ICCDCS2012 Capacitance

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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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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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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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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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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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ICCDCS2012 Capacitance

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