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米国における窒化物半導体開発動向- 超高周波GaN電子デバイス -
R&D activities of nitride semiconductors in USA
- High-frequency GaN electrical devices -
東脇 正高
Masataka Higashiwaki
情報通信研究機構 新世代ネットワーク研究センター
National Institute of Information and Communications Technology
Outline
1. Background
2. R&D projects of high-frequency GaN transistors in USA
3. Recent reports
4. Summary
Past background of R&D of high-frequency GaN HEMTs
Excellent GaN material properties for high-frequency applications
Large saturation electron velocity: ~3×107 cm/s (theory), ~ 2×107 cm/s (exp.)
High breakdown electric field: 3.3×106 V/cm
Past projects above mm-wave frequency
USA:
High-power GaN HEMTs at Ka band (27-40 GHz)
• 13.7 W/mm @ 30GHz, PAE 40-41% [Y.-F. Wu et al. (Cree), IEDM Tech. Dig. (2007)]
• 10 W/mm @ 30 GHz, PAE 55% [J. S. Moon et al. (HRL), IEEE EDL 29, 834 (2008)]
Japan:
FED (2002-2006)
• 5.8 W/mm @ 30 GHz, PAE 43.2% [ T. Inoue et al., IEEE TMTT 53, 74 (2005)]
`The research and development project for expansion of radio spectrum resources'„
the Ministry of Internal Affairs and Communications (2006-2010)
MMIC: 1.6 W/mm @ 75 GHz [Y. Nakasha et al. (Fujitsu), IEEE CSIC Symp. Dig. (2010)]
ONR MINE MURI program (2005-2010)
Multi-University Research Initiative (MURI) program
University of California, Santa Barbara (Profs. Mishra and York)
Cornell University (Prof. Shealy )
Notre Dame University (Prof. Xing)
University of North Carolina (Prof. Trew )
Ohio State University (Prof. Ringel)
University of Michigan (Prof. Singh)
Funding agency: Office of Naval Research (ONR)
Program: Millimeter-wave Initiative for Nitride Electronics (MINE)
Term: 2005-2010 (5+1=6 years)
Purpose: “Development of GaN-based mm-wave power sources”
DARPA NEXT program (2009- )
Ongoing US R&D program of high-frequency GaN HEMTs and circuits
Funding agency: US Defense Advanced Research Projects Agency (DARPA)
Microsystems Technology Office (MTO)
Program: Nitride Electronic NeXt-Generation Technology (NEXT)
Term: 2009-2011 (1st phase), 2011-2013 (2nd Phase ), 2013-2015 (3rd phase)
Purpose:
“Developing a revolutionary nitride transistor technology that simultaneously
provides extremely high-speed and high-voltage swing [Johnson Figure of Merit
larger (JFoM) than 5 THz-V] in a process consistent with large-scale integration in
enhancement/depletion (E/D) mode logic circuits of 1,000 or more transistors.”
Contractors: HRL Laboratories ($16,043,488)
Northrop Grumman + UCSB ($28,900,900)
TriQuint Semiconductor + Notre Dame + MIT ($16,188,131)
http://www.darpa.mil/Our_Work/MTO/Programs/Nitride_Electronic_NeXt-Generation_Technology_(NEXT).aspx
Purpose of DARPA NEXT program
http://www.darpa.mil/Our_Work/MTO/Programs/Nitride_Electronic_NeXt-Generation_Technology_(NEXT).aspx
GaN HEMT
InP HEMT, HBT
Target characteristics of DARPA NEXT program (1)
Before NEXT Requirements of NEXT
D-mode GaN HEMT:
fT = 300⇒400⇒500 GHz
fmax = 350⇒450⇒550 GHz
E-mode GaN HEMT:
fT = 200⇒300⇒400 GHz
fmax = 250⇒350⇒450 GHz
JFoM (fT×Vbr) > 5 (THz・V)
D-mode AlGaN/GaN HEMT (Lg = 60 nm):
fT = 190 GHz
fmax = 251 GHz
Vbr > 40 V
E-mode AlN/GaN HEMT (Lg = 80 nm):
fT = 111 GHz
fmax = 156 GHz
Ref. D-mode: M. Higashiwaki et al., APEX 1, 021103 (2008)
E-mode: M. Higashiwaki et al., IEEE TED 54, 1566 (2007)
http://www.darpa.mil/Our_Work/MTO/Programs/Nitride_Electronic_NeXt-Generation_Technology_(NEXT).aspx
Program Go/No-Go decision metrics
(1) Johnson Figure of Merit = (breakdown voltage) x (fT)
(2) Yield defined as fraction of devices tested that meet fT metric.
