Transcript Goal：GaN FETs

GaN-based Heterostructure
Field-Effect Transistors
Ting-Chi Lee
OES, ITRI
11/07/2005
Outline







Introduction to GaN
ICP etching of GaN
Low resistance Ohmic contacts to n-GaN
Narrow T-gate fabrication on GaN
Polarization effect in AlGaN/GaN HFETs
Thermal effect of AlGaN/GaN HFETs
Conclusion
Introduction

Unique material properties of GaN
 Wide
bandgap, 3.4 eV at RT
 High breakdown field, 3 MV/cm
 High electron saturation velocity, 1.3x107 cm/s
 Excellent thermal stability
 Strong polarization effect
Introduction

GaN-based devices
Great achievement in blue LEDs and laser diodes
 Potential microwave high power devices
 Next generation wireless communication system, especially
in the base station power amplifiers, high Vbk is required


Next generation wireless communication
Access multi-media information using cell phones or PDAs
at any time anywhere
 High-efficiency base station PAs
 Present base station PAs: Si LDMOS, low efficiency

Device Power Performance vs. Frequency
1000
100
SiC
GaN
10
O
u
tp
u
t
p
o
w
e
r
(W
)
1
0.1
HBT
pHEMT
10
Frequency (GHz)
100
The Wide Band Gap Device Advantages
GaN HEMT and Process
Suitable Specifications for
GaN-based Power Devices
Ron of GaN HEMT Switches
GaN-based devices for various applications
• For high-power switching applications




GaN
GaN
GaN
GaN
schottky diodes
p-i-n diodes
HEMT-based switching devices
MOSHFET-based switching devices
• For microwave power amplifications
 GaN Schottky diode
 AlGaN/GaN HEMTs
 AlGaN/GaN MOSHFETs
 GaN-based microwave circuits
• For pressure sensor application
 AlGaN/GaN HEMTs
R & D activity in GaN HFET



Company
 RF Micro Devices, Cree Inc., Sensor Electronic Technology, ATMI
 Epi wafers for GaN FET
Lab.
 USA: US Naval Research Lab., Hughes Research Lab., Lucent
Technologies Bell Lab., TRW, nitronex
 USA: Cornell U., UCSB, RPI, U. Texas, USC, NCSU
 Germany: Water-Schottky Institute, DaimlerChrysler lab.
 Sweden: Chalmers U., Linkopings U.,
 Japan: Meijo U., NEC and Sumitomo
Military contracting lab.
 Raytheon, GE, Boeing, Rockwell, TRW, Northrop Grumman, BAE
Systems North America
ICP Etching of GaN

GaN-based materials
Inert chemical nature
 Strong bonding energy
 Not easy to perform etching by conventional wet
etching or RIE


New technology
High-density plasma etching (HDP)
 Chemically assisted ion beam etching (CAIBE)
 Reactive ion beam etching (RIBE)
 Low electron energy enhanced etching (LE4)
 Photoassisted dry etching

ICP Etching of GaN

High density plasma etching (HDP)
 Higher
plasma density
 The capability to effectively decouple the ion
energy and ion density
 Inductively
coupled plasma (ICP)
 Electron cyclotron resonance (ECR)
 Magnetron RIE (MRIE)
Our work

