1

Design, Fabrication, and Characterization of GaN High Power

Rectifiers

Kwang H. Baik Materials Science and Engineering,

Univ. of Florida, Gainesville, FLNovember 2, 2004

Ph.D. Dissertation Defense

2

Outline

Motivation Theoretical Calculations

GaN Materials Parameters Intrinsic carrier concentration Breakdown Voltage (VB) On-state Resistance (RON) Forward Voltage Drop (VF) & Leakage Current (IR)

Device Modeling Breakdown Study with Edge Termination Techniques Electrical Characteristics of GaN Rectifiers

Experimental Data GaN High Voltage Diodes with Field Plate Termination High Power Schottky Diode Array (GaN & SiC)

3

Motivation

0

50

100

150

200

250

Si GaAs InP SiC GaN

Baliga's FOM

Johnson's FOM

Nor

mal

ized

fig

ure

of m

erit

The figure-of-merit for power microwave applications

Si GaAs GaN AlN 6H-SiC

Bandgap (eV) @300 ºC 1.1

indire

ct

1.4

direct

3.4

direct

6.2

direct

2.9

indirect

Electron mobility

(cm2/V·s), RT

1400 8500 1000 (bulk)

2000 (2D-gas)

135 600

Hole Mobility

(cm2/V·s), RT

600 400 30 14 40

Saturation velocity

(cm/s), 107

1 2 2.5 1.4 2

Breakdown field (V/cm),

106

0.3 0.4 >5 4

Thermal conductivity

(W/cm)

1.5 0.5 1.5 2 5

Melting temperature (K) 1690 1510 >1700 3000 >2100

High Temperature, High Power and High Frequency Applications

• Intrinsic wide bandgap energy• High breakdown field for power applications• Excellent electron transport properties• Heterostructure available and strong piezoelectric polarization effect

Johnson’s FOM (vsat EC)2/2

Baliga’s FOM (EC2)

4

High Power Rectifiers

SiC high power rectifier product

Current ratings of 1A to 20A at 600V, and 5A to 10A at 1200V

http://www.cree.com The applications of IGBT modules

- UPS Power Supply,Servo Drive, Medical Power Supply, Motor Drives, Inverters

http://www.pwrx.comObjectiveDevelop the technology base for GaN-based rectifiers at power levels above 1MW

5

GaN Material Parameters

Temperature dependence of bandgap energy of GaN and SiC. Ref. H. Teisseyre et al., 1994

)772772300

300(1039.9396.3)(

224

T

TeVEg

Temperature dependence of bandgap energy

0 100 200 300 400 500 600

3.10

3.15

3.20

3.25

3.30

3.35

3.40

3.45

3.50

3.55

6H-SiC

Eg = 3.396-4.5x10-4(T-300)

GaN

Ban

dgap

(eV

)

Temperature (K)

Density of states for GaN

GaN tefor wurzti 20.0 0* mme

GaN tefor wurzti 50.1 0* mmh

2

32

3

0

*19

2

3

2

*

3001050945.2

22)(

T

m

m

h

kTmTN ee

C

2

32

3

0

*19

2

3

2

*

3001050945.2

22)(

T

m

m

h

kTmTN hh

V

18103.2)300( CN

19106.4)300( VN

Incomplete ionization of impurity atoms

Sifor eV 016.0 DE

Mgfor eV 175.0 AE

kT

EEEg

NN

DCFnB

DD

exp1

kT

EEEg

NN

AFpVB

AA

exp1

6

Mobility & Recombination Models

Analytical Mobility Model

)()300

T(1

)300

T(

minmax

min

ref

tot

N

N

1E15 1E16 1E17 1E18 1E19 1E20

100

1000

Electron

Hole

Mob

ility

(cm

2 V-1s-1

)

Carrier concentration (cm-3)

Field-Dependent Mobility Model

ii

sat

n

nn

vE

E

/1

1

)(

600exp8.01

107.2 7

Tvsat

Shockley-Read-Hall Recombination

)()(

2

inip

iSRH npnn

nnpR

ns

SRHn

tot

nn

N

N

)(1

0

Auger Recombination

))(( 2inpAu npnnCpCR

7

Intrinsic Carrier Concentration

  

Intrinsic carrier concentration in SiC and GaN as a function of temperature. Ref. R. Kolessar et al., 2001.

)2

exp()(

ionconcentratcarrier intrinsic The

kT

ENNTn g

VCi

)20488

exp(1098.1)( 2

316

TTTni

GaN epitaxialfor 1025.2(300) 3-10i

cmn

The small intrinsic carrier concentration in GaN at room temperature enables the high power and temperature applications.

