Density-Dependent Electron Transport and … Electron Transport and Accurate Modeling of GaN HEMTs...
Transcript of Density-Dependent Electron Transport and … Electron Transport and Accurate Modeling of GaN HEMTs...
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Density-Dependent Electron
Transport and Accurate Modeling of
GaN HEMTs
Sanyam Bajaj1, O. F. Shoron1, P. S. Park1, S. Krishnamoorthy1, F.
Akyol1, T.-H. Hung1, S. Rajan1
1Department of Electrical and Computer Engineering
The Ohio State University, Columbus, OH USA
S. Reza2, E.M. Chumbes2
2Raytheon IDS Microelectronics, Andover, MA USA
J.B. Khurgin3
3Department of Electrical and Computer Engineering
Johns Hopkins University, Baltimore, MD USA
Acknowledgment:
Raytheon IDS Microelectronics, EXEDE MURI, DATE MURI
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Outline
2
• Motivation
• Density dependence of velocity
• Velocity measurement
• Simulation using measurement
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Outline
3
• Motivation
• Density dependence of velocity
• Velocity measurement
• Simulation using measurement
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
GaN HEMTs for RF amplification
4
Space Communications
Mobile/Wireless Communications
Military
Advantages of GaN HEMTs
High Power Density/Unit Area
High Frequency Operation
Harsh Environment Applications
THz Imaging
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Figures of Merit
5
• Maximum available gain (MAG)
• Power gain cutoff frequency (fmax)
• Current gain cutoff frequency (fT)
Transistor requirements
- High power gain over output contour
- Gain linearity to minimize signal distortion
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Small and Large Signal Issues
6
• Actual device shows reduction
in gm, fT and MAG as the VG
(charge density) increases
• Non-uniformity in power-gain,
SOFT GAIN COMPRESSION
David Meyer, WOCSEMMAD 2015
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
What causes this reduction?
7
gm,int
VG
• Lower 2DEG velocity as the density increases
• Critical to investigate density dependence of velocity
-5-4
-3-2
-10
0
10
20
30
40
50
60
5
10
15
2025
f T (
GH
z)
V DV
G
Intrinsic fT Intrinsic gm
• Intrinsic fT / gm profiles – NOT just an RS effect
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Outline
8
• Motivation
• Density dependence of velocity
• Velocity measurement
• Simulation using measurement
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Density dependence of velocity
9
Measured Hall
mobility
Low field: ve = μ*F
• Density dependent mobility
0 2 4 6 8 10 12
400
600
800
1000
1200
1400
1600
H
ALL
(cm
2/V
s)
ns (x10
12 cm
-2)
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Density dependence of velocity
10
• Density dependent velocity has NOT been measured
• Theoretical studies on density dependent velocity
• Models predict a decrease in vsat as sheet density increases
2 4 6 8 10
1.0
1.5
2.0
2.5
3.0
[ref]
New model
vS
at (
107cm
/s)
nS (10
12
/cm2
)
LO phonon-based model:
D. Jena and S. Rajan, arXiv preprint arXiv:1008.1154 (2010)
Fang et al., IEEE Electron Device Letters 33.5 (2012): 709
0 2 4 6 8 10 12
400
600
800
1000
1200
1400
1600
H
ALL
(cm
2/V
s)
ns (x10
12 cm
-2)
Low field: ve = μ*F
• Density dependent mobility
Measured Hall
mobility
High field: ve = vsat
• Density dependent vsat
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Density dependence of velocity
11
E (k) E (k)
k k
ky ky
• Simplified “toy” model based
on optical phonon emission
• Energy-band structure of
GaN
• 2DEG occupation in k-space:
Fermi circle
• Comparing lower and higher
sheet charge densities (ns)
• No applied field
D. Jena and S. Rajan, arXiv preprint arXiv:1008.1154 (2010)
Fang et al., IEEE Electron Device Letters 33.5 (2012): 709
No Applied Field
kxkxkF
kF
EF
EF
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Density dependence of velocity
12
E (k) E (k)
k k
D. Jena and S. Rajan, arXiv preprint arXiv:1008.1154 (2010)
Fang et al., IEEE Electron Device Letters 33.5 (2012): 709
kx
ky ky
APPLIED FIELD
kx
F
(-e)F
ħωop
k0
kF
k0
kF
• Simplified “toy” model based
on optical phonon emission
• Energy-band structure of
GaN
• 2DEG occupation in k-space:
Fermi circle
• Comparing lower and higher
sheet charge densities (ns)
• APPLIED FIELD
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Comparison
13
kx
ky
kOP
APPLIED FIELD
F
(-e)F
Lower ns
Higher ns
D. Jena and S. Rajan, arXiv preprint arXiv:1008.1154 (2010)
Fang et al., IEEE Electron Device Letters 33.5 (2012): 709
kx
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Comparison
14
ky
kOP
APPLIED FIELD
F
(-e)F
k0
k0
Lower ns
Higher ns
• k0 (centroid) – effective
momentum of 2DEG
• k0 for lower ns GREATER
THAN k0 for higher ns
• Strong density-dependence
D. Jena and S. Rajan, arXiv preprint arXiv:1008.1154 (2010)
Fang et al., IEEE Electron Device Letters 33.5 (2012): 709
kx
kx
2 4 6 8 10
1.0
1.5
2.0
2.5
3.0
[ref]
New model
vS
at (
107cm
/s)
nS (10
12
/cm2
)
LO phonon-based model:
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Outline