ICP etching

Ni mask fabrication

Dry etching parameters
Ni mask fabrication

Suitable etching mask for ICP etching of GaN
 PR,

Ni and SiO2
Ni mask fabrication
 Wet
chemical etching by HNO3: H2O (1:1)
 Lift-off
Ni mask fabrication
PR
Ni
20 um
Wet etching
Rough edge
Poor dimension control
20 um
Ni
Lift-off
Smooth edge
Good dimension control
Dry etching parameters: bias power
Larger bias power
-Increase the kinetic energy of
incident ions
-Enhance physical ion
bombardment
3500
ICP 300W
Pressure 15mtorr
Cl 50sccm
Ar 5 sccm
Etching rate (A/min.)
3000
2500
2000
1500
-More efficient bond breaking and
desorption of etched products
1000
500
0
0
5
10
15
20
Bias power (W)
25
30
Dry etching parameters: bias power
Ni: 2000 Å
GaN: 2 um
Bias power: 5 w
Bias power: 10 w
Bias power: 20 w
Bias power: 30 w
Dry etching parameters: Ar flow rate
3440
3400
etch rate (A/min)
Higher Ar flow rate
-Increase the density of
incident Ar ions
-Enhance physical ion
bombardment
Samco ICP
ICP 300w
Bias 30w
Pressure 15mTorr
Cl 50sccm
3420
3380
3360
Ar flow rate> 15 sccm
-Cl2/Ar flow ratio decrease
3340
3320
3300
0
5
10
15
20
Ar flow rate (sccm)
25
30
Dry etching parameters: Ar flow rate
Ni: 2000 Å
GaN: 2 um
Ar flow rate: 5 sccm
Ar flow rate: 15 sccm
Ar flow rate: 20 sccm
Ar flow rate: 25 sccm
Dry etching parameters: Cl2 flow rate
3600
Etching rate (A/min.)
3400
Higher Cl flow rate
-Generate more reactive Cl radicals
to participate in the surface
chemical reaction
ICP power: 300 W
Bias power: 30 W
pressure: 15 mtorr
Ar flow: 5 sccm
3200
3000
2800
2600
10
20
30
Cl2 flow rate (sccm)
40
50
Dry etching parameters: Cl2 flow rate
Ni: 2000 Å
GaN: 2 um
Cl flow rate: 10 sccm
Cl flow rate: 20 sccm
Cl flow rate: 30 sccm
Cl flow rate: 50 sccm
Summary

Good Ni mask fabrication by lift-off

Dry etching parameters
 Bias
power
 Ar flow rate
 Cl2 flow rate

Smooth etched surface and vertical sidewall
profile
Low resistance Ohmic contacts
to n-GaN

GaN-based materials
 Wide
bandgap
 Not easy to obtain low resistance Ohmic contacts

Approaches to improve the contact resistance
 Select
proper contact metal: Ti, Al, TiAl, TiAlTiAu,…
 Surface treatment: HCl, HF, HNO3: HCl (1:3),…
 Plasma treatment: Cl2/Ar, Cl2, Ar, …
Our work