(3) Test sample: at least 100 devices on a single wafer.
(4) Test sample: at least 100 devices/wafers over a lot of at least 5 wafers.
(5) Yield defined as fraction of process control monitors (PCMs) tested that achieve at least 80% of designed frequency.
(6) PCM to be a 5-stage ring oscillator. Test sample: at least 20 PCMs on a single wafer.
(7) PCM to be a 51-stage ring oscillator. Test sample: at least 20 PCMs on a single wafer.
(8) PCM to be a 501-stage ring oscillator. Test sample: at least 20 PCMs/wafer over a lot of at least 5 wafers.
(9) The standard deviation of the stated parameter.
(10) Minimum test time for PCM under normal operating conditions until failure condition observed.
Test sample: 20 PCMs.
Failure condition: Failure of a single PCM or degradation of average frequency changes by 20%.
Target characteristics of DARPA NEXT program (2)
HRL Laboratories, LLC
Device structure (HRL)
• S/D electrodes: Non-alloy metal + n+-GaN re-growth
• Thin barrier layer to keep high aspect ratio
• D-mode: AlN (3.5 nm), E-mode: AlN (2.0 nm)
• AlGaN back barrier
K. Shinohara et al., IEDM Tech. Dig (2010)
A. L. Corrion et al., IEEE EDL 31, 1116 (2010)
AlN/GaN D/E-mode HEMTs
DC characteristics (HRL)
Idmax = 1.62 A/mm
Peak gm = 723 mS/mm
Idmax = 0.92 A/mm
Peak gm = 700 mS/mm
D-mode (Lg = 40 nm) E-mode (Lg = 80 nm)
D-mode: K. Shinohara et al., IEDM Tech. Dig (2010)
E-mode: A. L. Corrion et al., IEEE EDL 31, 1116 (2010)
Small-signal characteristics (HRL)
D-mode (Lg = 40 nm) E-mode (Lg = 80 nm)
fT = 220 GHz
fmax = 400 GHz
fT = 112 GHz
fmax = 215 GHz
D-mode: K. Shinohara et al., IEDM Tech. Dig (2010)
E-mode: A. L. Corrion et al., IEEE EDL 31, 1116 (2010)
University of California, Santa Barbara (UCSB)
Ga and N-face (NRG & UCSB )
Polarization reverses direction with respect to the surface
Bulk GaN
in vacuum
+ + + + + + +
PSP
N-terminated surface
----------
PSP
+ + + + + + +
Ga-terminated surface
----------
Top figure taken from M. J. Murphy et al.,
MRS Internet J. Nitride Semicond. Res. 4S1, G8.4 (1999) Courtesy: Ms. Nidhi & Prof. Mishra (UCSB)
Better electron
confinement during pinch-off
→ larger Rds
Why do N-polar GaN HEMTs? (NRG & UCSB )
dsmgddsdsgdgs
mT
RRgCRRRCC
gf
1
2
GaN buffer
GaN
AlGaN
2DEG Gate can be placed very close to the
2DEG
More gate control due to charge
centroid shifted towards the gate
Low ohmic contact resistance Natural Back-barrier
N-face
