GaN based power transistors:

Proposals for low-loss operations

Kazuo Tsutsui

Dept. of Electronics and Applied Physic,

Tokyo Institute or Technology

WIMNACT-39, Tokyo, Feb.7, 20141

Acknowledgement

The present works are carried out with the collaborations;

Y. Takei, K. Terayama, M. Kamiya, H. Yonezawa,

K. Kakushima, H. Wakabayashi, Y. Kataoka, H. Iwai

(Tokyo Institute of Technology)

W. Saito (Toshiba Corp.)

A. Nakajima, S. Nishizawa, H. Ohashi, M. Shimizu

(Advanced Industrial Science and Technology)

2

Outline

1. Introduction

2. Problems on AlGaN/GaN HEMT power devices

3. Trade-off properties on ohmic contact formation

4. Proposal of new contact technique: Uneven AlGaN layers

5. GaN base FinFETs

6. Conclusion

3

Importance of power devices

Saving energy consumption High efficiency of energy

conversion system

Low loss operation of power devices

used in inverter systems is required.

Requirement for power devices:

- low on-state resistance

- low off-state leakage current

- high speed switching

under necessary high voltage

operation (necessary

breakdown voltage).

Wide band gap

semiconductors such as SiC

and GaN are expected.

4

AlGaN

GaN

Source Gate Drain

AlGaN/GaN HEMT (high electron mobility transistor)

Substrate (Si, Al2O3, etc.)

buffer layers

two-dimensional electron gas (2DEG)

Met

al

AlG

aN

GaN

ΦB

EcEF

Ev

Electric field produced by

polarization

5

12

10

8

6

4

2

0

2D

EG C

on

cen

trat

ion

[1

01

2cm

-2]

AlGaN Thickness [nm] [1]0 5 10 15 20 25

Al：24%T：300 K

[1] D. Qiao, et al, JAP, 89, (2001) 5543.

AlGaN

GaN

Source Gate Drain

2DEG Produced by Polarization Effects

2DEG

Thickness

6

Present

state

Normalized On-resistance vs. Breakdown Voltage

AlGaN

GaN

S G D

LSD

Long LSD is

necessary for high

breakdown voltage.

Increase in Ron

7

VB=600V VB=100V

Breakdown of On-state Resistance

contact resistance

8

Problems of AlGaN/GaN HEMTs for Power Devices

AlGaN

GaN

RCH

RC

Source Gate Drain

AlGaN

GaN

Source Gate Drain

On-state

Off-state

leakage

On-state resistance: further decrease.

- Contact resistance (Rc) is a key

issue.

- Improvement of crystalline quality:

decrease of RCH.

Stability of on current (current collapse).

- Improvement of crystalline quality.

- Control of surface state.

Off-state leakage current.

- Significant on scaling down: short

channel effects.

- Improvement of crystalline quality.

9

Formation of Conventional Contacts on AlGaN/GaN

Au

Mo

Al

Ti

after annealing @850℃ [2]

AlGaN

as deposite

GaN

GaN

AlGaN

Au/X/Al/TiTiN

island

GaN

AlGaN

Au/X/Al/Ti

TiN

[2] L. Wang, et al,

JAP, 103, 093516 (2008)

Au shell

GaN

Au/X/Al/Ti

dislocation

2DEG

Better crystallinity with

lower dislocation densityHard to form contacts

10

Contact Formation Independent of Dislocations

[3] B. Daele, APL, 89, 201908 (2006) [4] K. Tsuneishi, et al. ECS Transactions, Vol.50(3), pp. 447-450.

AlGaN

GaN

Non-alloy or uniform reaction types are desirable.

Example: Al/Ti/Si3N4[3], TiN/TiSi2

[4]

TiN/TiSi2/AlGaN/GaN after annealing at 950oC

11

Trade-off on AlGaN Layer Thickness

AlGaN AlGaN

AlGaN layer thickness

Resis

tance

2D

EG

concentr

ation

Rc

Rc=Rc2DEG + RcAlGaN

Rc2DEG

RcAlGaN

Rc

Rc

2DEG 2DEG

Rc2DEG

RcAlGaN

2DEG

12

80 μm Metal300 μm250 μm

Si（111） sub.

Al0.25Ga0.75N

Buffer layer

GaN

SiO2

Thinning by the step-by-step

etching using ozone oxidation

: 7.0～30.0 nm

Rc Depending on AlGaN Thickness: Experiment

TiN 45nm/TiSi2 20nm

or

Mo 35nm/Al 60nm/Ti 15nm

Annealing in N2 at 400oC -->1100oC

TLM measurement

Used TLM structure

13

after annealing at 650oCafter annealing at

950oC (Mo/Al/Ti)

or 1100oC (TiN/TiSi2)

Rc Depending on AlGaN Thickness: Experiment

The trade-off property

was observed.

Conductance through

the AlGaN layer was

reduced under high

temperature annealing.14

GaN

AlGaN

2DEG

GaN

metal

AlGaN

2DEG

Introduction of uneven AlGaN layer

Proposal for low Rc overcoming the “trade-off”

Increase in contact area density.

