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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
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
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
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
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
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
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
VB=600V VB=100V
Breakdown of On-state Resistance
contact resistance
8
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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