AlGaNGaN
EF
EC
VP
N-polar nitrides are the best choice for high frequency applications
Psp-
+
Contacting the 2DEG through GaN
(lower bandgap) instead of AlGaN
InN based contacts feasible. Metal
to InN contact resistance ~ 5 Ω-μm
Already achieved RC = 23 Ω-μm by
InGaN regrowth on GaN
Courtesy: Ms. Nidhi & Prof. Mishra (UCSB)
Device structure (NRG & UCSB)
Gate first process:
• Refractory metal gate
• SiN sidewall spacer
Graded InGaN regrowth:
• Grade from GaN to InN
• Very little growth on sidewalls
Gate
AlGaN
GaN
Source Drain
High K dielectric
n+ GaN
graded to InN
SiN
n+ GaN
graded to InN
GaN
N-polar GaN HEMT structure
Courtesy: Ms. Nidhi & Prof. Mishra (UCSB)
Excellent DC characteristics achieved by self-aligned technology
DC characteristics ofD-mode N-polar GaN HEMTs (NRG & UCSB )
Idmax = 2 A/mm Peak gm = 434 mS/mm
-7 -6 -5 -4 -3 -2 -1 00.0
0.4
0.8
1.2
1.6
2.0
IDS
gm
VGS
(V)I D
S (
A/m
m)
0
100
200
300
400
500
gm (m
S/m
m)
LG = 130 nm
LSD
= 1.0 m
VDS
= 2.5 V
0.0 0.5 1.0 1.5 2.0 2.50.0
0.4
0.8
1.2
1.6
2.0
I DS (
A/m
m)
VDS
(V)
LG = 130 nm
VGS
: 2V ... -6V
Courtesy: Ms. Nidhi & Prof. Mishra (UCSB)
Small-signal characteristics ofD-mode N-polar GaN HEMTs (NRG & UCSB )
RG 560 Ω/mm
CGD 168.4 fF/mm
RG 26 KΩ/mm
CGD 92.8 fF/mm
fMAX value is state-of-the-art for the
voltages relevant for a self-aligned
structureFurther gate design is needed
to improve RG while keeping
CGD low to prevent fT drop
0.1 1 10 100
0
10
20
30
40
fT = 100 GHz
fMAX
= 126 GHz
LG = 130 nm
WG = 50 m
LSD
= 0.5 m
VD = 6 V
VG = -4 V
Gain
(dB
)
Frequency (GHz)
h21
U
fT/fMAX:
137/27 GHz
100/126 GHz Courtesy: Ms. Nidhi & Prof. Mishra (UCSB)
Device structure ofE-mode N-polar GaN HEMTs (NRG & UCSB )
Under sidewall
20 40 60 80 100
-4
-3
-2
-1
0
1
2
3
4
GaN
channel
SiN sidewall
EF
n(x)n
(x10
19 c
m-3)
Depth (nm)
En
erg
y (
eV
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
∫ n(x)dx
= 4.5×1012 cm-2
AlN removed under sidewall
Under gate
0 10 20 30 40 50 60-5
-4
-3
-2
-1
0
1
2
3
4
5
GaN:SiGaN
channel
AlNAlN
SiN
Depth (nm)E
nerg
y (
eV
)
EF
Top AlN depletes 2-DEG under gate
0 20 40 60 80 100
-4
-3
-2
-1
0
1
2
3
4
InN
n+ Graded InGaN
(In: 0% to 65%)
GaN:SiAlN
GaN
channel
EF
En
erg
y (
eV
)
Depth (nm)
Under S/D
contacts*
U. Singisetti et al., APEX 4, 024103 (2011)
Courtesy: Dr. Singisetti & Prof. Mishra (UCSB)
• Idmax=0.74 A/mm, peak gm = 260 mS/mm at Vds = 3.0 V
• Vth = 0.7 V at Vds = 3.0 V.
