Ting-Chi Lee

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GaN-based Heterostructure Field-Effect Transistors

• Ting-Chi Lee
• OES, ITRI
• 11/07/2005

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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

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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

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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

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Device Power Performance vs. Frequency
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The Wide Band Gap Device Advantages
GaN HEMT and Process
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Suitable Specifications for GaN-based Power
Devices
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Ron of GaN HEMT Switches
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GaN-based devices for various applications

• For high-power switching applications
• ? GaN schottky diodes
• ? GaN p-i-n diodes
• ? GaN HEMT-based switching devices
• ? GaN 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

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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

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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

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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)

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Our work

• ICP etching
• Ni mask fabrication
• Dry etching parameters

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Ni mask fabrication

• Suitable etching mask for ICP etching of GaN
• PR, Ni and SiO2
• Ni mask fabrication
• Wet chemical etching by HNO3 H2O (11)
• Lift-off

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Ni mask fabrication
PR
Ni
20 um
Wet etching
Rough edge Poor dimension control
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Dry etching parameters bias power
Larger bias power -Increase the kinetic energy of
incident ions -Enhance physical ion
bombardment -More efficient bond breaking and
desorption of etched products
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Dry etching parameters bias power
Ni 2000 Å GaN 2 um
Bias power 5 w
Bias power 10 w
Bias power 30 w
Bias power 20 w
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Dry etching parameters Ar flow rate
Higher Ar flow rate -Increase the density of
incident Ar ions -Enhance physical ion
bombardment Ar flow rategt 15 sccm -Cl2/Ar flow
ratio decrease
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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 25 sccm
Ar flow rate 20 sccm
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Dry etching parameters Cl2 flow rate
Higher Cl flow rate -Generate more reactive Cl
radicals to participate in the surface chemical
reaction
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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 50 sccm
Cl flow rate 30 sccm
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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

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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 (13),
• Plasma treatment Cl2/Ar, Cl2, Ar,

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Our work

• Plasma treatment
• n-GaN with Nd8.7x1016, 3.3x1018 cm-3
• Cl2/Ar and Ar plasma
• Thermal stability issue
• Forming gas ambient treatment

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Plasma treatment
n-GaN
sapphire
Plasma treatment -gt create N vacancies (native
donors) -gt increase surface electron
concentration
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Plasma treatment Cl2/Ar, ArND8.7x1016 cm-3
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Plasma treatment ArND3.3x1018 cm-3
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Plasma treatment Ar flow rate Before annealing
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Plasma treatment Ar flow rate After annealing
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Plasma treatment time
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Plasma treatment time
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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

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Thermal aging tests N2 ambient
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Thermal aging tests Air ambient
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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

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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 ??

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Forming gas ambient treatment
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Summary

• Proper plasma treatment by Cl2/Ar or Ar
• Very effective in the improvement of contact
  resistance
• Thermal stability issue
• Plasma-treated Ohmic contacts to n-GaN exhibited
  excellent thermal stability
• Forming gas ambient treatment
• No electrical degradation observed
• Even lower contact resistance obtained

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Narrow T-gate fabrication on GaN

• To realize high performance devices especially
  for high-frequency 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

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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

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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

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Bi-layer PMMA/Copolymer process
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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
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Foot width v.s. central dose
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40 nm Narrow T-gate
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Discussion

• As the spacing between the central stripe and the
  side stripeltlt 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

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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
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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

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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

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High performance GaN HFETs

• In addition to develop device processing
  technologies
• Design rules
• High sheet charge density
• High carrier mobility
• Maintain high breakdown voltage
• -gt high Al composition AlGaN/GaN heterostructures

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High Al composition AlGaN/GaN heterostructures

• Higher band discontinuity
• Better carrier confinement
• Al0.3, ?Ec0.5 eV
• Higher spontaneous polarization and piezoelectric
  effect
• Higher 2DEG sheet charge density
• Higher bandgap of AlGaN
• higher breakdown field

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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

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Polarization, polarization-induced sheet charge
and formation of 2DEG
Ga-face
N-face
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Comparison of calculated and measured 2DEG ns
AlGaN 200Å, /? undoped/doped
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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
• -gt decrease in electron mobility with high Al
  composition

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Our work

• Design AlGaN/GaN heterostructures with different
  Al compositions, different AlGaN thickness and
  modulation-doping
• Surface morphology
• Electron transport properties
• Device characteristics