2.5 3.0 3.5 4.010-20

10-15

1x10-10

1x10-5

1x100

1x105

GaN

ni(T)=1.98x1016T(3/2)exp(-20488/T)

SiC

Intri

nsic

con

cent

ratio

n (c

m-3

)

Temperature (1000/K)

8

Impact Ionization Coefficient

2.5x10-7 3.0x10-7 3.5x10-7 4.0x10-7 4.5x10-7 5.0x10-7100

101

102

103

104

105

106

a = 4.55x10-35 E6

a = 9.1x10-43 E7GaN

6H-SiC

GaN electron & hole Power-law fit for GaN 6H-SiC electron 6H-SiC hole Power-law fit for 6H-SiC

Impa

ct io

niza

tion

rate

(cm

-1)

Inverse electric field (V/cm)-1

743eff 101.9 EAE n

10

dxCW

eff

Simplified breakdown condition

1])(exp[00

dxdxpn

xW

p

BD

)(cm )106.2

exp(1085.8 1-7

6, Epn

Impact ionization integral

2.5x10-7 3.0x10-7 3.5x10-7 4.0x10-7 4.5x10-7 5.0x10-7

101

102

103

104

105

Wurtzite GaN 300K

Electron-initiated i

Hole-initiated p

Hole-initiated n

Impa

ct io

niza

tion

rate

(cm

-1)

Inverse electric field (V/cm)-1

Impact ionization coefficient Fulop’s form (Power law expression)

9

Ec & VB

1E15 1E16 1E17 1E181x106

2x106

3x106

4x106

5x106

6x106

7x106

6H-SiC

GaN

Crit

ical

ele

ctric

fiel

d (V

/cm

)

Doping concentration (cm-3)

02

2

BqN

dx

dE

dx

Vd

7

14

6

1

7

1

0

1016.177

Bc

Bc N

Aw

qN

AE

1-D Poisson’s equation 8

7

08

1

8

Bc qNA

W

1E15 1E16 1E17 1E18101

102

103

104

6H-SiC

GaN

Bre

akdo

wn

volta

ge (V

)

Doping concentration (cm-3)

4

3151087.2 BB NV

100

dxEAdxCC W nW

eff

10

Breakdown Voltage

The calculated reverse breakdown voltage of punch-through diode as a function of doping concentration and standoff region thickness

where EC is critical electric field, WP drift region thicknessNA doping concentration, and permittivity

1E15 1E16 1E17

102

103

104

Non-punchthrough theoretical limit

GaN punchthrough diodeTheoretical breakdown voltage50 µm

30 µm

20 µm

10 µm

5 µm

3 µm

1 µm

Bre

akdo

wn

volta

ge (V

)

Doping concentration (cm-3)

0

2

2PTB

PTcPT

WqNWEBV

GaN punchthrough diode

n- n n+

3 µm GaN epi can give more than 900V of reverse breakdown voltage with the doping concentration of 1016 cm-3

11

On-state Resistance

2000 4000 6000 8000 10000

0.005

0.010

0.015

0.020

0.025

0.030

GaN

On-

stat

e re

sist

ance

(-

cm2 )

Reverse breakdown voltage (V)

B

D

Bd Nq

W

Nq

dxR

GaNfor 104.2 5.212 BVRON

On-state resistance (RON)

contsubdON RRRR

102 103 1041E-4

1E-3

0.01

0.1

1

GaN Schottky rectifiers AlGaN-UF

AlGaN-UF

AlGaN-UF

GaN-UF

GaN-UFGaN-UF

GaN-UF

GaN-UF

GaN-UT

GaN-Caltech

GaN

6H-SiC

Si

Spec

ific

on-s

tate

resi

stan

ce (-

cm2 )

Reverse breakdown voltage (V)

12

VF & IR

Forward Voltage Drop (VF)

102 103 104

1

2

3

4

5GaN-UF,'00GaN-UT,'00

GaN-CalTech,'99

eVeV

For

war

d V

olta

ge D

rop

(V)

Breakdown Voltage (V)

JF=100 A/cm2

300 K

eV

GaN-UF,'02

FONBF

F JRnΦTA

J

q

nkTV )ln(

2**

5.210 )(104.226.0 BVV BF

500 1000 1500 200010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

GaN-UF,'00

GaN-CalTech,'99

Theoretical reverse leakage currentGaN Schottky Rectifiers

Without Barrier Lowering

Rev

erse

Cur

rent

(A

/cm

2 )

Reverse Bias (V)

With Barrier Lowering

Reverse Leakage Current (IR)

)(exp2**

BBR kT

qTAJ

s

mB

qE

4 )(

2biR

s

Dm VV

qNE

13

Device Modeling Scheme

Bandgap (Eg)

Density of States

IncompleteIonization

Impact Ionizationcoefficients

Recombination Models

Mobility Models

Device designEdge terminationBreakdown analysisI-V characteristicsReverse recoveryThermal analysis

Medici

14

Edge Termination

Edge termination is critical for obtaining high breakdown voltage and reduced on-state resistance.