Plasma treatment
with Nd=8.7x1016, 3.3x1018 cm-3
 Cl2/Ar and Ar plasma
 n-GaN

Thermal stability issue

Forming gas ambient treatment
Plasma treatment
Cl2/Ar or Ar plasma
n+-GaN
n-GaN
n-GaN
sapphire
sapphire
Plasma treatment
-> create N vacancies (native donors)
-> increase surface electron concentration
Plasma treatment: Cl2/Ar, Ar
ND=8.7x1016 cm-3
No.
1
2
3
4
5
6
7
8
9
ICP power (w)
Bias power (w)
Pressure (mtorr)
Cl2 flow (sccm)
Ar flow (sccm)
Time (min.)
-
300
5
15
50
30
1
300
5
15
50
10
2
300
5
15
50
15
2
300
5
15
50
20
2
300
5
15
50
30
2
300
5
15
10
1
300
5
15
30
1
300
5
15
50
1
0.638
621.0
6.6
0.614
656.3
5.7
0.48
692.2
3.4
0.45
696.3
2.8
0.21
668.3
0.68
0.28
671.5
1.2
0.87
673
11
0.57
649.3
5.0
0.3
803
0.87
Rc (Ω‧mm)
ρs (Ω/□)
ρc (mΩ‧cm2)
Plasma treatment: Ar
ND=3.3x1018 cm-3
ICP power (W)
Bias power (W)
Pressure (mTorr)
Ar flow (sccm)
Time (min.)
-
300
5
15
10
1
300
5
15
30
1
300
5
15
50
1
300
5
15
50
2
300
5
15
50
3
Plasma treatment: Ar flow rate
Before annealing
1.6
Current (A)
0.02
0.00
-0.02
-0.04
-0.06
-3
C
1.2
1.0
-1
0
Voltage (V)
1
2
3
1E-4
0.8
0.6
0.4
0.2
-2
Pure Ar, 1min.
RC
1.4
2
before annealing
No treatment
Ar=10 sccm
Ar=30 sccm
Ar=50 sccm
Contact resistance ( -mm)
0.04
1E-3
Specific contact resistance ( -cm )
0.06
1E-5
0
10
20
30
Ar flow (sccm)
40
50
Plasma treatment: Ar flow rate
After annealing
-0.02
-0.04
-0.5
0.0
Voltage (V)
0.5
1.0
C
0.16
0.14
0.12
0.10
0.08
0
10
20
30
Ar flow (sccm)
40
50
2
0.18
0.00
-0.06
-1.0
1E-5
Pure Ar,1min.
RC
no treatment
Current (A)
0.02
After annealing
No treatment
Ar=10 sccm
Ar=30 sccm
Ar=50 sccm
Contact resistance (-mm)
0.04
0.20
Specific contact resistance (-cm )
0.06
1E-6
Plasma treatment: time
0.06
0.06
Current (A)
0.02
0.04
0.02
Current (A)
0.04
Before annealing
No treatment
Ar=50, 1min.
Ar=50, 2min.
Ar=50, 3min.
0.00
-0.02
-0.04
After annealing
No treatment
Ar=50, 1min.
Ar=50, 2min.
Ar=50, 3min.
0.00
-0.02
-0.04
-0.06
-3
-2
-1
0
Voltage (V)
1
2
3
-0.06
-1.0
-0.5
0.0
Voltage (V)
0.5
1.0
Plasma treatment: time
0.20
0.16
0.14
0.12
1E-6
0.10
0.0
1E-5
2
C
no treatment
Contact resistance (-mm)
0.18
Specific contact resistance(-cm )
Pure Ar,50sccm
RC
0.5
1.0
1.5
2.0
Etching time (min.)
2.5
3.0
Thermal stability issue



Important for devices
Several studies on the thermal stability of Ohmic
contacts to n-GaN have been performed
Thermal stability of plasma-treated Ohmic contacts to
n-GaN
If the damages created or defects generated by plasma
treatment have any effect on the device reliability ??
 Thermal aging tests at different temperatures for 2h were
performed to observe it

2
2
Specific contact resistance (-cm )
1E-4
Specific contact resistance (-cm )
Thermal aging tests: N2 ambient
N2 ambient
No treatment
Cl2/Ar=50/20, 2min.
Cl2/Ar=50/30, 1min.
1E-5
1E-6
0
50
300
400
500
0
Temperature ( C)
600
N2 ambient
No treatment
Ar=10 sccm, 1 min.
Ar=50 sccm, 1 min.
1E-4
1E-5
1E-6
0
50
300
400
500
0
Temperature ( C)
600
Thermal aging tests: Air ambient
Air ambient
No treatment
Cl2/Ar=50/30, 2 min.
Ar=30sccm, 1 min.
2
Specific contact resistance (-cm )
1E-4
1E-5
1E-6
0
50
300
400
500
0
Temperature ( C)
600
Discussion

After thermal annealing
TiN form at M/GaN interface, thermodynamically stable
over a wide temperature
 N vacancies and other defects form at interface


High-temperature thermal aging
Improve the crystal quality
 Reduce the defect density


No obvious electrical degradation observed

Plasma-treated Ohmic contacts exhibited excellent
thermal stability
Forming gas ambient treatment

Thermal annealing in N2 ambient for nitride
processing
 To
avoid hydrogen passivation of dopants
 Especially for p-GaN