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Structure Al0.17
i-AlGaN 18 nm (Al0.17)
i-GaN 3 µm
Buffer layer
Sapphire
i-AlGaN 50 nm (Al0.17)
i-GaN 3 µm
Buffer layer
Sapphire
Undoped
Undoped
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Structure Al0.3
i-AlGaN 28 nm (Al0.3)
i-GaN 3 µm
Buffer layer
Sapphire
i-AlGaN 5 nm
n-AlGaN 5E18 20 nm
i-AlGaN 3 nm
i-GaN 3 µm
Buffer layer
Sapphire
Undoped
Modulation-doped
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Surface morphology Al0.17
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
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Surface morphology Al0.3
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
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Discussion

• Surface morphology
• Step-like structure
• Surface roughness 0.15 nm
• Very smooth surface, indicating good crystal
  quality
• Comparable to previous reports

Step like
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Hall data Al composition
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Hall data AlGaN thickness
Strain relaxation ??
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Hall data Al0.3, structure
Thermal activation of Si donors
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Discussion

• Higher Al composition
• Higher ns, lower mobility
• Larger AlGaN thickness
• 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

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Carrier profile Al0.3
Undoped
Modulation-doped
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0.15 um AlGaN/GaN HFETs
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DC characteristics
-Good dc performance -Vt -7 -wide gm profile
over Vg, good linearity
0.15x75 Al0.3 undoped
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Schottky I-V
0.15x75 Al0.3 undoped
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Small-signal characteristics
0.15x75 Al0.3 Undoped Vgs -3.5 Vds 6
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DC characteristics
0.15x75 Al0.3 Modulation-doped
-Good dc performance -Vt -9 -narrow gm profile
over Vg
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Schottky I-V
0.15x75 Al0.3 Modulation-doped
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Small-signal characteristics
0.15x75 Al0.3 M-doped Vgs -6 Vds 6
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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

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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/cmK), SiC (4.5 W/cmK)
• 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

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Our work

• Compare undoped and modulation-doped AlGaN/GaN
  HFETs, Al0.3
• Temperature-dependent electron transport
  properties
• Device high temperature performance

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Electron transport properties v.s. T
Thermal activation of Si donors
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Calculation of Ed
--- (1) (charge neutrality condition)
--- (2)
Charge neutrality condition give more accurate
Ed n electron concentration (exp. data, eq
(2)) NA acceptor concentration (NAltlt ND) ND
donor concentration (ND5e18) Nc effective
density of state in conduction band
(T3/2) gd donor spin-degeneracy factor
(gd2) Ed activation energy (fit
parameter) dAlGaN effective AlGaN
thickness (fit parameter)
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Thermal activation energy
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Si donor in GaN, AlGaN

• Si level in GaN
• Ed20 meV (for n1e17 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
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DC characteristics v.s. T
-Good dc performance from RT to 200C -Id
reduction due to 2DEG mobility degradation -Vt
-7, const over temperature, stable gate -wide gm
profile over Vg, good linearity
0.15x75 Al0.3 undoped
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Schottky I-V
0.15x75 Al0.3 undoped
Slight increase in Ig, stable Schottky gate High
Rg
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DC characteristics V.s. T
-Good dc performance from RT to 200C -Id
reduction due to 2DEG mobility degradation -Vt
-9, const over temperature, stable gate -narrower
gm profile over Vg
0.15x75 Al0.3 Modulation-doped
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Schottky I-V
0.15x75 Modulation-doped
Slight increase in Ig, stable Schottky gate Lower
Rg than undoped
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Comparison dc
Larger gm in M-doped structure -smaller parasitic
Rs -Rs at RT(undoped/M-doped) 3.4/2.6 Omm
Larger Id in M-doped structure -additional
modulation doping
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Comparison small-signal
No obvious degradation observed as Tlt 100ºC -weak
temperature dependence of the electron
transport property higher fT for M-doped -
smaller parasitic Rs
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Comparison

• undoped M-doped
• Ns constant increase with T
• Mobility comparable at high T for both
• Id (T) lower higher
• gm (T) lower higher
• gm profile wider narrower
• Rs (T) higher lower
• Rg (T) higher lower
• fT (T) lower higher

Modulation-doped structure better performance
over temperatures
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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

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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

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Comparison GaN HFETs on sapphire
2DEG Ns 2DEG µ Lg (mm) Id, max (mA/mm) Gm,ext (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
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Comparison GaN HFETs on SiC
2DEG Ns 2DEG µ Lg (mm) Id, max (mA/mm) Gm,ext (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 200 75 90