HIPER Lab

Harris Integrative Photonics and Electronics Research Laboratory

High Voltage GaN Devices for

Photovoltaics and High

Frequency Switched Power

Supplies

H. Rusty Harris

Texas A&M University

Depts. Of Physics and ECE

SLAC Advanced Instrumentation Seminars July 27, 2011

Outline

• Motivation

• GaN design for 5kV

• Additional device and integration needs

• Summary

http://www.tamu.edu/http://engineering.tamu.edu/

Students and Collaborators

• PhD Students

– Iman Rezanejhad

– Derek Johnson

– Mary Coan

– Jung Hwan Woo

• Collaborators

– Mark Holtz and Sergey Nikishin – Texas Tech

University

– Eddie Piner – Texas State University

http://www.tamu.edu/http://engineering.tamu.edu/

Outline

• Motivation

• GaN design for 5kV

• Additional device and integration needs

• Summary

http://www.tamu.edu/http://engineering.tamu.edu/

Area Needs for advanced HV

switches

Photovoltaics

High Voltage drives RF and microwave communications

http://www.tamu.edu/http://engineering.tamu.edu/

• We’re getting there

PV Progress

http://www.tamu.edu/http://engineering.tamu.edu/http://upload.wikimedia.org/wikipedia/commons/e/ed/PVeff(rev110408U).jpg

• Higher voltage in the

arrays will reduce

resistive losses

– Residential: 600V

– Commercial: 1000V

Switched power supplies in PV

http://www.tamu.edu/http://engineering.tamu.edu/

E

E

E

E

A

B

C

D

A

B

C

D

Higher Voltage requires more

components

Minority carriers in large drift regions

cause reverse current after switching

http://www.tamu.edu/http://engineering.tamu.edu/

• Clear Advantage in

terms of scaling for MV

applications

6 MW LV (690 V) full-power converter system for

wind turbine

6 MW MV (3300 V) full-power converter for wind turbine

Medium Voltage energy

advantage

Microwave Switches- RF Switches

http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/

Outline

• Motivation

• GaN design for 5kV

• Additional device and integration needs

• Summary

http://www.tamu.edu/http://engineering.tamu.edu/

0.1

1

10

100

100 1000 10000

On

-Sta

te R

esis

tan

ce (

milli O

hm

s.C

m^

2)

Breakdown Voltage (V)

GaN SiC

US

SankeMatsushit

Furukawa

Toshiba

Denso CELEGY

Siemens

Purdue

CREE SICE

D

Siemens Mitsubishi SICE

D

Kansai Power

Electronics & CREE Si-

Limit

SiC-

Limit

GaN-

Limit

Huang

[26]

[27] [28]

[29]

Current Status of Alternate

switching technology

http://www.tamu.edu/http://engineering.tamu.edu/

• AlGaN/GaN forms a high

density 2D Electron Gas

– Low on state resistance!

• We can grow high quality

GaN on a Si platform

– Up to 300mm possible!

Benefits of GaN HEMT in HV

applications

http://www.tamu.edu/http://engineering.tamu.edu/

Our Goal

• Use the HEMT architecture and all its benefits, but

still retain high voltage blocking characteristics

http://www.tamu.edu/http://engineering.tamu.edu/

Spontaneous and Piezoelectric

Polarizations

jijp

i eP

]0,0,0,,,[ 321

^

31

^

0

0

e

e

31

0

0

e 33

0

0

e 0

0

14e

0

0

15e

0

0

0

]2,0,0[. 333131

^^

eeeP p

)(2233

133331333131

c

cee

a

aaeePP

AlGaN

AlGaNGaNp

zPE

http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/

Degree of Relaxation

• Any strained epitaxial films

will have partial, spatially

dependant relaxation below

the critical thickness (Tc)

• Above Tc, full relaxation

occurs and the only

polarization left is PSP

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0

0.2

0.4

0.6

0.8

1

Al Mole Fraction, x

De

gre

e o

f R

ela

xa

tio

n, r(

x)

r(x) = 3.5 (x - 0.35)

