln3
2
3
g go
go th gog goII
s
g go
go th
C VV V V
C V qn
q C VV V
q
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GaN HEMT Unified ns Model
Regional Models are combined to produce oneunified analytical solution
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2-DEG Modeling Highlights
Analytical and Physics-Based
No empirical/fitting parameters
Developed for both AlGaN/GaN and
AlGaAs/GaAs HEMT devices
Excellent Model Agreement with Numerical
Solutions
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Gate Capacitance Modeling
For useful operating range the metal-AlGaN isreverse biased
Displacement Current flows through the metal-
AlGaN junction
Metal
AlGaN
GaN 2-DEG
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Gate Capacitance Modeling
Variation in 2-DEG charge with the applied biass
ch
g
dqnC
dV
2 2
'( 1)
ln( ) ' '
2 2( 1) ( 1)
ge
go th gege go th gege
ch th g th g
go th ge go th ge
VG V V V
V G V V V V
C V C V C G V V V G V V V
3/2
0
3/2
0
3
21
3ln1
q
VC
V
VV
q
VCVVV
VH
gog
go d
thgo
gog
go nthgo
go
/ 2 / 21/ , 1 , 1go th go thV V V V go go ge geG V H V V e V e
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Gate Capacitance Modeling
In un-doped GaN layer Cch will be the gatecapacitance
In devices with doped GaN layer addional
capacitance needs to be accounted for
Metal
AlGaN
GaN 2-DEG
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Gate Capacitance Modeling
Doped GaN layer gives rise to a p-n junction likedepletion capacitance
Capacitance behavior depends on the dopingprofile
0
1
2
1
dep
dep
m
bi
go
CC
VV
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Gate Capacitance: Results
In high Vg regions, capacitance in due the 2-DEG charge
Below Voffthe capacitance is mainly due to p-n junction depletion
capacitance
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Conclusions
Analytical physics-based 2-DEG charge density
model presented
Model has no empirical or fitting parameters and
is valid in all the regions of device operation
Gate capacitance model derived from 2-DEGcharge model
Model is in excellent agreement with experimental
data
Model can serve as basis for developing a
complete physics-based compact model for these
devices
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References
1. S. Kola, J. M. Golio, and G. N. Maracas An analytical expression for Fermi Level
versus sheet carrier concentration for HEMT modeling,IEEE Electron device Lett.,vol. 9, pp. 136-138, March 1988.
2. Device Modeling for Analog and RF Circuit Design, T. Ytterdal, Y Cheng and T. A.
Fjeldly, John Wiley and Sons, pp. 31-32, 2003.
3. BSIMSOI4.3 Users Manual, BSIM Group, Dec. 2009
4. Geoffrey J. Coram, How to and how not to write a compact model in Verilog-A, Proc
of. IEEE International Behavioural Modeling and Simulation Conference, pp. 97-106,Oct. 2004.
5. Y.-F. Wu, S. Keller, P. Kozodoy, B. P. Keller, D. Kapolnek, S. P. Denbaars, and U. K.
Mishra, Bias dependent microwave performance ofAlGaN/GaN MODFETs up to
100V,IEEE Electron Dev. Lett. vol. 18, no. 6, pp. 290-292, June 1997.
6. J.W. Lee and K. J. Webb, A temperature dependent non-linear analytic model for
AlGaN-GaN HEMT on SiC,IEEE Trans. Microw. Theory Tech. vol. 52, no. 1, pp. 2-9,Jan. 2004.
7. M. A. Khan, X. Hu, G. Simin, A. Lunev, J. Yang, R. Gaska, and M. S.
Shur,AlGaN/GaN Metal Oxide Semiconductor heterostructure Field Effect Transistor,
IEEE Electron Dev. Lett. vol. 21, no. 2, Feb. 2000.
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Thank You for Attention!
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