Ting-Chi Lee OES, ITRI 11/07/2005 GaN-based Heterostructure Field-Effect Transistors.

Ting-Chi LeeOES, ITRI11/07/2005

GaN-based Heterostructure Field-Effect Transistors

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

0.1

1

10

100

1000

10 100

HBT

SiC

pHEMT

GaN

Frequency (GHz)

The Wide Band Gap Device Advantages

GaN HEMT and Process

Suitable Specifications for GaN-based Power Devices

Ron of GaN HEMT Switches

• 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

GaN-based devices for various applications

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

Wet etching

20 um

Rough edgePoor dimension control

Lift-off

20 um

Smooth edgeGood dimension control

Ni

Ni

PR

Dry etching parameters: bias power

0 5 10 15 20 25 30

0

500

1000

1500

2000

2500

3000

3500

Etc

hing

rat

e (A

/min

.)

Bias power (W)

ICP 300WPressure 15mtorrCl 50sccmAr 5 sccm

Larger bias power-Increase the kinetic energy of incident ions-Enhance physical ion bombardment

-More efficient bond breaking and desorption of etched products

Dry etching parameters: bias power

Bias power: 5 w

Bias power: 30 w

Bias power: 10 w

Bias power: 20 w

Ni: 2000 ÅGaN: 2 um

Dry etching parameters: Ar flow rate

Higher Ar flow rate-Increase the density of incident Ar ions-Enhance physical ion bombardment

Ar flow rate> 15 sccm-Cl2/Ar flow ratio decrease

0 5 10 15 20 25 303300

3320

3340

3360

3380

3400

3420

3440

etc

h r

ate

(A

/min

)

Ar flow rate (sccm)

Samco ICPICP 300wBias 30wPressure 15mTorrCl 50sccm

Ar flow rate: 5 sccm

Ar flow rate: 25 sccm

Ar flow rate: 15 sccm

Ar flow rate: 20 sccm

Dry etching parameters: Ar flow rate

Ni: 2000 ÅGaN: 2 um

Dry etching parameters: Cl2 flow rate

Higher Cl flow rate-Generate more reactive Cl radicals to participate in the surface chemical reaction

10 20 30 40 502600

2800

3000

3200

3400

3600

Etc

hin

g r

ate

(A

/min

.)

Cl2 flow rate (sccm)

ICP power: 300 WBias power: 30 Wpressure: 15 mtorrAr flow: 5 sccm

Cl flow rate: 10 sccm

Cl flow rate: 50 sccm

Cl flow rate: 20 sccm

Cl flow rate: 30 sccm

Dry etching parameters: Cl2 flow rate

Ni: 2000 ÅGaN: 2 um

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 n-GaN with Nd=8.7x1016, 3.3x1018 cm-3

Cl2/Ar and Ar plasma

Thermal stability issue

Forming gas ambient treatment

Plasma treatment

n-GaN

sapphire

n-GaN

sapphire

n+-GaN

Plasma treatment -> create N vacancies (native donors) -> increase surface electron concentration

Cl2/Ar or Ar plasma

Plasma treatment: Cl2/Ar, ArND=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.)

------

3005

1550301

3005

1550102

3005

1550152

3005

1550202

3005

1550302

3005

15-

101

3005

15-

301

3005

15-

501

Rc (Ω mm)‧ρs (Ω/□)

ρc (Ω cm‧ 2)

0.638621.0

6.6

0.614656.3

5.7

0.48692.2

3.4

0.45696.3

2.8

0.21668.30.68

0.28671.5

1.2

0.8767311

0.57649.3

5.0

0.38030.87

Plasma treatment: ArND=3.3x1018 cm-3

ICP power (W)Bias power (W)Pressure (mTorr)Ar flow (sccm)Time (min.)

