Improving the Performance of GaN Power Devices for HighBreakdown Voltage and High Temperature Operation
by Hiroshi Kambayashi *, Shin-ichi Kamiya *, Nariaki Ikeda *, Jiang Li *, Sadahiro Kato *,Sonomi Ishii *, Yasumasa Sasaki *, Seikoh Yoshida * and Mitsuru Masuda **GaN, like SiC and AlN and other wide bandgap semiconductors, has outstanding features such as being able to operate at high breakdown voltages, high frequencies and high temperatures, and it is therefore expected to find wide application in power conversion devices, etc. In this work the authors have investigated a unique Ti/
AlSi/Mo electrode as the ohmic electrode of an AlGaN/GaN HFET. This has made it possible toachieve a contact line resistance less than one-third that of Ti/Al. Fabricating an AlGaN/GaN
HFET with a gate width of 200 mm using this electrode, we have achieved operating currents of
20 A or more at room temperature and 10 A or more in a 500-K environment, and off breakdown
voltages in excess of 450 V at 500 K. And by using high-refractive index SiNx as the surface passivation film, it was possible to achieve a reduction in gate leakage current of about two ordersof magnitude compared to SiO2.

ABSTRACT1.

INTRODUCTION

In recent years there has been a growing need for energy
development that takes account of environment policies
like reduction of carbon dioxide. Specifically, the Kyoto
Accords, issued on February 16, 2005 with the aim of stabilizing greenhouse gases in the atmosphere, have
imposed on Japan a duty to reduce greenhouse gases by
6 % relative to 1990 levels from 2008 to 2012. This is the
background against which the need for more efficientelectric power and energy-saving measures has arisen.
Currently power conversion devices use silicon-based
switching elements, but in recent years the physical limitations of silicon are being approached, creating a desire
for alternative materials.
GaN, like SiC and AlN and other wide bandgap semiconductors, has a bandgap width about three times
greater than Si and an insulation breakdown voltage
about an order of magnitude greater, giving it superior
thermal insulation and breakdown voltage properties,
thereby enabling higher frequency operation and various
other superior performance indices 1)~6).
Particular noteworthy is the fact that the on-state resistance (Ron) during FET operation can theoretically be
reduced to more than an order of magnitude less thanthat of Si-based devices. This promises simpler, smaller
and lighter cooling systems, and is drawing much atten* Yokohama Research Lab., R&D Division** Ecology & Energy Lab., R&D Division

tion as an alternative to Si as a semiconductor material for
ultra-low-loss power devices.
Early development of GaN was carried out in the areaof light-emitting devices—green LEDs, lasers, etc.—but
its superior performance as noted above and the needsof society led to R&D work for electronic devices being
conducted world-wide. Specifically, the AlGaN/GaN HFET
structure permits the formation of a high-density electron
layer known as a two-dimensional electron layer on an
AlGaN/GaN hetero interface by adding impurities to thecarrier supply layer of AlGaN. Thus this AlGaN/GaN HFET
structure holds out the possibility of achieving a high-frequency output device surpassing the characteristics ofexisting devices.
Furukawa Electric has been involved in this development from the earliest stages, and has reported theworld's first prototype inverter circuit using an AlGaN/GaN
HFET. This inverter is made up of a DC converter using an
AlGaN/GaN HFET and an AC inverter. Its operating output
power was 50 W and maximum output power was
200 W 7). More recently a device with an output power inexcess of 200 W has been reported 8), and its application
as a high-output power device is anticipated.
In this paper we describe the results of investigations in
an AlGaN/GaN HFET, of, first of all, a Ti/AlSi/Mo structure
for a unique ohmic electrode having reduced on-state
resistance. We also describe how this ohmic electrodeevolved into a larger device (gate width: 200 mm) andmounted in a package designed for operation in a hightemperature environment, and its characteristics at 500 K

Furukawa Review, No. 29

2006

7

Improving the Performance of GaN Power Devices for High Breakdown Voltage and High Temperature Operation

2.

