A Novel GaN Device with Thin AlGaN/GaN Heterostructure forHigh-power Applications
by Nariaki Ikeda *, Jiang Li *, Sadahiro Kato *, Mitsuru Masuda * and Seikoh Yoshida *GaN electron devices are expected to contribute significantly toward efficiency
improvement and downsizing of power supplies since the devices have thepotential of realizing higher breakdown voltages and lower on-state resistancesin comparison to Si electron devices conventionally used. The authors have investigated, bythinning the AlGaN layer and simultaneously inserting an AlN layer, GaN/AlGaN heterojunction
Field Effect Transistor (HFET) structures aimed at realization of normally-off type devices that arehigh in breakdown voltage yet comparatively low in on-state resistance characteristics. And,
using AlGaN/GaN heterostructure epitaxial layers on a Si substrate-- one of the prerequisites forcost reduction, normally-off operation with a threshold voltage of 0 V has been achieved. A proprietary diode structure has also been proposed to enable loss reduction, and operation with alow on-state voltage has been confirmed where a current begins to flow at approximately 0 V.
Moreover, an epitaxial AlGaN structure with a reduced thickness has been applied to this structure, and a diode featuring low leakage current as well as low on-state voltage operation has
been obtained.

ABSTRACTINTRODUCTION

GaN-based semiconductor devices have long been underdevelopment with the expectation that they would havesuperior characteristics in comparison to conventional Sibased devices since they, like SiC devices, belong to thefamily of wide band-gap semiconductors. In particular,
GaN-based field effect transistors (FETs) are capable ofoperating in high output power, high frequency and hightemperature conditions exhibiting many outstanding figures of merits 1)~5). Figure 1 shows the correlation between
power capacity and frequency of various power devices
together with the so-called “Si limit.” The wide band-gap
semiconductor devices such as SiC and GaN can be
expected, unlike conventional Si devices, to exceed theperformance limit, so that expectations are growing thatthey would realize high-performance power supplies in
the high-frequency range. In particular, the on-state resistance of wide band-gap FETs can be by two orders ofmagnitude lower than that of Si FETs. It is thus possible
for GaN devices to reduce significantly, in comparison to
Si devices, the power loss in switching devices such as
inverters and converters thereby downsizing or eliminating the cooling devices included. In addition, high-speed
switching and high-frequency operation using GaN FETs
makes it possible to improve the efficiency of switchingcircuits, resulting in downsizing and high-density mounting of the circuits.

Power capacity (kVA)1.

Frequency (kHz)

Figure 1

Power capacity versus frequency for several types ofpower devices.

We have reported the results of prototyping an inverteras one of the GaN-based power supplies. The inverter
comprised a DC converter circuit and AC inverter circuit,and the operating output power was 50 W, reaching amaximum power of 200 W 6) . However, the devices usedin these inverters were of the normally-on type. In thiswork, normally-off type devices were fabricated making
full use of GaN HFET structures, developing into novel
devices such as normally-off FETs and low-loss diodes.
The developmental results will be presented in this report.

* Yokohama Research Lab., R&D Div.

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A Novel GaN Device with Thin AlGaN/GaN Heterostructure for High-power Applications

2.

DEVELOPMENT OF NORMALLY-OFF
FETThis section presents the achievements of normally-off
FET using GaN FET.
2.1 Past Reports on Normally-off GaN FETIn order to develop a general-purpose device for power
supplies, it is essential to use a normally-off type elementfrom the standpoint of making the equipment fail-safe.
Normally-on elements, if they were used in the circuits,
can lead to a complicated circuit structure. Despite this,traditionally, very few have reported on normally-off type
GaN power devices. And most of the traditional studies
made a positive use of the two-dimensional electron gas(2DEG) of high density that can be formed at the AlGaN/
GaN interface in the common GaN devices.
Several candidate structures are conceivable for achieving normally-off operation. The first one is to dispose, likeSi devices, an insulating layer such as SiO2 between thegate electrode and the semiconductor. According to pastreports, however, this structure was unable to achieve
normally-off operation since gate bias could not beapplied due to the interface state. It is considered that
reducing the interface state is necessary for this technique to be successful. Another technique which makes
full use of HFET structure is to reduce the thickness of the
AlGaN layer near the gate to obtain the so-called recess
structure as shown in Figure 2, thereby shifting the pinchoff voltage to achieve normally-off operation. Whereas anumber of studies have reported the experimental resultsof this technique, it is true that none of them have sufficiently achieved normally-off operation 7). Also, the problem with this technique is that there remains an issue ofdamage removal after etching in the recess etching process and that fluctuations in the threshold voltage mightoccur due to etching depth variations thus leading to low
yields.
D

S
G
AlGaN

Recess etched

GaN

2DEG

BufferSub.

Figure 2

Recess-gate structure for HFET device proposed byothers.

