![Page 1: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/1.jpg)
GaN based power transistors:
Proposals for low-loss operations
Kazuo Tsutsui
Dept. of Electronics and Applied Physic,
Tokyo Institute or Technology
WIMNACT-39, Tokyo, Feb.7, 20141
![Page 2: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/2.jpg)
Acknowledgement
The present works are carried out with the collaborations;
Y. Takei, K. Terayama, M. Kamiya, H. Yonezawa,
K. Kakushima, H. Wakabayashi, Y. Kataoka, H. Iwai
(Tokyo Institute of Technology)
W. Saito (Toshiba Corp.)
A. Nakajima, S. Nishizawa, H. Ohashi, M. Shimizu
(Advanced Industrial Science and Technology)
2
![Page 3: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/3.jpg)
Outline
1. Introduction
2. Problems on AlGaN/GaN HEMT power devices
3. Trade-off properties on ohmic contact formation
4. Proposal of new contact technique: Uneven AlGaN layers
5. GaN base FinFETs
6. Conclusion
3
![Page 4: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/4.jpg)
Importance of power devices
Saving energy consumption High efficiency of energy
conversion system
Low loss operation of power devices
used in inverter systems is required.
Requirement for power devices:
- low on-state resistance
- low off-state leakage current
- high speed switching
under necessary high voltage
operation (necessary
breakdown voltage).
Wide band gap
semiconductors such as SiC
and GaN are expected.
4
![Page 5: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/5.jpg)
AlGaN
GaN
Source Gate Drain
AlGaN/GaN HEMT (high electron mobility transistor)
Substrate (Si, Al2O3, etc.)
buffer layers
two-dimensional electron gas (2DEG)
Met
al
AlG
aN
GaN
ΦB
EcEF
Ev
Electric field produced by
polarization
5
![Page 6: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/6.jpg)
12
10
8
6
4
2
0
2D
EG C
on
cen
trat
ion
[1
01
2cm
-2]
AlGaN Thickness [nm] [1]0 5 10 15 20 25
Al:24%T:300 K
[1] D. Qiao, et al, JAP, 89, (2001) 5543.
AlGaN
GaN
Source Gate Drain
2DEG Produced by Polarization Effects
2DEG
Thickness
6
![Page 7: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/7.jpg)
Present
state
Normalized On-resistance vs. Breakdown Voltage
AlGaN
GaN
S G D
LSD
Long LSD is
necessary for high
breakdown voltage.
Increase in Ron
7
![Page 8: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/8.jpg)
VB=600V VB=100V
Breakdown of On-state Resistance
contact resistance
8
![Page 9: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/9.jpg)
Problems of AlGaN/GaN HEMTs for Power Devices
AlGaN
GaN
RCH
RC
Source Gate Drain
AlGaN
GaN
Source Gate Drain
On-state
Off-state
leakage
On-state resistance: further decrease.
- Contact resistance (Rc) is a key
issue.
- Improvement of crystalline quality:
decrease of RCH.
Stability of on current (current collapse).
- Improvement of crystalline quality.
- Control of surface state.
Off-state leakage current.
- Significant on scaling down: short
channel effects.
- Improvement of crystalline quality.
9
![Page 10: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/10.jpg)
Formation of Conventional Contacts on AlGaN/GaN
Au
Mo
Al
Ti
after annealing @850℃ [2]
AlGaN
as deposite
GaN
GaN
AlGaN
Au/X/Al/TiTiN
island
GaN
AlGaN
Au/X/Al/Ti
TiN
[2] L. Wang, et al,
JAP, 103, 093516 (2008)
Au shell
GaN
Au/X/Al/Ti
dislocation
2DEG
Better crystallinity with
lower dislocation densityHard to form contacts
10
![Page 11: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/11.jpg)
Contact Formation Independent of Dislocations
[3] B. Daele, APL, 89, 201908 (2006) [4] K. Tsuneishi, et al. ECS Transactions, Vol.50(3), pp. 447-450.