Severe electric field crowding around metal contact periphery.

High leakage current and breakdown at the highest electric field

Depletion contour

Potential contour

nDepletion region contour

Electrode Oxide

15

Field Plate Termination

nDepletion region contour

0.0 0.2 0.4 0.6 0.8 1.0 1.2

1.0

1.2

1.4

1.6

1.8

Dielectric egde termination10 µm metal overlap

Nor

mal

ized

bre

akdo

wn

volta

ge (

V)

Oxide thinkness (µm)

0 5 10 15 20 25 30

1.0

1.2

1.4

1.6

1.8

Dielectric egde termination0.7 µm oxide thickness

Nor

mal

ized

bre

akdo

wn

volta

ge (

V)

Metal overlap (µm)

Oxide breakdown up to 0.7 µm thick Metal contact corner breakdown more than 0.7 µm thick oxide No further improvement in VB beyond 10 µm overlap

16

Field Plate Termination

1.3

1.4

1.5

1.6

1.7

MgO Sc2O

3AlNSi3N

4SiO2

Nor

mal

ized

bre

akdo

wn

volt

age

(V)

Dielectric edge termination10 µm metal overlap0.7 µm dielectric film

Oxide Nitride AlN MgO Sc2O3

3.9 7.5 8.5 9.8 14

Eg (eV) 9 4.7 6.2 8 6.3

4 6 8 10 12

1.4

1.6

1.8

2.0

Nor

mal

ized

bre

akdo

wn

volta

ge (

V)

Ramp oxide angle (°)

Dielectric edge termination10 µm metal overlap1.0 µm ramp oxide

17

Guard Ring Termination

Junction spacing and doping should be optimized.

Maximum E-field should be induced at the outside of the junction.

0 1 2 3 4 5

600

700

800

900

1000

1100

1200

1300

Bre

akdo

wn

Vol

tage

(V

)

Guard ring Spacing (µm)

18

Junction Termination Extension

VB values are highly sensitive to the charge in the JTE layer.

Multiple JTE termination technique (JTE1 + JTE2).

3x1017 4x1017 5x1017

1800

2000

2200

2400

2600

2800

Rev

erse

bre

akdo

wn

volt

age

(V)

JTE doping concentration (cm-3)

p+

N

p-

JTE layer

Depletion boundary

19

Comparison of Edge Termination Methods

1 2 3 4 5

500

1000

1500

2000

2500

3000Single JTE

2 p-rings1 p-ring

Planar junction with the field plate

Planar Junction

Rev

erse

Bre

akdo

wn

Vol

tage

(V

)

Edge Termination Method

Reverse breakdown voltage as a function of edge termination techniques.

JTE the highest VB values. (4-fold increase)

The choice of edge termination should be based on the device type, size, and the effectiveness of termination method. The edge termination designs with a numerical solution technique.

20

Forward I-V characteristics

0 1 2 3

1x103

2x103

3x103

4x103

5x103

GaN Schottky diode

300 K 423 K 573 K

Cur

rent

Den

sity

(A

/cm

2 )

Forward Bias (V)

Temperature dependence of I-V- Mobility degradation effects

Forward turn-on voltage▫ 1.6 V @ 100 A·cm-2

▫ Experimental values ~ 3.5 V ▫ Materials issues (defects)

0 1 2 3 410-5

10-4

10-3

10-2

10-1

100

101

102

103

p-n diode Schottky diode

573 K

423 K 300 K

Cur

rent

den

sity

(A

/cm

2 )

Forward bias (V)

5 µm n(11016 cm-3)

1 µm n+ (51019 cm-3)

Anode (Pt)

Cathode

VF for pin diodes (even @573K) > VF for Schottky diodes The absolute value of VF is also much higher than typical experimentally reported values, which are 5 V.