Forming gas annealing ambient
 Better
reduction capability due to the H2
 Reduce the oxidation reaction of metal at high T
 Cause carrier reduction of n-GaN due to the H
passivation ??
Forming gas ambient treatment
0.06
2
0.00
-0.02
-0.04
-0.06
-1.0
-0.5
Annealing ambient
N2 gas
Forming gas
0.0
Voltage (V)
0.5
1.0
1E-5
no treatment
Current (A)
0.02
Forming gas ambient
No treatment
Ar=10 sccm
Ar=30 sccm
Ar=50 sccm
Specific contact resistance (-cm )
0.04
1E-6
0
10
20
30
Ar flow (sccm)
40
50
Summary

Proper plasma treatment by Cl2/Ar or Ar


Thermal stability issue


Very effective in the improvement of contact resistance
Plasma-treated Ohmic contacts to n-GaN exhibited
excellent thermal stability
Forming gas ambient treatment
No electrical degradation observed
 Even lower contact resistance obtained

Narrow T-gate fabrication on GaN


To realize high performance devices especially for highfrequency application
Conventional approach
A high accelerating voltage of around 40-50 kV
 Much reduced forward scattering effect


A lower accelerating voltage for e-beam lithography
Less backscattering from the substrate
 Lower doses needed
 Much reduced radiation damage
 But larger forward scattering effect

Our work

E-beam system

E-beam resist processing


PMMA (120 nm)/Copolymer (680 nm)
Narrow T-gate fabrication using a lower accelerating
voltage, 15 kV
Writing pattern design
 Especially for the reduction of forward scattering with a
lower accelerating voltage

E-beam system

JEOL 6500 SEM + nano pattern generation
system (NPGS)
 Max.
acceleration voltage: 35 kV
 Beam current: tens of pA ~ 1 nA
 Thermal field emission (TFE) gun

Thermal field emission gun
 Large
beam current
 Good beam current stability
Bi-layer PMMA/Copolymer process
Write strategy
Central stripe (50 nm): foot exposure
Side stripe (75 nm): head exposure
Spacing between the central stripe and the side stripe: key point
Foot width v.s. central dose
Foot width (nm)
200
150
100
50
140
150
160
170
180
2
Central dose ( mC/cm )
190
200
40 nm Narrow T-gate
Discussion

As the spacing between the central stripe and
the side stripe<< stripe width
 Sub
100 nm T-gate can be easily obtained
 Forward scattering effect was dramatically
improved
 Thus side exposure influences significantly the
final e-beam energy density profile
Comparison: dose, dose ratio
50 kV
15 kV
PMMA (uC/cm2)
600
140-200
PMMA-MAA (uC/cm2)
200
40
Dose ratio
Copolymer/PMMA
3-4
3.5-5
Lower dose, higher sensitivity
Summary


Narrow T-gate fabrication using a lower
accelerating voltage of 15 kV is practical
Specially designed writing pattern
 Can
significantly improve the forward scattering
problem with a lower accelerating voltage

Lower doses are needed for a lower
accelerating voltage
Polarization effect in AlGaN/GaN HFETs





Design rules for realizing high performance
GaN HFETs
High Al content AlGaN/GaN heterostructure
Crystal structure
Polarization-induced sheet charge, 2DEG
Difficulties in the growth of AlGaN
High performance GaN HFETs


In addition to develop device processing
technologies
Design rules
High sheet charge density
 High carrier mobility
 Maintain high breakdown voltage

-> high Al composition AlGaN/GaN heterostructures
High Al composition
AlGaN/GaN heterostructures

Higher band discontinuity
Better carrier confinement
 Al=0.3, Ec=0.5 eV


Higher spontaneous polarization and
piezoelectric effect


Higher 2DEG sheet charge density
Higher bandgap of AlGaN

higher breakdown field
Crystal structure and polarity


JAP, 1999
Wurtzite crystal
structure
Hexagonal Bravais
lattice (a, c, u)
 Both spontaneous
and piezoelectric
polarization