LOWER RESISTANCE USING AlSiBASED OHMIC ELECTRODESIn order to improve FET performance, a reduction in onstate resistance is essential, and this requires decreased
contact resistance between the ohmic electrode and the
AlGaN layer. According to examples reported in the literature, methods such as implantation of Si, an n-type dopant, in the AlGaN layer have been used 9)~11). In our effort
to reduce on-state resistance, we adopted selective area
growth (SAG) of a high-density n-GaN layer immediately
beneath the ohmic electrode, thereby achieving lower
contact resistance 12). This technique, however, is subject
to reduced yields due to faulty deposition of the SAG
layer, and to reductions in breakdown voltage thought to
be attributable to degraded crystallization and to theoccurrence of damage. Taking these problems into consideration, we may conclude that it is necessary to form a
low-contact-resistance ohmic electrode directly, with nospecial processing on the AlGaN layer.
In the literature, Ti/Al-based ohmic electrodes havebeen commonly used and it has been reported that, using
Ti as the contact electrode on AlGaN, the forming of nitrogen holes by Al diffusion under high-temperature heat
treatment contributes to reducing contact resistance 13).
Since the melting point of Al is low, around 660°C, hightemperature heat treatment is difficult, and sufficient
reduction in contact resistance cannot be obtained. Thereis also a problem in that high-temperature heat treatment
gives rise to deterioration of the electrode surface.
Accordingly we tried investigating an electrode with amultilayer structure, with a barrier metal of high melting
point sandwiched between, by which means reduced
contact resistance and improvements to surface conditions were achieved 14)~17).
In this work, in trying to achieve reduced contact resistance we applied a Ti/AlSi/Mo electrode structure using
AlSi instead of Al in the frequently reported TiAl, and disposing Mo, which has a higher melting point and comparatively low metallic resistivity, on the uppermost surface,
and we examined the results.
All the epitaxial wafers in this report were fabricated on
2-inch sapphire (0001) substrates by successive deposition by MOCVD of a buffer layer, a GaN layer (thickness 3μm), and AlxGa1-xN layer (x = 0.22 and thickness 30 nm).
The structure of the ohmic electrode examined was Ti(25 nm) / AlSi (100 nm) / Mo (200 nm), fabricated by sputtering. After the ohmic electrode was formed, it was heat
treated at 850°C for 30 sec.
The contact line resistance Rc and channel sheet resistance Rsh were evaluated by transfer length measurement(TLM). TLM pad gaps were 10, 20, 30, 40, 50 and 100 μm.

Table 1 shows the structure of the ohmic electrodes investigated and the results of TLM evaluation. In the one withthe lowest contact line resistance we were able to obtain
0.48 Ωmm, representing less than one-third compared to
Ti/Al.
With respect to the surface morphology of the Ti/AlSi/
Mo structure before and after heat treatment, observations were made by atomic force microscopy (AFM).
Virtually no change in surface morphology was observed
before and after heat treatment, and compared to an RMS
value of 5.5 nm before heat treatment, the value afterwards, at 5.7 nm, was virtually the same.
Table 1Results of investigation of Ohmic electrodes.

Barrier metal

Mo

None

Mo

Al or Al-Si

Al-Si

Al

Al

Rc (Ωmm)

0.48

1.66

NG

Rsh (Ω/)

432

426

—

Ti/AlSi/Mo

100

AlGaN/GaN

90
Mo

80
C

70
mol%

have been evaluated, confirming satisfactory operation athigh temperatures. We also describe the results obtained
from investigations focusing on the refractive index of SiNx
surface passivation films with the aim of reducing gateleakage current.

O

60

Si

50

Ti

40

Ga

30

N

N
Ti

Al

20

Mo

10
0

Ga

Al

0

100

Si

200

300

400

500

Depth (nm)Figure 1

Analysis of Ti/AlSi/Mo ohmic electrode after heat
treatment by AES.