• High Ron = 2.7 W-mm, due insufficient InN growth
DC characteristics ofE-mode N-polar GaN HEMTs (NRG & UCSB )
U. Singisetti et al., APEX 4, 024103 (2011)
Courtesy: Dr. Singisetti & Prof. Mishra (UCSB)
Small-signal characteristics ofE-mode N-polar GaN HEMTs (NRG & UCSB )
fT = 120 GHz, fmax = 11 GHz
U. Singisetti et al., APEX 4, 024103 (2011)
Courtesy: Dr. Singisetti & Prof. Mishra (UCSB)
University of Notre Dame
Device structure (TriQuint and Notre Dame)
InAlN/AlN/GaN D/E-mode HEMTs
Y. Tang et al., IEDM Tech. Dig (2010)
D-mode: non-recessed gate
E-mode: recessed gate
DC characteristics (TriQuint and Notre Dame)
Idmax = 1.9 A/mm
Peak gm = 840 mS/mm
D-mode (Lg = 144 nm) E-mode (Lg = 144 nm)
Idmax = 1.84 A/mm
Peak gm = 920 mS/mm Y. Tang et al., IEDM Tech. Dig (2010)
D-mode (Lg = 144 nm) E-mode (Lg = 144 nm)
Y. Tang et al., IEDM Tech. Dig (2010)
fT = 94 GHz
fmax = 174 GHz
fT = 94 GHz
fmax = 176 GHz
Small-signal characteristics (TriQuint and Notre Dame)
Ring oscillator (TriQuint and Notre Dame)
Y. Tang et al., IEDM Tech. Dig (2010)
D/E-mode InAlN/GaN HEMT ring oscillators
a) 5-stage and b) 51-stage
5-stage circuit
Inferred stage delay
15.3 ps/stage @ 6.533 GHz
Massachusetts Institute of Technology (MIT)
Device structure (MIT)
D-mode AlGaN/AlN/GaN HEMTsSi/Ge/Ti/Al/Ni/Au alloyed ohmic
Novel T-gate process
J. W. Chung et al., IEDM Tech. Dig (2010)
D-mode AlGaN/GaN HEMT (Lg = 55 nm)
DC and small-signal characteristics (MIT)
Id = 1.0 A/mm @ Vg =0V
Peak gm = 500 mS/mm
fT = 225 GHz
fmax = 120 GHz
J. W. Chung et al., IEDM Tech. Dig (2010)
Current fT and fmax of GaN HEMTs (until Dec 2010)
D-mode E-mode
fT (GHz) fmax (GHz) fT (GHz) fmax (GHz)
HRL 220 400 112 225
UCSB 137 126 120 11
Notre Dame 94 174 94 176
MIT 225 120 NA NA
DARPA
required
value
(Phase I)
300 350 200 250
Evolution of fT in GaN HEMTs
J. W. Chung et al., IEDM Tech. Dig (2010)
Delay time analysis (HRL)
K. Shinohara et al., IEDM Tech. Dig (2010)
Drain delay has to be decreased
Self-aligned device structure is essential!!
How to make GaN HEMT faster
Total delay = Transit delay + Drain delay + Charging delay
• Transit delay: Transit time for passing through the region under the gate
• Drain delay: Delay time related to the drain depletion region
• Charging delay: Channel charging + parasitic charging
Transit delay can be decreased by reduction of Lg
Charging delay can be decreased by reduction of parasitic R and C
Drain delay is almost proportional to Vd (td = 0.05 ps×Vd)
Self-aligned device structure is the only solution for this problem
Summary
R&D activities of high-frequency GaN HEMTs in USA
DARPA NEXT program (2009-2013)
Current status
D-mode: fT = 225 GHz, fmax = 400 GHz
E-mode: fT = 120 GHz, fmax = 225 GHz
Self-aligned device structure is necessary to decrease drain delay
Conceivable tasks:
• Fabrication process in itself
• Suppression of increase in parasitic gate capacitance
• Suppression of decrease in breakdown voltage
Requirements by 2011
D-mode: fT = 300 GHz, fmax = 350 GHz
E-mode: fT = 200 GHz, fmax = 250 GHz
JFoM (fT×Vbr): > 5 (THz・V)