Polarity
Ga-face: MOCVD or PIMBE
N-face: PIMBE only
Polarization, polarization-induced
sheet charge and formation of 2DEG
Ga-face
N-face
Comparison of calculated and
measured 2DEG ns
AlGaN: 200Å, ■/□: undoped/doped
Difficulties in the growth of AlGaN



Atomically smooth surface is not easy to obtain,
especially in high Al content
Local variation in the alloy composition
Strain in the AlGaN layer due to the lattice mismatch
bet. AlGaN and GaN
Formation of structural defects
 Island growth mode
 Electrical property of heterostructure, piezoelectric effect

-> decrease in electron mobility with high Al composition
Our work

Design AlGaN/GaN heterostructures with
different Al compositions, different AlGaN
thickness and modulation-doping

Surface morphology

Electron transport properties

Device characteristics
Structure: Al=0.17
i-AlGaN
18 nm
(Al=0.17)
i-GaN
3 µm
i-AlGaN
50 nm
(Al=0.17)
i-GaN
3 µm
Buffer layer
Buffer layer
Sapphire
Sapphire
Undoped
Undoped
Structure: Al=0.3
i-AlGaN 5 nm
i-AlGaN
28 nm
(Al=0.3)
i-GaN
3 µm
Buffer layer
Sapphire
Undoped
n-AlGaN: 5E18 20 nm
i-AlGaN
3 nm
i-GaN
3 µm
Buffer layer
Sapphire
Modulation-doped
Surface morphology: Al=0.17
Top AlGaN: 18 nm
Undoped AlGaN/GaN structure
Step flow structure
RMS: 0.176 nm
Other location
0.108 nm
0.161 nm
Top AlGaN: 50 nm
Undoped
AlGaN/GaN structure
Step flow structure
RMS: 0.176 nm
Surface morphology: Al=0.3
undoped
AlGaN/GaN structure
Step flow structure
RMS: 0.096 nm
Contact mode
Modulation-doped
AlGaN/GaN structure
Step flow structure
RMS: 0.131 nm
Contact mode
Discussion

Surface morphology
 Step-like
structure
 Surface roughness ~ 0.15 nm
 Very smooth surface, indicating good
crystal quality
 Comparable to previous reports
Step like
Hall data: Al composition
7000
1.6
Al=0.17
Al=0.3
-2
cm )
1.4
1.2
5000
mobility (cm /Vs)
13
1.0
2
electron concentration (10
Al=0.17
Al=0.3
6000
0.8
0.6
4000
3000
2000
0.4
1000
0.2
0
0.0
100
200
300
Temperature (K)
400
500
100
200
300
Temperature (K)
400
500
Hall data: AlGaN thickness
7000
Al=0.17
top AlGAN: 18 nm
top AlGAN: 50 nm
-2
electron concentration (10 cm )
6
5
Al=0.17
top AlGaN: 18 nm
top AlGaN: 53 nm
6000
2
monility (cm /Vs)
12
5000
4
3
4000
3000
2000
1000
2
100
200
300
400
Temperature (K)
Strain relaxation ??
500
100
200
300
Temperature(K)
400
500
Hall data: Al=0.3, structure
1.40E+013
Al=0.3
undoped
modulation-doped
2500
mobility (cm /Vs)
-2
electron concentration (cm )
1.30E+013
3000
Al=0.3
modulation-doped
undoped
2000
2
1.20E+013
1.10E+013
1500
1000
1.00E+013
500
9.00E+012
100
200
300
400
500
Temperature (K)
Thermal activation of Si donors
100
200
300
Temperature (K)
400
500
Discussion