To understand whether or not contact line resistance
was improved by Ti/AlSi/Mo, we analyzed samples after
annealing using Auger electron microscopy (AES). Figure
1 shows the results, and, as can be seen, we found that
the Si of the AlSi was diffused as far as the interface layer
between the AlGaN and the electrode. We may conclude
that this contributes to the improvement in contact line
resistance.
Thus we see that, by means of our unique Ti/AlSi/Mo
electrode structure, we have been able to provide a highly
superior ohmic electrode, with a contact line resistancesignificantly better than that of the commonly used Ti/Al,
and in which electrode surface degradation caused byheat treatment was inhibited.
Accordingly we moved to actually fabricate an AlGaN/
GaN HFET device using the Ti/AlSi/Mo ohmic electrode.
Figure 2 shows a cross-sectional view of this device,
which has a gate length of 2 μm, a gate-to-drain distanceof 10 μm, a gate-to-source distance of 3 μm, and a gate
width of 400 μm.

Furukawa Review, No. 29

2006

8

Improving the Performance of GaN Power Devices for High Breakdown Voltage and High Temperature Operation

Gate (Ni/Au)
Drain (Ti/Al-Si/Mo)

Source (Ti/Al-Si/Mo)

Passivation film

AlGaN layer

GaN
BufferSapphire sub.

Figure 2

Cross-section of AlGaN/GaN HFET.

3.5E-01

Ron=11.6Ωmm

Ids (A/mm)

3.0E-01

Figure 4

2.5E-01
Vgs=0 V, -1 V step

2.0E-01

Photograph of AlGaN/GaN HFET with 200-mm gate
width.

1.5E-01
1.0E-01
5.0E-02
0.0E+00

0

5

10

15

20

25

Vds (V)

Figure 3

I-V characteristic of AlGaN/GaN HFET with gate widthof 400 μm.

Figure 3 shows the I-V characteristic of a device with agate width of 400 μm. With respect to on-state characteristics, we were able to obtain a current of 250 mA/mm ormore at a gate voltage of 0 V. On-state resistance of
11.6 Ωmm was obtained, a satisfactory characteristic.

3.

Figure 5

AlGaN/GaN HFET WITH 200-mm GATE
WIDTH

Using the unique Ti/AlSi/Mo ohmic electrode structure
discussed above, we fabricated an AlGaN/GaN HFET witha gate width increased to 200 mm. Figure 4 is a photograph of this device. Its cross-sectional structure is fundamentally as shown in Figure 2, with a gate length of 2 μm,
a gate-to-drain distance of 10 μm, and a gate-to-source
distance of 3 μm. It was possible to form a large-scale
AlGaN/GaN HFET with a gate width of 200 mm by linking
the source, gate and drain electrodes, respectively, of1000 unit FETs each having a gate width of 200 μm.
3.1 On-State Resistance
Figure 5 shows the results of an evaluation of the on-state
resistance of an AlGaN/GaN HFET with 200-mm gate
width at room temperature. The on-state resistance was
55.1±1.9 mΩ, and uniformity within the surface wasgood. Since the effective surface area of the device was
12 mm2, we obtain an on-state resistance per unit area of
6.3 mΩ·cm2.
3.2 Package
We designed a package to accommodate the AlGaN/GaN
HFET with the aim of supporting operation in high-temperature environments. Figure 6 shows a photograph of

Results of evaluation of on-state resistance(Ids = 5 A).