15
• Motivation
• Density dependence of velocity
• Velocity measurement
• Simulation using measurement
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Velocity measurement
16
Khan et al., IEEE Trans. on Electron Devices 47(2), 269 (2000)
Danilchenko et al., Appl. Phys. Lett. 104(7), 072105 (2014)
• Non-gated test structures [ref]
• Isolation used to make thin transport
channels
• Negligible resistance from
access regions (RC=0.4 ohm.mm)
• Gated-structures (Moll’s method)
cause abrupt peaks in field profile
• Non-uniform ns profile
Velocity directly from 2-terminal pulsed I-V:
J = qnsveve = J / qns
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
To vary 2DEG sheet density
17
• AlGaN recess to vary 2DEG sheet density
(ns)
• Extremely low-power Cl2-based plasma etch
• On-wafer hall measurement (Van der Pauw)
to confirm ns
• Multiple devices measured to confirm
uniformity
S
D
AlGaN GaNS
heet
density
AlGaN thickness
EF
EC
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Hall measurement
18
• Standard AlGaN/GaN HEMT
• ns = 7.4 x 1012 cm-2
• μ = 1423 cm2/Vs
• RSH = 590 ohm/sq
• On-wafer hall measurements for different charge density values
ve = J / qns
0.7 μm GaN
20 nm Al0.2Ga0.8N
2DEG
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Measurement setup
19
• Pulsed-iV measurements with 2-Terminal G-S probe
• Accent DiVA (Dynamic i-V Analyzer)
• Pulse width 500 ns; Duty Cycle ~ 0.01%
• SEM to confirm dimensions:
• Error < ~2%
2 μm x 2 μm
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Density-dependent velocity
20
• Velocity-field as a function of varying density
ve = J / qns
• ns-dependent saturation velocity:
• strong dependence
• vsat decreases as ns increases
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Outline
21
• Motivation
• Density dependence of velocity
• Velocity measurement
• Simulation using measurement
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Simulation using measurement
22
• To validate velocity measurement results
• Simulation of DC and RF characteristics using measured
2DEG density-dependent velocity
• Multiple devices simulated with varying gate-lengths
• 2-D TCAD simulator Silvaco ATLAS
GaN
1 nm AlN21 nm AlGaN
2DEG
Lg=60nm
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Simulated device characteristics
23
• Simulated DC and RF characteristics for
GaN HEMT with LG = 1 μm [ref]
• All device parameters used from the
report
• Simulation results show excellent
agreement with experimental data
-8 -6 -4 -2 00.0
0.3
0.6
0.9
1.2
1.5
gm (
S/m
m)
I D (
A/m
m)
VG (V)
Experiment
Simulation
VD = 8 V
LG = 1 m
0.0
0.1
0.2
0.3
1 10 1000
5
10
15
20
25
Curr
ent
Gain
(dB
)
Frequency (GHz)
Experiment [19]
Simulation
Experiment: ft = 15 GHz
Simulation: ft = 14 GHz
GaN
33 nm AlGaN
2DEG
Lg=1μm
Ping et al., IEEE Electron Device Lett. 19(2),
54-56 (1998)
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Simulated device characteristics
24
• Simulated DC and RF characteristics for
GaN HEMT with LG = 60 nm [ref]
• Simulation results show excellent
agreement with experimental data
• First report of velocity-field-density
curves which can be used for physics based
modeling of GaN HEMTs
-5 -4 -3 -2 -1 0 10.0
0.2
0.4
0.6
0.8
1.0
1.2
gm (
S/m
m)
I D (
A/m
m)
VG (V)
Experiment
Simulation
VD = 5 V
LG = 60 nm
0.0
0.1
0.2
0.3
0.4
0.5
0.6
1 10 1000
10
20
30
40
50
Experiment
Simulation
Curr
ent
Gain
(dB
)
Frequency (GHz)
Experiment: ft = 70 GHz
Simulation: ft = 66 GHz
0 2 4 6 8 100.0
0.5
1.0
1.5 Experiment
Simulation
I D (
A/m
m)
VD (V)
VG = +1V
VG = -1V
GaN
1 nm AlN21 nm AlGaN
2DEG
Lg=60nm
Chung et al., IEEE Electron Device
Lett. 31(3), 195-197 (2010)
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Summary
25
• Investigated density and field dependence of 2DEG
velocity in GaN HEMTs
• Pulsed I-V measurement on non-gated test structures
• Electron velocity strongly dependent on density-
decreases with increasing density
• Simulation of DC and RF characteristics using
measured velocity characteristics
• Excellent agreement with experimental results
• Validation of measurement
• Accurate model for GaN HEMTs
1 10 1000
10
20
30
40
50
Experiment
Simulation
Cu
rre
nt
Ga
in (
dB
)
Frequency (GHz)
Experiment: ft = 70 GHz
Simulation: ft = 66 GHz
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Pulsed iV with 2-T RF probes
27
• Pulsed measurements with 2-Terminal G-S probe
• Measurements done using Accent DiVA (Dynamic i-V Analyzer)
• Pulse width 500 ns; Duty Cycle ~ 0.01%
• Multiple devices measured for each unique charge density
• Adjacent isolated structure measured and subtracted
DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]
Simulation of test structure
28
• We assume uniform field in
the test structures; hence
uniform sheet charge density
• Simulated potential profile:
• Linear profile showing
uniform potential drop
• Simulated electric field
profile:
• No abrupt field peaks
• Field fairly uniform
across the active region
Simulated Potential Profile
Simulated Electric Field Profile