AlGaN
GaN
Non-alloy or uniform reaction types are desirable.
Example: Al/Ti/Si3N4[3], TiN/TiSi2
[4]
TiN/TiSi2/AlGaN/GaN after annealing at 950oC
11
![Page 12: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/12.jpg)
Trade-off on AlGaN Layer Thickness
AlGaN AlGaN
AlGaN layer thickness
Resis
tance
2D
EG
concentr
ation
Rc
Rc=Rc2DEG + RcAlGaN
Rc2DEG
RcAlGaN
Rc
Rc
2DEG 2DEG
Rc2DEG
RcAlGaN
2DEG
12
![Page 13: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/13.jpg)
80 μm Metal300 μm250 μm
Si(111) sub.
Al0.25Ga0.75N
Buffer layer
GaN
SiO2
Thinning by the step-by-step
etching using ozone oxidation
: 7.0~30.0 nm
Rc Depending on AlGaN Thickness: Experiment
TiN 45nm/TiSi2 20nm
or
Mo 35nm/Al 60nm/Ti 15nm
Annealing in N2 at 400oC -->1100oC
TLM measurement
Used TLM structure
13
![Page 14: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/14.jpg)
after annealing at 650oCafter annealing at
950oC (Mo/Al/Ti)
or 1100oC (TiN/TiSi2)
Rc Depending on AlGaN Thickness: Experiment
The trade-off property
was observed.
Conductance through
the AlGaN layer was
reduced under high
temperature annealing.14
![Page 15: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/15.jpg)
GaN
AlGaN
2DEG
GaN
metal
AlGaN
2DEG
Introduction of uneven AlGaN layer
Proposal for low Rc overcoming the “trade-off”
Increase in contact area density.
Two-dimensional effect at pattern edges:
- intrusion of 2DEG under thin-AlGaN region
- metal closing higher 2DEG density region15
![Page 16: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/16.jpg)
0.0E+00
2.0E+12
4.0E+12
6.0E+12
8.0E+12
1.0E+13
1.2E+13
180 190 200 210 220Position [nm]0 10 20-10-20
0
2
4
6
8
10
12
Thickness
10/25 nm
Thickness
5/25 nm
2DEGi-GaN
20 nm20 nm
5 or 10 nm25 nm
40 nmpitch
i-Al0.3Ga0.7N
Uniform thickness 25 nm
Uniform thickness 10 nm
Uniform thickness 5 nm
2D
EG
co
nce
ntr
atio
n [1
01
2cm
-2]
Lateral distribution of 2DEG concentration
on uneven AlGaN layer
16
![Page 17: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/17.jpg)
0.0E+00
2.0E+12
4.0E+12
6.0E+12
8.0E+12
1.0E+13
1.2E+13
160 170 180 190 200 210 220 230 240
Position [nm]0 10 20-10-20 30 40-30-40
35/ 5 nm 30/10 nm20/20 nm
10 nm25 nm
10 nm
5 nm
0
2
4
6
8
10
12
i-Al0.3Ga0.7N
2DEG
5/10/20 nm35/30/20
nm25 nm
40 nmpitch
i-GaN
5 nm
2D
EG
co
nce
ntr
atio
n [1
01
2cm
-2]
Lateral distribution of 2DEG concentration
on uneven AlGaN layer
17
![Page 18: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/18.jpg)
0.0E+00
2.0E+12
4.0E+12
6.0E+12
8.0E+12
1.0E+13
1.2E+13
160 170 180 190 200 210 220 230 240Position [nm]0 10 20-10-20 30-30 40-40
0
2
4
6
8
10
12 25 nm
10 nm
5 nm
5 nm25 nm
40 nm 40 nmpitch
20 nm 20 nm
i-Al0.3Ga0.7N
i-GaN
20 nm 20 nm
5 nm25 nm
20 nm
5 nm25 nm
20 nm
Dome
Pyramid
Pi
t
2D
EG
co
nce
ntr
atio
n [1
01
2cm
-2]
Effects of shapes of Unevenness
18
![Page 19: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/19.jpg)
Fabrication of Contacts using Uneven AlGaN Layers
5 μm 5 μm5 μm 10 μm
150 μ
m
14
0 μ
m
metal
80 μm
TLM pattern
AlGaN
Process: photo lithography, partial etching of AlGaN layer by RIE
perpendicular to
current flow
parallel to
current flow
square close packing
5 μ
m5 μ
m
Current
direction
5 μ
m
10 μ
m
PERP5 PARA5 SQU5 CP5
Thin AlGaN
region
19
![Page 20: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/20.jpg)
Rc Reduced by Uneven AlGaN Layers
GaN2DEG
AlGaN30 nm
TiN/TiSi2
20 nm5 nm
Reduced Rc on the uneven
AlGaN contacts with proper
patterns was observed,
compared with the flat AlGaN
structure with optimized
thickness.