Cu-W (90-10)

BeO

O.F.H.C

Fe-Ni52

Figure 6

Photograph of package for high-temperature operation.

this package. The package material was selected for considerations of heat-diffusion. With respect to the thermal
resistance of the package, a thermal analysis simulation
resulted in an estimate of 0.45 K/W, showing that heat dissipation is satisfactory.
3.3

Evaluation of Characteristics of a Packaged
AlGaN/GaN HFETThe characteristics of the AlGaN/GaN HFET were then

Furukawa Review, No. 29

2006

9

Improving the Performance of GaN Power Devices for High Breakdown Voltage and High Temperature Operation

evaluated when mounted in the package we designed,
and Figure 7 shows the I-V characteristic. It was possible
to obtain a drain current of more than 20 A at room temperature (300 K), and even at 500 K more than 10 A was
obtained. As a result we conclude that the use of ourunique Ti/AlSi/Mo ohmic electrode was the major factor in
our success in reducing on-state resistance. Furthermore,
our achievement of high-output operation, even in environments as hot as 500 K, is indicative of the superiortemperature characteristics of the GaN material itself.
Figure 8 shows the results obtained when off-state characteristics were measured in a high-temperature environment and the off breakdown voltage evaluated. An
increase in off breakdown voltage to over 450 V wasachieved.
25
20

Ids (A)

300 K (R.T)
500 K

Vgs= 0 V, -1 V step

15
Vgs= 0 V, -1 V step10
Wg = 200 mm
Lg = 2 µm
Ldg = 10 µm
Lgs = 3 µm

5
0

0

2

4

6

8

10

Vds (V)

Figure 7

I-V characteristic of AlGaN/GaN HFET with gate widthof 200 mm.

2.0E-04

Wg = 200 mm
Lg = 2 µm
Ldg = 10 µm
Lgs = 3 µm

1.5E-04

Ids (A)largely caused by surface morphology 18) .
Accordingly consideration was given to a recessed gate
structure 19), a field plate structure 20), MIS structures 21), 22),
surface passivation films 23)~25), and so on. There havebeen a particularly large number of reports of the use of
SiNx in passivation films, thereby suppressing current collapse. These may be attributed to the difference in interface level densities between a passivation film of SiN x,
SiO2, etc. and the AlGaN (GaN), and there is a report of
the use of SiNx resulting in a reduction in interface level
density of an order of magnitude or more compared to
SiO2 25). It was therefore concluded that the use of SiNx as
the passivation film is effective in suppressing current collapse.
On the other hand it has been reported that in GaAs
FETs the piezoelectric charges, which occur because ofpassivation film stress and are concentrated at the gate
terminal, cause degradation in characteristics 29) .
Accordingly we focused our attention on SiNx stress, andinvestigated the effect that film stress has on gate leakage
current.
4.1 Evaluation of SiNx StressFirst we grew a 300-nm film of SiNx on a silicon substrateusing the PCVD method to vary the refractive index, andinvestigated the relationship between stress and refractive
index for SiNx. We evaluated stress by measuring theamount of substrate flexure before and after deposition of
the SiNx film and calculating the stress. Figure 9 shows
the results.
The stress for SiNx was all tensile stress, and with a shift
to a higher refractive index the stress decreased, becoming substantially constant at a refractive index of about
2.1. For purposes of comparison we also evaluated SiO2
in the same way, and found that for SiO2 the stress was
2.46 Pa, and, contrary to SiNx, was compressive.
6

1.0E-04

5.0E-05

0.0E+00

Vgs= -6 V

0

100

200

300

400

500

600

Vds (V)

Figure 8

Off breakdown voltage characteristic of AlGaN/GaN
HFET (large scale).

Tensile stress (Pa)

5
4
3
2
1
0
1.8

4.

INVESTIGATION OF SiNx PASSIVATIONFILMS FOR DECREASING GATE LINKCURRENT

Among the problems arising in the commercialization of
the AlGaN/GaN HFET, we may mention suppression of
the so-called current collapse phenomenon in which thedrain current is drastically decreased during high-voltage
operation, and the reduction of the extremely large gateleakage current. These problems are considered to be

Figure 9

1.9

2
2.1
Refractive Index

2.2

2.3

Relationship between stress and refractive index for
SiNx.