-----

3005

15101

3005

15301

3005

15501

3005

15502

3005

15503

Plasma treatment: Ar flow rate Before annealing

-3 -2 -1 0 1 2 3-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Cur

rent

(A

)

Voltage (V)

before annealing No treatment Ar=10 sccm Ar=30 sccm Ar=50 sccm

0 10 20 30 40 500.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Co

nta

ct r

esi

sta

nce

(-

mm

)

Ar flow (sccm)

Pure Ar, 1min. R

C

1E-5

1E-4

1E-3

Sp

eci

fic c

on

tact

re

sist

an

ce (-

cm2 )

C

Plasma treatment: Ar flow rate After annealing

-1.0 -0.5 0.0 0.5 1.0-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Cur

rent

(A

)

Voltage (V)

After annealing No treatment Ar=10 sccm Ar=30 sccm Ar=50 sccm

0 10 20 30 40 500.08

0.10

0.12

0.14

0.16

0.18

0.20Pure Ar,1min.

RC

Co

nta

ct r

esi

sta

nce

(-

mm

)

Ar flow (sccm)

1E-6

1E-5

C

no

tre

atm

en

t

Sp

eci

fic c

on

tact

re

sist

an

ce (-

cm2 )

Plasma treatment: time

-3 -2 -1 0 1 2 3

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Cur

rent

(A

)

Voltage (V)

Before annealing No treatment Ar=50, 1min. Ar=50, 2min. Ar=50, 3min.

-1.0 -0.5 0.0 0.5 1.0-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Cur

rent

(A

)

Voltage (V)

After annealing No treatment Ar=50, 1min. Ar=50, 2min. Ar=50, 3min.

Plasma treatment: time

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.10

0.12

0.14

0.16

0.18

0.20

Pure Ar,50sccm R

Cn

o tr

ea

tme

nt

Co

nta

ct r

esi

sta

nce

(-

mm

)

Etching time (min.)

1E-6

1E-5

C

Sp

eci

fic c

on

tact

re

sist

an

ce(

-cm

2 )

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

Thermal aging tests: N2 ambient

0 50 300 400 500 600

1E-6

1E-5

1E-4

Spe

cific

con

tact

res

ista

nce

(-c

m2 )

Temperature (0 C)

N2 ambient

No treatment Cl

2/Ar=50/20, 2min.

Cl2/Ar=50/30, 1min.

0 50 300 400 500 600

1E-6

1E-5

1E-4

Sp

eci

fic c

on

tact

re

sist

an

ce (-

cm2 )

Temperature (0 C)

N2 ambient

No treatment Ar=10 sccm, 1 min. Ar=50 sccm, 1 min.

Thermal aging tests: Air ambient

0 50 300 400 500 600

1E-6

1E-5

1E-4

S

pe

cific

con

tact

res

ista

nce

(-

cm2 )

Temperature (0 C)

Air ambient No treatment Cl

2/Ar=50/30, 2 min.

Ar=30sccm, 1 min.

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

-1.0 -0.5 0.0 0.5 1.0-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Cur

rent

(A

)

Voltage (V)

Forming gas ambient No treatment Ar=10 sccm Ar=30 sccm Ar=50 sccm

0 10 20 30 40 501E-6

1E-5

no

tre

atm

en

t

Sp

eci

fic c

on

tact

re

sist

an

ce (-

cm2 )

Ar flow (sccm)

Annealing ambient N

2 gas

Forming gas

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

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

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 exposureSide stripe (75 nm): head exposureSpacing between the central stripe and the side stripe: key point

Foot width v.s. central dose

140 150 160 170 180 190 200

50

100

150

200

F

oo

t w

idth

(n

m)

Central dose (C/cm2)

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

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

High Al composition AlGaN/GaN heterostructures

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 PIMBEN-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

Buffer layer

Sapphire

Undoped Undoped

i-AlGaN 50 nm(Al=0.17)

i-GaN 3 µm

Buffer layer

Sapphire

Structure: Al=0.3

i-AlGaN 28 nm(Al=0.3)

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

Surface morphology: Al=0.17Top AlGaN: 18 nmUndoped AlGaN/GaN structure

Step flow structureRMS: 0.176 nm

Other location0.108 nm0.161 nm

Top AlGaN: 50 nmUndoped AlGaN/GaN structure

Step flow structureRMS: 0.176 nm

undoped AlGaN/GaN structure

Step flow structureRMS: 0.096 nmContact mode

Surface morphology: Al=0.3

Modulation-doped AlGaN/GaN structure

Step flow structureRMS: 0.131 nmContact 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