21

Fabrication of GaN Rectifiers

GaN high power rectifiers

• GaN Schottky & PiN rectifiers

- GaN epi layer on sapphire

- GaN epi layer on freestanding GaN

(Vertical geometry)

SiO2

Pt/Au

Ti/Al/Pt/AuTi/Al/Pt/Au

3m n+ GaN

Al2O3 substrate

3m undoped GaN

Device processing

• Mesa etch (ICP dry etch)

• Oxide deposition (PECVD)

• p-guard rings (Implantation)

• Window opening (RIE)

• Ohmic metal formation (RTA)

• Schottky metal deposition (E-beam)

22

ICP mesa etch: Plasma-Therm ICPMesa Lithography:

AZP 4330 -- 4k RPM, 30 sec.(~3.75 µm straight wall resist)

Etch Conditions:

10 sccm Cl2 5.0 sccm Ar2 mTorr, 25° CICP: 300 WRF: 150 W electrod eG A S

I C P rf s o u rc e(2 M H z)

s u b s tra te b ia srf s o u rc e(1 3 .5 6 M H z)

GaN Rectifier Processing: Mesa

23

GaN High Voltage Diodes

Unterminated diodeDiameters of Schottky metal

54/72/98/134 µm

Dielectric edge terminated diodeDiameters of Schottky metal

44/62/88/124 µm

24Emcore substrate # K1645

-140 -120 -100 -80 -60 -40 -201E-7

1E-6

1E-5

1E-4

1E-3

0.01

10 A/cm2

1 A/cm2

0.1 A/cm2

Circular diode134 um diameter

Cur

rent

(A

)

Voltage (V)

Diode Forward I-V

0.0 0.5 1.0 1.5 2.00

10

20

30

40

50

Experimental Simulated

GaN Schottky diode134 m diameter

Cur

rent

(m

A)

Voltage (V)

Device breakdown after JR=10/cm2

VB=150 – 240 V

RON=~2.2 mcm2

Very close to simulation results

25

Breakdown voltage at 10A/cm2

Sample #: M5221sc4

50 60 70 80 90 100 110 120 130 140130

140

150

160

170

180 Unterminated diodes Terminated diodes

Bre

akdo

wn

volta

ge (

V)

Diameter of diode (µm)

40 60 80 100 120 140

130

140

150

160

170

180

190

200

210

Unterminated diodes Terminated diodes

Bre

akdo

wn

volta

ge (

V)

Diameter of diode (µm)

Sample #: M5217sc3

Breakdown Voltage

26

Pulse Measurements of Large-Area Diode

0 2 4 60.0

0.4

0.8

1.2

1.6

2.0

Experimental Simulated

1.72 A

Cur

rent

(A

)Voltage (V)

100-10,000 Hz10% duty cycle

Independent of measurement frequencies RON=3.4 cm2 ≥ 3.31 mcm2

The total defect density determined by TEM is ~106 cm-2.

27

GaN Schottky Diode Array

Schottky (Pt/Au)Nitride

Oxide (1500 Å)

Electroplated Au (3 µm)

Freestanding GaN (200 µm)

The schematic of high power GaN diode

Schottky diode array with the size of 500 µm×500 µm.

Nitride windows interconnected with electroplated Au (~3µm)

500 m

500 m

500µm

500µm

GaN Schottky diode array layout

28

GaN Power Diode Array

0 1 2 3 4 5 6 7-20

0

20

40

60

80

100

120

140

160

180

Free-standing GaN Schottky diode

161 A @ 7.12 V

Cur

rent

(A)

Bias voltage (V)

161 A forward output current @ 7.12 V RON (On-state resistance) = 8 mΩ·cm2

1.1 kW for 66 mm2 (active device area)

0 1 2 3 4 5 6 70.00

0.25

0.50

0.75

1.00

1.25

Experimental Simulated

500×500 µm2 GaNSchottky diode

Cur

rent

(A

)

Bias voltage (V)

Promising results for practical “on-state current” Very close to simulated RON values (3.3 mΩ·cm2)

29

SiC Power Diode Array

-20 -15 -10 -5 0 51E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

10

100

1000

Cur

rent

(A

)

Bias voltage(V) 430 A forward output current @ 5.7 V RON (On-state resistance) = 5.8 mΩ·cm2

2.45 kW for 99 mm2 (active device area)

30

Acknowledgement

Y. Irokawa, J. R. LaRoche, B. S. Kang, J. Kim, and K.P. Lee Professors F. Ren and S. J. Pearton S. S. Park and S. K. Lee for GaN substrates D. Sheridan and G. Y. Chung about device modeling