Higher Al composition


Larger AlGaN thickness


Higher ns, lower mobility
Higher ns, lower mobility
Ns: 2DEG formation mechanism
Spontaneous polarization and piezoelectric effect
 Strain relaxation
 Thermal activation (modulation-doped structure)


Mobility: scattering mechanism
Phonon scattering dominates at high T
 Interface roughness scattering dominates at low T

Carrier profile: Al=0.3
8.00E+019
1.50E+019
i-AlGaN
i-GaN
i-GaN
AlGaN:Si
carrier distribution (cm )
-3
-3
carrier distribution (cm )
6.00E+019
4.00E+019
2.00E+019
0.00E+000
10
15
20
25
30
35
depth (nm)
Undoped
40
45
50
1.00E+019
5.00E+018
0.00E+000
10
15
20
25
30
35
40
45
depth (nm)
Modulation-doped
50
0.15 um AlGaN/GaN HFETs
DC characteristics
60
60
undoped_RT
Vg,top=2 V
step=-1 V
50
140
undoped_RT
Vds= 5 V
50
120
100
40
Id (mA)
Id (mA)
80
30
20
30
60
20
40
10
10
0
20
0
0
2
4
6
Vds (V)
0.15x75
Al=0.3
undoped
8
10
0
-8
-6
-4
-2
0
2
Vgs (V)
-Good dc performance
-Vt ~ -7
-wide gm profile over Vg, good linearity
gm (mS/mm)
40
Schottky I-V
1E-4
0.10
0.00000
1E-5
0.08
-0.00001
1E-6
1E-7
-0.00003
0.06
1E-8
0.04
1E-9
-0.00004
0.15x75
Vbk>100V
-0.00005
-120
-100
-80
-60
Vg (V)
0.15x75
Al=0.3
undoped
-40
-20
0
1E-10
0.02
undoped
1E-11
1E-12
0.00
0.0
0.5
1.0
Vg (V)
1.5
2.0
Ig (mA)
Ig (A)
Ig (A)
-0.00002
Small-signal characteristics
30
0.15x75
Al=0.3
Undoped
fT
fmax
undoped
25
fT, fmax (dB)
20
Vgs: -3.5
Vds: 6
15
10
fT
fmax
5
0
1
10
frequency (GHz)
100
DC characteristics
90
70
M-doped
Vg,top=0.5V
step=-1.5V
70
60
M-doped
Vds=4V
180
160
50
140
120
40
50
Id (mA)
ID (mA)
60
40
100
30
80
30
60
20
20
40
10
10
20
0
0
2
4
6
VDS (V)
0.15x75
Al=0.3
Modulation-doped
8
10
0
-12
0
-10
-8
-6
-4
-2
0
Vgs (V)
-Good dc performance
-Vt ~ -9
-narrow gm profile over Vg
2
gm (mS/mm)
80
200
Schottky I-V
10
0
25
Vbk:53.4 V
RT
M-doped
20
-10
-20
15
Ig (mA)
Ig (mA)
-30
-40
-50
10
-60
5
-70
-80
0
-120
-100
-80
-60
-40
Vg (V)
0.15x75
Al=0.3
Modulation-doped
-20
0
0.0
0.5
1.0
Vg (V)
1.5
2.0
Small-signal characteristics
35
0.15x75
Al=0.3
M-doped
fT
fmax
M-doped
30
25
fT, fmax (dB)
fT
20
Vgs: -6
Vds: 6
15
fmax
10
5
0
1
10
frequency (GHz)
100
Summary



Surface morphology
 Step-like structure
 Surface roughness ~ 0.15 nm, indicating that very smooth surface
and good crystal quality
Electron transport properties
 For undoped structure, due to the strong spontaneous and
piezoelectric polarization, high 2DEG density obtained, ~1e13 cm-2
 Additional doping, modulation doping or channel doping, is not
necessary
Device characteristics of AlGaN/GaN HFETs
 Very large output drain current available, the undoped (~700 mA/mm)
and modulation-doped structures (~1000 mA/mm)
 High breakdown voltage
 High operation frequency
Thermal effect of AlGaN/GaN HFETs