4.2 Evaluation of Properties of AlGaN/GaN HFET
The four types of passivation films that we evaluated, as
shown in Table 2, were three types of SiNx with varying
refractive indices and SiO2. The stress values shown in
Table 2 are the results of this evaluation. The devices
evaluated had a gate length of 2 μm, a gate-to-drain dis-

Furukawa Review, No. 29

2006

10

Improving the Performance of GaN Power Devices for High Breakdown Voltage and High Temperature Operation

Refractive index and stress of passivation films.

Film

Refractive index

Stress (Pa)SiNx (1)

1.85

-4.98

SiNx (2)

2.02

-2.19

SiNx (3)

2.09

-1.65

SiO2

1.46

+2.46

5.0E-04
Vgs=-6 V
4.0E-04
SiNx (1)
Ids (A/mm)

Table 2* + = compressive; - = tensile

3.0E-04

SiO2

2.0E-04

Gate leakage current Idg (A/m)

1.0E-03

1.0E-05

1.0E-04

SiNx (1)SiNx (2)SiO2SiNx (2)1.0E-07

0.0E+00

SiNx (3)SiNx (3)
0

200

400
Vds (V)

600

800

Figure 11 Off breakdown voltage characteristic.
1.0E-09

1.0E-11
-5

5.
-4

-3

-2

-1

0

1

Gate voltage Vdg (V)Figure 10 Source-to-drain I-V characteristic.

tance of 10 μm, a gate-to-source distance of 3 μm, and agate width of 400 μm.
Figure 10 shows the gate leakage current characteristic
measured between source and drain (Idg). Focusing on
SiNx, it can be seen that, with a shift toward a higher
refractive index (i.e., as stress decreases), gate leakage
current decreases. With respect to the characteristics of
SiO 2, which in absolute value of stress is positioned
between SiNx(1) and SiNx(2), the gate leakage current islower than that of SiNx(1) and higher than SiNx(2). These
results suggest that gate leakage current is dependent on
stress. It also suggests that the lower the stress in termsof absolute values, that is to say the higher the refractive
index for SiN x , the more gate leakage current will
decrease. It was further found that by using SiNx with ahigher refractive index as the passivation film, values
would be some two orders of magnitude smaller than for
SiO2.
Figure 11 shows the off breakdown voltage characteristic of the device fabricated in this work, when a gate voltage of -6 V is applied. As a result SiNx(1), with a low
refractive index, has low values for both leakage current
and off breakdown voltage. It can also be seen that, as
the absolute values of stress decrease further—SiO 2,SiNx(2) and SiNx(3)—leakage current goes down and off
breakdown voltage goes up. It is thought that theseresults are reflective of gate leakage current. Thus wemay say that using SiNx with a low gate leakage current
and high refractive index for the passivation film makes
for a device with lower leakage current and higher off
breakdown voltage.

CONCLUSION

We investigated increasing the performance of AlGaN/
GaN HFETs, which hold promise in applications as power
conversion devices. First of all, by applying an ohmic
electrode using our unique Ti/AlSi/Mo structure we wereable to achieve a contact line resistance of 0.48 Ωmm.
This factor technology was applied to a large-scale device
with a gate width of 200 mm. As a result it was possible toobtain an on-state resistance per unit area of 6.3 mΩ·cm2.
We conducted investigations with respect to packages for
operation in high-temperature environments, mounted
AlGaN/GaN HFETs, and carried out evaluations, with theresult that we achieved operating currents of 20 A or moreat room temperature and 10 A or more in a 500-K environment, and off breakdown voltages in excess of 450 V in a
500-K environment. Further, by using SiNx having a highrefractive index as the passivation film, we achieved a
reduction in gate leakage current of approximately twoorders of magnitude compared to SiO2. The above results
suggest that the AlGaN/GaN HFET holds promise as ahigh-efficiency power conversion device, and broader
application is to be expected.