)()](1[233

133331

c

cee

a

aaxrP

AlGaN

AlGaNGaNPE

GaNrelaxedAlGaN

GaNstrainedAlGaN

aa

aaxr

,

,)(

]1),(5.3min[

0)(

1xxxr

1

0

1

1

xx

xx

http://www.tamu.edu/http://engineering.tamu.edu/

Parametric numerical modeling

of polarization

• Any field above Ecrit

will result in device

failure

• Collapse to universal

polarization above Tc

)/(3.342.8, CmMVxE AlGaNcrit

http://www.tamu.edu/http://engineering.tamu.edu/

Resulting Sheet Carrier Density

• Consistently greater

than 1012 cm-3 sheet

charge density

• We now have a

parametric design

template for

concentration in terms

of Al fraction and

thickness

0 0.1 0.2 0.3 0.4 0.5 0.6 0.71012

1013

Al Mole Fraction, x

2D

EG

Sh

ee

t C

arr

ier

Co

nce

ntr

atio

n (

1/C

m2

)

d = 6nm

d = 10nm

d = 20nm

d = 30nm

d = 40nm

d = 50nm

][)(2

0CFb

AlGaNs EEe

deen

GaNSPAlGaNPEAlGaNSP PPP ,,,

)(84.03.1 eVxe b

)(7.02.1 2 eVxxEC

s

AlGaN

sF n

m

n

m

eE

*

23/2

*

0

2

]88

9[

http://www.tamu.edu/http://engineering.tamu.edu/

And (almost) finally…2DEG

resistivity

• Now we can design

our device

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7300

500

700

900

1100

1500

1900

2300

2700

3500

4300

5000

Al Mole Fraction, x

2D

EG

Sh

ee

t R

esis

tivity (

Oh

ms)

d = 6nm

d = 10nm

d = 20nm

d = 30nm

d = 40nm

d = 50nm

ss

DEGen

1

2

http://www.tamu.edu/http://engineering.tamu.edu/

Device consideration

• Our goal is to use the

2DEG sheet resistance to

create a device that

uniformly drops 5kV

without any formation of

electric field above Ecrit

gdgdx LVE /

22

xPAlGaN EEE

EAlGaN = Ecrit,AlGaN

2222222

, )/()/( gdBRPgdgdPxPAlGaNcrit LVELVEEEE

22

,/ PAlGaNcritBRgd EEVL

http://www.tamu.edu/http://engineering.tamu.edu/

Breakdown in GaN

• Peizoelectric field is

negligible in GaN

• Smaller subset of

previous Lgd

parameters due to

substrate breakdown

2

0

,22'2'2 )()(GaN

GaNSP

gd

dg

SPxGaN

P

L

VEEE

2

0

,22

, )()(GaN

GaNSP

gd

BRGaNcrit

P

L

VE

http://www.tamu.edu/http://engineering.tamu.edu/

0 0.1 0.2 0.3 0.4 0.5 0.6 0.710

-1

100

101

102

103

104

Al Mole Fraction, xD

rain

Re

sis

tan

ce

(m

illi O

hm

s.C

m2

)

d = 6nm

d = 10nm

d = 20nm

d = 30nm

d = 40nm

d = 50nm

The result

• A full set of design

parameters that allow

HV design with ultra-

low on-state resistance

– AlGaN Thickness

– AlGaN concentration

– Gate-drain distance

• Need corroboration!

http://www.tamu.edu/http://engineering.tamu.edu/

Outline

• Motivation

• GaN design for 5kV

• Additional device and integration needs

• Summary

http://www.tamu.edu/http://engineering.tamu.edu/

Caveat Emptor…

• Other consideration that we are researching

– Enhancement mode operation

– GaN breakdown to substrate

– Alternate electric field smoothing techniques

– Thermal management

http://www.tamu.edu/http://engineering.tamu.edu/

Depletion HEMT to Enhancement MOSHFET

• HEMT has Negative Threshold

Voltage!

• Application of a dielectric and

high work function metal lifts

the Fermi level, removing the

equilibrium 2DEG

GaN AlyGa1-yN

ΔPInAlN-InGaN

2DEG

GaN AlyGa1-yN Metal

Φ~5.1eV

http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/

Alternate Field Smoothing

26 July 2011

Drain Field

Plate

Gate Gate

Dielectric

Gate Field

Plate (FP)

Passivation

Dielectric

(translucent)

Source

http://www.tamu.edu/http://engineering.tamu.edu/

Summary

• Pushing high voltage design of low Ron,sp GaN

HEMT devices opens up miniaturization,

integration and efficiency options

• A first-time complete design spec outlined for HV

GaN HEMTs

• Alternate means being developed to eliminate

additional device design constraints

http://www.tamu.edu/http://engineering.tamu.edu/http://www.tamu.edu/http://engineering.tamu.edu/