For microwave high-power devices, the stability of
device over temperature is extremely important
The thermal conductivity of substrate
Sapphire (0.5 W/cm·K), SiC (4.5 W/cm·K)
 Self-heating effect


Device structure
Undoped structure
 Modulation-doped structure
 Channel-doped structure
 Exhibit different electrical behavior at high temperature
due to their different transport properties

Our work



Compare undoped and modulation-doped
AlGaN/GaN HFETs, Al=0.3
Temperature-dependent electron transport
properties
Device high temperature performance
Electron transport properties v.s. T
1.40E+013
Al=0.3
undoped
modulation-doped
2500
mobility (cm /Vs)
-2
Electron concentration (cm )
1.30E+013
3000
Al=0.3
modulation-doped
undoped
2000
2
1.20E+013
1.10E+013
1500
1000
1.00E+013
500
9.00E+012
100
200
300
400
500
Temperature (K)
Thermal activation of Si donors
100
200
300
Temperature (K)
400
500
Calculation of Ed
Nc
Ed
n( N A  n)

exp( )
N D  N A  n gd
kT
--- (1) (charge neutrality condition)
N s (T )  N s (100K )
n
d AlGaN
--- (2)
Charge neutrality condition: give more accurate Ed
n: electron concentration
NA: acceptor concentration
ND: donor concentration
Nc: effective density of state in conduction band
gd: donor spin-degeneracy factor
Ed: activation energy
dAlGaN: effective AlGaN thickness
(exp. data, eq (2))
(NA<< ND)
(ND=5e18)
(~T3/2)
(gd=2)
(fit parameter)
(fit parameter)
Thermal activation energy
1E18
fit
exp.data
Ed= 83.2 meV
1E16
2
n /(ND-n)
1E17
1E15
1E14
500 400
300
200
Temperature (K)
100
Si donor in GaN, AlGaN