REFERENCES
1) S. Yoshida: Journal of The Japan Society of Applied Physics, 68,(1999), 787. (in Japanese)
2) E. O. Johnson: RCA Rev. 26, (1965) 163.
3) R. W. Keys: Proc. IEEE, 60, (1972) 225.
4) B. J. Baliga: IEEE Electron Device Lett. 10, (1989) 455.
5) B. J. Baliga: J.Appl.Phys., 53, 1759 (1982).
6) K. Shenai, R. S. Scott, and B. J. Baliga: IEEE Trans. Electron
Device, 41, (1994) 1481.
7) S. Yoshida, J. Li, T. Wada, and H. Takehara: Proc. 15th ISPSD,(2003) 58.

Furukawa Review, No. 29

2006

11

Improving the Performance of GaN Power Devices for High Breakdown Voltage and High Temperature Operation

8) T.Kikkawa, T. Maniwa, H. Hayashi, M. Kanamura, S. Yokokawa, M.
Nishi, N. Adachi, M. Yokoyama, Y. Tateno, and K. Joshin: IEEE Int.
Microwave Sympo. Dig., (2004) 1347.
9) Haijiang Yu, L. McCarthy, H. Xing, P. Waltereit, L. Shen, S. Keller,
S. P. Denbaars, J. S. Speck, and U. K. Mishra: Appl. Phys. Lett. 85,(2004) 5354.
10) J. Burm, K. Chu, W. A. Davis, W. J. Schaff, L. F. Eastman, and T. J.
Eustis: Appl. Phys. Lett., 70, (1997) 464.
11) D. Qiao, Z. F. Guan, J. Carlton, S. S. Lau and G. J. Sullivan: Appl.
Phys. Lett, 74, (1999) 2652.
12) S. Yoshida, H. Ishii: Phys. Stat. Sol. (a), Vol.188, (2001) 243.
13) V. Kumar, L. Zhor, D. Selnanathan, and I. Adesida: J. Appl.Phys., 92,(2002) 1712.
14) D. Selvanathan, L. Zhou, V. Kummar, and I. Adesida: Phys. Status
Solidi A 194, (2002) 583.
15) D. Selvanathan, F. M. Mohammed, A. Tesfayesus, and I. Adesida: J.
Vac. Sci. Technol. B, 22, 2409 (2004).
16) J. Gillespie, A. Crespo, R. Fitch, and D. Via: Solid-State Electron.,49, (2005) 670.
17) T. Nakayama, H. Miyamoto, Y. Ando, T. Inoue, K.Hataya, and M.
Kuzuhara: Appl. Phys. Lett., 85, (2004) 3775.
18) T. Hashizume, H. Hasegawa: Technical Research Reports of IEICE,
ED2003-204 (2004) (in Japanese)
19) H. Okita, K. Kaifu, J. Mita, T, Yamada, Y. Sano, H. Ishikawa, T.
Egawa, and T. Jimbo: Phys. Stat. Sol. (a) 200, No.1, (2003) 187.
20) Y. Okamoto, Y. Ando, K. Hataya, H. Miyamoto, T. Inoue, and M.
Kuzuhara: Electron Lett. 39, (2003) 1474.
21) S. C. Binari, and H. B. Dietrich: Proc. 21st Int. Symp. On Compound
Semiconductors, IOP Publishing, Bristol, (1995) 459.
22) T. Hashozume, T. Ootomo and H. Hasegawa: Appl. Phys. Lett., 83,(2003) 2952.
23) R. Nakasaki, T. Hashozume, H. Hasegawa: Physica E 7 (2000) 953.
24) R. Vetury, N. Q. Zhang, S. Keller, and U. K. Mishra: IEEE Trans.
Electron Device, 48, (2001) 560.
25) T. Hashozume, and R. Nakasaki: Appl. Phys. Lett., 80, (2002) 4564.
26) P. M. Asbeck, C. P. Lee, and M. F. Chang: IEEE Trans. Electron
Device, 31, (1984) 1377.

Furukawa Review, No. 29

200612