2.2 Control of Normally-off Threshold Voltage
The threshold voltage of HFETs is expressed by Equation
1, where the threshold voltage is determined mainly by
three parameters; ΦB: height of Schottky barrier, d: thickness of AlGaN layer and N d: carrier concentration of
2DEG.
∆EqN ( x)Vth = Φ B − q c − ⌠⌡⌠⌡ εD dx2
d(1)

Heretofore, we have achieved normally-off operation by
doping the GaN layer of the HFET structure with carbon
in order to obtain high resistivity 8). The mechanism is thatcarriers in the 2DEG are compensated by carbon resulting in a decrease of Nd value, which generates a shift in
the pinch-off voltage. However, carbon doping has limitations in terms of large-current operation. Accordingly, thistime, we embarked on decreasing the d value by reducing
the thickness of the AlGaN layer as a whole. Figure 3
shows schematics of the HFET structures of conventional
normally-on type and normally-off type. It is thought that,
due to the depletion region that extends to beneath thegate electrode, whereas in the case of normally-on typeshown in (a) the gate potential has to be turned into negative in order to pinch off the channel by 2DEG, in the caseof normally-off type shown in (b) the channel can be
pinched off without applying a voltage on the gate
because the AlGaN layer has originally a reduced thickness.
Depletion region
S
AlGaN
i-GaN

G

D
2DEG

(a) Conventional HFET
(Thick AlGaN layer)

Figure 3

Depletion region
G

S
i-GaN

D

AlGaN

2DEG

(b) Normally-off HFET
(Thin AlGaN layer)

Different HFET structures of: (a) conventional HFET
with thick AlGaN layer and (b) normally-off HFET withthin AlGaN layer.

2.3 Fabrication Process of HFET Device
This time, the reduced-thickness AlGaN structure wasused to carry out prototype fabrication of HFETs. The epitaxial layer was grown on an Si(111) substrate by metalorganic chemical vapor deposition (MOCVD) method
using ammonia, trimethyl gallium (TMGa) and trimethyl
aluminum (TMA) as raw materials. The epitaxial layer
structure was formed by layer deposition of AlN/GaN buffer layer, thick GaN layer (0.8 µm), AlN (0.5~1 nm) andAlGaN (5~40 nm) in this order. Devices having the above
mentioned structure were fabricated followed by characteristics evaluation. A layered structure of Ti/Al was used
for ohmic electrode and Pt/Au for Schottky electrode.
Device dimensions were 2 µm in the gate length and 10µm in the gate-drain distance, and devices with different
gate widths were evaluated. The maximum gate lengthwas 200 mm. After the fabrication processes, the devices
were evaluated for their characteristics using a curve tracer by Sony Tectronics and a semiconductor parameter
analyzer by Agilent.
2.4 Results of Device Evaluation
Figure 4 plots the dependence of the threshold voltage
on the thickness of the AlGaN layer. As can be seen, thethreshold voltage increases as the AlGaN layer decreases
in thickness, reaching 0 V at a thickness of 5 nm. Schottky
characteristics were evaluated between the gate-drain terminals, and Figure 5 shows the reverse characteristics of

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A Novel GaN Device with Thin AlGaN/GaN Heterostructure for High-power Applications
10

Vth (V)

-1
-2
-3
-4
-5
-6

0

10

20

30

40

50

AlGaN thickness (nm)Figure 4

Dependence of threshold voltage Vth on AlGaN thickness.

Current (A/mm)-5.0E-03

0.0E+00

0

-100

-200

-300

-400

-500

Voltage (V)Reverse characteristics of Schottky diode with a thin
AlGaN structure.

Lg=2 µm, Wg=400 µm, Lgd=10 µm
1.5E-01
Gm>70 mS/m

1.0E-03
Vgs=+2

1.0E-01

Ids (A/mm)

1.0E-01

Log (Ids)(A/mm)

Figure 5

starting from 2 V with a 1-V step, exhibiting a good normally-off characteristic. A maximum current gain of 70
mS/mm has been obtained. Figure 7 shows the gate voltage versus drain current characteristics of similar devices
using a logarithmic scale. An excellent pinch-off characteristic has been obtained, i.e. pinch-off voltage is virtuallynear 0 V and the leakage current then is not more than 10
nA/mm.
The multilayer interconnection method was used to
combine 500 elements of the device with a gate width of
400 µm to form a 200-mm device, and the drain current
versus voltage characteristics of the device are shown in
Figure 8. From the Figure where the cases of Vgs=1 V and
0 V are plotted, it can be seen that a good normally-off
characteristic has be obtained. Figure 9 shows the offstate characteristic of the same device, demonstrating a
breakdown voltage of 300 V or higher. Since the area of
the active region is 0.15 cm2, the specific on-resistance isestimated as 30 mΩ·cm2. Figure 10 shows, summarizing
the results described above, the relationship between the
breakdown voltage and on-state resistance, where theresults with the normally-on type are represented by red
circle, while those with the normally-off type by blacksquare. The results obtained this time are seen to havesignificantly outperformed the Si limit, demonstrating thata GaN-based normally-off type HFET has been realizedthat is higher in breakdown voltage and lower in loss incomparison to conventional Si devices.