100 200 300 400 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

ele

ctro

n c

on

cen

tra

tion

(1

013 c

m-2)

Temperature (K)

Al=0.17 Al=0.3

100 200 300 400 5000

1000

2000

3000

4000

5000

6000

7000

mo

bili

ty (

cm2 /V

s)

Temperature (K)

Al=0.17 Al=0.3

Hall data: AlGaN thickness

100 200 300 400 5002

3

4

5

6

ele

ctro

n c

on

cen

tra

tion

(1

012

cm

-2)

Temperature (K)

Al=0.17 top AlGAN: 18 nm top AlGAN: 50 nm

100 200 300 400 500

1000

2000

3000

4000

5000

6000

7000

mo

nili

ty (

cm2 /V

s)

Temperature(K)

Al=0.17 top AlGaN: 18 nm top AlGaN: 53 nm

Strain relaxation ??

Hall data: Al=0.3, structure

100 200 300 400 5009.00E+012

1.00E+013

1.10E+013

1.20E+013

1.30E+013

1.40E+013

ele

ctro

n c

on

cen

tra

tion

(cm

-2)

Temperature (K)

Al=0.3 modulation-doped undoped

100 200 300 400 500

500

1000

1500

2000

2500

3000

mo

bili

ty (

cm2 /V

s)

Temperature (K)

Al=0.3 undoped modulation-doped

Thermal activation of Si donors

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

Carrier profile: Al=0.3

10 15 20 25 30 35 40 45 500.00E+000

2.00E+019

4.00E+019

6.00E+019

8.00E+019

i-GaNi-AlGaN

carr

ier d

istri

butio

n (c

m-3

)

depth (nm)

10 15 20 25 30 35 40 45 500.00E+000

5.00E+018

1.00E+019

1.50E+019

i-GaNAlGaN:Si

carr

ier d

istri

butio

n (c

m-3

)

depth (nm)

Undoped Modulation-doped

0.15 um AlGaN/GaN HFETs

DC characteristics

0 2 4 6 8 100

10

20

30

40

50

60

I d (m

A)

Vds

(V)

undoped_RTVg,top=2 Vstep=-1 V

-8 -6 -4 -2 0 20

10

20

30

40

50

60

I d (m

A)

Vgs

(V)

undoped_RTV

ds= 5 V

0

20

40

60

80

100

120

140

gm (

mS

/mm

)

0.15x75Al=0.3undoped

-Good dc performance-Vt ~ -7-wide gm profile over Vg, good linearity

Schottky I-V

-120 -100 -80 -60 -40 -20 0

-0.00005

-0.00004

-0.00003

-0.00002

-0.00001

0.00000

Ig (

A)

Vg (V)

0.15x75 Vbk>100V

0.0 0.5 1.0 1.5 2.01E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

Ig (

A)

Vg (V)

undoped

0.00

0.02

0.04

0.06

0.08

0.10

Ig (

mA

)

0.15x75Al=0.3undoped

Small-signal characteristics

1 10 1000

5

10

15

20

25

30

fmax

fT

f T, f

max

(d

B)

frequency (GHz)

fT

fmax

undoped

0.15x75Al=0.3Undoped

Vgs: -3.5Vds: 6

DC characteristics

-12 -10 -8 -6 -4 -2 0 20

10

20

30

40

50

60

70

I d (m

A)

Vgs

(V)

M-dopedVds=4V

0

20

40

60

80

100

120

140

160

180

200

gm (

mS

/mm

)

0.15x75Al=0.3Modulation-doped

0 2 4 6 8 100

10

20

30

40

50

60

70

80

90

ID (

mA

)

VDS (V)

M-dopedVg,top=0.5Vstep=-1.5V

-Good dc performance-Vt ~ -9-narrow gm profile over Vg

Schottky I-V

-120 -100 -80 -60 -40 -20 0

-80

-70

-60

-50

-40

-30

-20

-10

0

10

Ig (A

)

Vg (V)

Vbk:53.4 V

0.0 0.5 1.0 1.5 2.00

5

10

15

20

25

Ig (

mA

)

Vg (V)