Two-dimensional effect at pattern edges:

- intrusion of 2DEG under thin-AlGaN region

- metal closing higher 2DEG density region15

0.0E+00

2.0E+12

4.0E+12

6.0E+12

8.0E+12

1.0E+13

1.2E+13

180 190 200 210 220Position [nm]0 10 20-10-20

0

2

4

6

8

10

12

Thickness

10/25 nm

Thickness

5/25 nm

2DEGi-GaN

20 nm20 nm

5 or 10 nm25 nm

40 nmpitch

i-Al0.3Ga0.7N

Uniform thickness 25 nm

Uniform thickness 10 nm

Uniform thickness 5 nm

2D

EG

co

nce

ntr

atio

n [1

01

2cm

-2]

Lateral distribution of 2DEG concentration

on uneven AlGaN layer

16

0.0E+00

2.0E+12

4.0E+12

6.0E+12

8.0E+12

1.0E+13

1.2E+13

160 170 180 190 200 210 220 230 240

Position [nm]0 10 20-10-20 30 40-30-40

35/ 5 nm 30/10 nm20/20 nm

10 nm25 nm

10 nm

5 nm

0

2

4

6

8

10

12

i-Al0.3Ga0.7N

2DEG

5/10/20 nm35/30/20

nm25 nm

40 nmpitch

i-GaN

5 nm

2D

EG

co

nce

ntr

atio

n [1

01

2cm

-2]

Lateral distribution of 2DEG concentration

on uneven AlGaN layer

17

0.0E+00

2.0E+12

4.0E+12

6.0E+12

8.0E+12

1.0E+13

1.2E+13

160 170 180 190 200 210 220 230 240Position [nm]0 10 20-10-20 30-30 40-40

0

2

4

6

8

10

12 25 nm

10 nm

5 nm

5 nm25 nm

40 nm 40 nmpitch

20 nm 20 nm

i-Al0.3Ga0.7N

i-GaN

20 nm 20 nm

5 nm25 nm

20 nm

5 nm25 nm

20 nm

Dome

Pyramid

Pi

t

2D

EG

co

nce

ntr

atio

n [1

01

2cm

-2]

Effects of shapes of Unevenness

18

Fabrication of Contacts using Uneven AlGaN Layers

5 μm 5 μm5 μm 10 μm

150 μ

m

14

0 μ

m

metal

80 μm

TLM pattern

AlGaN

Process: photo lithography, partial etching of AlGaN layer by RIE

perpendicular to

current flow

parallel to

current flow

square close packing

5 μ

m5 μ

m

Current

direction

5 μ

m

10 μ

m

PERP5 PARA5 SQU5 CP5

Thin AlGaN

region

19

Rc Reduced by Uneven AlGaN Layers

GaN2DEG

AlGaN30 nm

TiN/TiSi2

20 nm5 nm

Reduced Rc on the uneven

AlGaN contacts with proper

patterns was observed,

compared with the flat AlGaN

structure with optimized

thickness.

0

0.2

0.4

0.6

0.8

1

10 nm squ5 photo5 yoko5

Conta

ct r

esis

tance

[10

-3Ω

cm2]

1000℃

10 nm -

uniform AlGaN 20

Ohmic contacts using uneven AlGaN Layer

Pit structure Pyramid structure

Optimized size and structure formed by using lithography.

Application of self-assemble

processes.

Merit on practical device

process.S.K. Hong et al., J. of

Crystal Growth 191

(1998) 275.

anisotropic etching by H3PO4

21

S G D

AlGaNGaN

S

G

D

GaNAlGaN

S

G1

D

GaN AlGaN

G2

Conventional planar HEMT FinFET

Separated

double gate

GaN based FinFET

Higher function

High gate controllability

Reduction of off-leakage current

Low RonA

High aspect ratio of Fin

Possible advantages:

bulk conduction (not 2DEG)

ohmic contact to bulk

High crystalline quality

Use of selective growth22

GaNGaN

mask

GaN Fin structures

formed by selective growth

Dislocation

density is

reducedHigh dislocation density

Original dislocations remained.

Etching damage is added.

× ×××

×

Lowering Defect Density in Fin Structure

by GaN Selective Growth

GaN Fin structures

formed by etching

23

GaN

AlGaN

Side wall: Polar

2DEG 2DHG

Side wall: Non polar

EcEc

Bulk conduction

electrons

holeselectrons

metal

Variation of Possible Channel Conduction Modes

Rather high electron mobility 〜1500 cm2/Vs

S/D ohmic contacts to bulk channel may be easy

compared with those to 2DEG ?

Bulk conduction is promising ?

24

Problems of AlGaN/GaN HEMTs, which should be solved

for further low-loss operation of power transistors.

- Further decrease in ohmic contact resistances for S/D.

- Control of short channel effects; leakage current

in off-states.

Ohmic contacts for future devices

- Contact formations depending on dislocations in

AlGaN layers will not be useful in future. Contact

formations with uniform reaction (not necessary non-

alloy) are desirable.

Conclusion

25

Development of contact formation technology based on

clarifying its mechanism is important.

- Evaluation of the contact properties depending on AlGaN

thickness will be a useful tool.

- A new ohmic contact fabrication technique, the uneven

AlGaN layer, was proposed, and its advantage was

demonstrated by simulation and primitive experiment.

GaN based FinFETs are promising for future power

transistors.

- Not only 2DEG channels but also bulk channels are worth

to be investigated.

- Challenge of selective growth to form Fin structure with

low defect density will open future high performance

power transistors.

26