Si level in GaN:
 Ed~20 meV (for n=1e17 cm-3)
Si level in AlGaN:
 Al composition and Si doping concentration dependent
Si level in Al0.3Ga0.7N
Ed (meV)
Growth
1997, MSE-B
110
MBE
1998, SSE
40
MOCVD
2000, PRB
100
MBE
2002, MSE-B
40
MBE
2002, APL
50
Calculation
DC characteristics v.s. T
140
60
undoped
RT
o
100 C
o
200 C
Vds=5 V
40
Id (mA)
Id (mA)
40
30
120
100
80
30
60
20
20
gm (mS/mm)
50
undoped
RT
o
200 C
50
40
10
10
0
20
0
0
2
4
6
Vds (V)
0.15x75
Al=0.3
undoped
8
10
0
-8
-6
-4
-2
0
2
Vgs (V)
-Good dc performance from RT to 200°C
-Id reduction due to 2DEG mobility degradation
-Vt ~ -7, const over temperature, stable gate
-wide gm profile over Vg, good linearity
Schottky I-V
1E-4
0.08
1E-5
1E-6
0.06
1E-8
0.04
1E-9
1E-10
RT
o
200 C
undoped
1E-11
1E-12
0.00
0.0
0.15x75
Al=0.3
undoped
0.02
0.5
1.0
1.5
2.0
Vg (V)
Slight increase in Ig, stable Schottky gate
High Rg
Ig (mA)
Ig (A)
1E-7
DC characteristics V.s. T
80
modulation-doped
RT
o
200 C
70
70
60
60
200
50
Id (mA)
50
Id (mA)
250
modulation-doped
RT
o
100 C
o
200 C
Vds=5 V
40
30
150
40
100
30
20
20
10
10
gm (mS/mm)
80
50
0
0
0
2
4
6
Vds (V)
0.15x75
Al=0.3
Modulation-doped
8
10
0
-10
-8
-6
-4
-2
0
2
VgS (V)
-Good dc performance from RT to 200°C
-Id reduction due to 2DEG mobility degradation
-Vt ~ -9, const over temperature, stable gate
-narrower gm profile over Vg
Schottky I-V
0.1
25
0.01
RT
o
200 C
M-doped
1E-3
1E-4
20
1E-5
Ig (mA)
1E-7
1E-8
10
Ig (mA)
15
1E-6
1E-9
1E-10
5
1E-11
1E-12
1E-13
0
0.0
0.5
1.0
1.5
2.0
Vg (V)
0.15x75
Modulation-doped
Slight increase in Ig, stable Schottky gate
Lower Rg than undoped
Comparison: dc
220
1100
undoped
modulation-doped
1000
undoped
modulation-doped
200
180
gm, max (mS/mm)
Id, max (mA/mm)
900
800
700
160
140
120
100
600
80
500
0
50
100
150
o
Temperature ( C)
Larger Id in M-doped structure
-additional modulation doping
200
0
50
100
150
200
o
Temperature ( C)
Larger gm in M-doped structure
-smaller parasitic Rs
-Rs at RT(undoped/M-doped): 3.4/2.6 Ωmm
Comparison: small-signal
100
undoped
modulation-doped
90
80
fT (GHz)
70
60
No obvious degradation
observed as T< 100ºC
-weak temperature
dependence of the electron
transport property
50
higher fT for M-doped
- smaller parasitic Rs
40
30
20
10
0
50
100
150
o
Temperature ( C)
200
Comparison
Ns
Mobility
undoped
M-doped
constant
increase with T
comparable at high T for both
Id (T)
gm (T)
gm profile
Rs (T)
Rg (T)
fT (T)
lower
lower
wider
higher
higher
lower
higher
higher
narrower
lower
lower
higher
Modulation-doped structure: better performance over temperatures
Conclusion

ICP etching of GaN


Smooth etched surface and vertical sidewall profile
obtained
Low resistance Ohmic contacts to n-GaN
Plasma-treated Ohmic contacts exhibit low Rc and
excellent thermal stability
 Even lower Rc obtained using forming gas ambient


Narrow T-gate fabrication
40 nm narrow T-gate was successfully fabricated using
a lower accelerating voltage, 15 kV
 A specially designed writing pattern

Conclusion

Polarization effect
Design different structures
 Electron transport properties: high 2DEG concentration
 Device characteristics: high output current
 Polarization effect plays a crucial role


Thermal effect
In addition to the substrate, device structure plays a
significant role
 Compared undoped and modulation-doped structure
 Electron transport properties: thermal activation of Si donors
 Device high temperature performance: modulation-doped
devices exhibit better performance

Comparison: GaN HFETs on sapphire
2DEG Ns
2DEG µ
Lg
(mm)
Id, max
Gm,ext
(mA/mm) (mS/mm)
fT
(GHz)
fmax
(GHz)
2002
EDL
1.3E13
1330
0.18
920
212
101
140
2002
EL
1.2E13
1200
0.25
1400
401
85
151
2001
IEDM
1.2E13
1200
0.15
recess
1310
402
107
148
2001
EL
1.5E13
1170
0.25
1390
216
67
136
Our best
result
1.23E13
953
0.15
1060
200
75
90
Comparison: GaN HFETs on SiC
2DEG Ns
2DEG µ
Lg
(mm)
Id, max
Gm,ext
(mA/mm) (mS/mm)
fT
(GHz)
fmax
(GHz)
2003
EL
1.61E13
993
0.13
1250
250
103
170
2002
EDL
1.1E13
1300
0.12
1230
314
121
162
2001
ED
1.2E13
1200
0.12
1190
217
101
155
2000
EL
1.1E13
1100
0.05
1200
110
140
Our best
result
1.23E13
953
0.15
1060
75
90
200