RTM-doped

0.15x75Al=0.3Modulation-doped

Small-signal characteristics

1 10 1000

5

10

15

20

25

30

35

fmax

fT

f T, f

ma

x (d

B)

frequency (GHz)

fT

fmax

M-doped

0.15x75Al=0.3M-doped

Vgs: -6Vds: 6

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

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

Thermal effect of AlGaN/GaN HFETs

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

100 200 300 400 5009.00E+012

1.00E+013

1.10E+013

1.20E+013

1.30E+013

1.40E+013

Ele

ctro

n c

on

cen

tra

tion

(cm

-2)

Temperature (K)

Al=0.3 modulation-doped undoped

100 200 300 400 500

500

1000

1500

2000

2500

3000

mo

bili

ty (

cm2 /V

s)

Temperature (K)

Al=0.3 undoped modulation-doped

Thermal activation of Si donors

Charge neutrality condition: give more accurate Ed

n: electron concentration (exp. data, eq (2))NA: acceptor concentration (NA<< ND)ND: donor concentration (ND=5e18)Nc: effective density of state in conduction band (~T3/2)gd: donor spin-degeneracy factor (gd=2)Ed: activation energy (fit parameter)dAlGaN: effective AlGaN thickness (fit parameter)

Calculation of Ed

)exp()(

kT

E

g

N

nNN

nNn d

d

c

AD

A

--- (2)

--- (1) (charge neutrality condition)

AlGaN

ss

d

KNTNn

)100()(

Thermal activation energy

500 400 300 200 1001E14

1E15

1E16

1E17

1E18

n2 /(N

D-n

)

Temperature (K)

fit exp.data

Ed= 83.2 meV

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

0 2 4 6 8 100

10

20

30

40

50

60

I d (m

A)

Vds

(V)

undoped RT

200oC

-8 -6 -4 -2 0 20

10

20

30

40

50

I d (m

A)

Vgs

(V)

undoped RT 100oC 200oC

Vds

=5 V

0

20

40

60

80

100

120

140

gm (m

S/m

m)

0.15x75Al=0.3undoped

-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

0.0 0.5 1.0 1.5 2.01E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

Ig (

A)

Vg (V)

RT

200oCundoped

0.00

0.02

0.04

0.06

0.08

Ig (

mA

)

0.15x75Al=0.3undoped

Slight increase in Ig, stable Schottky gateHigh Rg

0 2 4 6 8 100

10

20

30

40

50

60

70

80

I d (m

A)

Vds

(V)

modulation-doped RT

200oC

-10 -8 -6 -4 -2 0 20

10

20

30

40

50

60

70

80

I d (m

A)

VgS

(V)

modulation-doped RT

100oC

200oCV

ds=5 V

0

50

100

150

200

250

gm (

mS

/mm

)

0.15x75Al=0.3Modulation-doped

DC characteristics V.s. T

-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

0.0 0.5 1.0 1.5 2.01E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

Ig

(m

A)

Vg (V)

RT

200oCM-doped

0

5

10

15

20

25

Ig

(m

A)

0.15x75Modulation-doped

Slight increase in Ig, stable Schottky gateLower Rg than undoped

Comparison: dc

0 50 100 150 200500

600

700

800

900

1000

1100

I d, m

ax (

mA

/mm

)

Temperature (oC)

undoped modulation-doped

0 50 100 150 200

80

100

120

140

160

180

200

220

g m, m

ax (

mS

/mm

)

Temperature (oC)

undoped modulation-doped

Larger Id in M-doped structure-additional modulation doping

Larger gm in M-doped structure-smaller parasitic Rs-Rs at RT(undoped/M-doped): 3.4/2.6 Ωmm

Comparison: small-signal

0 50 100 150 20010

20

30

40

50

60

70

80

90

100

f T (

GH

z)

Temperature (oC)

undoped modulation-doped

No obvious degradation observed as T< 100ºC-weak temperature dependence of the electron transport property

higher fT for M-doped - smaller parasitic Rs

Comparison undoped M-doped

Ns constant increase with TMobility comparable at high T for both

Id (T) lower higher

gm (T) lower higher

gm profilewider narrower

Rs (T) higher lower

Rg (T) higher lower

fT (T) 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

(m)

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

Comparison: GaN HFETs on SiC 2DEG Ns

2DEG µ

Lg

(m)

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