0
0.2
0.4
0.6
0.8
1
10 nm squ5 photo5 yoko5
Conta
ct r
esis
tance
[10
-3Ω
cm2]
1000℃
10 nm -
uniform AlGaN 20
![Page 21: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/21.jpg)
Ohmic contacts using uneven AlGaN Layer
Pit structure Pyramid structure
Optimized size and structure formed by using lithography.
Application of self-assemble
processes.
Merit on practical device
process.S.K. Hong et al., J. of
Crystal Growth 191
(1998) 275.
anisotropic etching by H3PO4
21
![Page 22: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/22.jpg)
S G D
AlGaNGaN
S
G
D
GaNAlGaN
S
G1
D
GaN AlGaN
G2
Conventional planar HEMT FinFET
Separated
double gate
GaN based FinFET
Higher function
High gate controllability
Reduction of off-leakage current
Low RonA
High aspect ratio of Fin
Possible advantages:
bulk conduction (not 2DEG)
ohmic contact to bulk
High crystalline quality
Use of selective growth22
![Page 23: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/23.jpg)
GaNGaN
mask
GaN Fin structures
formed by selective growth
Dislocation
density is
reducedHigh dislocation density
Original dislocations remained.
Etching damage is added.
× ×××
×
Lowering Defect Density in Fin Structure
by GaN Selective Growth
GaN Fin structures
formed by etching
23
![Page 24: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/24.jpg)
GaN
AlGaN
Side wall: Polar
2DEG 2DHG
Side wall: Non polar
EcEc
Bulk conduction
electrons
holeselectrons
metal
Variation of Possible Channel Conduction Modes
Rather high electron mobility 〜1500 cm2/Vs
S/D ohmic contacts to bulk channel may be easy
compared with those to 2DEG ?
Bulk conduction is promising ?
24
![Page 25: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/25.jpg)
Problems of AlGaN/GaN HEMTs, which should be solved
for further low-loss operation of power transistors.
- Further decrease in ohmic contact resistances for S/D.
- Control of short channel effects; leakage current
in off-states.
Ohmic contacts for future devices
- Contact formations depending on dislocations in
AlGaN layers will not be useful in future. Contact
formations with uniform reaction (not necessary non-
alloy) are desirable.
Conclusion
25
![Page 26: GaN based power transistors: Proposals for low-loss operations presen.pdf · AlGaN/GaN HEMT (high electron mobility transistor) Substrate (Si, Al 2 O 3, etc.) buffer layers two-dimensional](https://reader030.vdocument.in/reader030/viewer/2022041018/5ecb7dbb05853f2c713fe755/html5/thumbnails/26.jpg)
Development of contact formation technology based on
clarifying its mechanism is important.
- Evaluation of the contact properties depending on AlGaN
thickness will be a useful tool.
- A new ohmic contact fabrication technique, the uneven
AlGaN layer, was proposed, and its advantage was
demonstrated by simulation and primitive experiment.
GaN based FinFETs are promising for future power
transistors.
- Not only 2DEG channels but also bulk channels are worth
to be investigated.
- Challenge of selective growth to form Fin structure with
low defect density will open future high performance
power transistors.
26