1.0E-05

1.0E-07<10 nA/mm

Vgs=+1

1.0E-09-3

5.0E-02

0.0E+00

Figure 7

Vgs=0
0

5

10

15

20

Vds (V)

Schottky diodes with an AlGaN thickness of 5 nm. From
the Figure, it can be seen that a breakdown voltage of
450 V or higher is obtained. The leakage current here isapproximately 0.1 µA/mm, which indicates that the HFET
structure with an AlGaN layer thickness of 5 nm has
improved the pinch-off performance, and as a result, thereverse leakage current has been sufficiently suppressed.
Figure 6 shows the drain current versus voltage characteristics of selected devices having a gate length of 2 µm,
gate width of 400 µm and gate-drain distance of 10 µm.
The drain currents are plotted while the Vgs was changed-10
Vgs (V)1

2

3

Vgs versus Ids of selected HFET of normally-off operation.

5.0

Ids versus Vds of selected HFET of normally-off operation.

Vgs=+1 V
4.0
Current (A)Figure 6

-2

3.0
2.0
1.0
0.0

Vgs=0 V
0

Figure 8246Voltage (V)810Large current characteristics of normally-off HFET
device.

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A Novel GaN Device with Thin AlGaN/GaN Heterostructure for High-power Applications

Drain current (A/mm)

1.0E-01

5.0E-02

Vb > 300 V

Vgs = 0 V
0.0E+00

0

100

200

300

AlGaN/GaN HFET structure. With regard to the forward
characteristics, the current flows through the 2DEG channel formed on an AlGaN/GaN structure, so that due to themetal SM1 having a low barrier, an operation with lower
on-state voltage compared to the case of conventional
SBDs can be expected.
Meanwhile, the reverse characteristics of the FESBD
are shown in Figure 12. Since the channel is pinched off
due to the SM2 having a high barrier, the reverse characteristics are thought to be comparable with those of conventional SBDs. Thus, it is anticipated that the new structure results in an SBD of low on-state voltage while maintaining a high breakdown voltage.

Drain voltage (V)Figure 9Gate-drain breakdown characteristics of normally-off
HFET device.

Low Φ B metal (SM1)

On-state resistance (Ω·cm2)

2DEG

High Φ B metal (SM2)

Ohmic electrode

AlGaN
Undoped GaN

Deboy et al.

Current

Forward :Ohmic -like property
by low Φ B metal

Substrate

SJMOS
Onishi et al.

this work(normally-off)I-V property of FESBD

I

FESBD

Conventional(normally-on)

V
Figure 11 Mechanism for forward characteristics of FESBD.

Breaddown voltage (V)Figure 10 Correlation between on-state resistance and breakdown voltage.

3.

DEVELOPMENT INTO FESBD WITH
THIN AlGaN STRUCTURE

In order to realize a high-efficiency power supply, a highspeed diode is needed as well as an FET, and GaN-based
diodes are expected to offer such a high switching speed
due to their solid state properties. To this end, we havedeveloped a diode capable of operation that is low in onstate voltage. The diode, which is commonly called field
effect Schottky barrier diode (FESBD) 9), has been developed by application of reduced-thickness AlGaN structure
based on our proprietary technology. The results will bepresented below.
3.1

Mechanism for High Breakdown Voltage and Low
On-state Voltage Operation of FESBDIn FESBDs, low on-state voltage is achieved by using a
new electrode structure to replace the Schottky electrodeof ordinary Schottky barrier diodes (SBDs), where a metal
having a low Schottky barrier is embedded in a metal having a high Schottky barrier. Figure 11 shows a schematicof FESBD together with a schematic description of theworking principle in the forward direction. The electrodestructure comprises a metal SM1 having a low Schottky
barrier enfolded in another metal SM2 having a highSchottky barrier, and the structure is formed on the

Low ΦB metal (SM1)

Pinch-off effect byhigh ΦB metal

Suppression ofleakage current(High VB)High Φ B metal (SM2)

Undoped GaN

Ohmic electrode

Leakage current
BufferSubstrate

I-V property of FESBD
FESBDConventional SBD

Figure 12 Mechanism for reverse characteristics of FESBD.

3.2 Fabrication Method of Device
The FESBD device is fabricated using a similar method asfor HFETs described earlier except for the formation of
Schottky electrode. In this development, Ti-based electrode having a low Schottky barrier and Pt-based electrode having a high barrier were used for SM1 and SM2,
respectively. Since the rest of the fabrication process iscompatible with both FESBD and HFET, it would be possible to integrate both of them in the future.
3.3 Results of Characterization of FESBD Device
Figure 13 shows the dependence of the device breakdown voltage on the SM1/SM2 electrode width ratio. The
SBD structure using SM2 only (excluding SM1) is seen tohave a breakdown voltage of 400 V, which shows, as the

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1.E-01
AlGaN
i-GaN

400
300

Structure

200

Ohmic

100
0

SM2
0

20

40

60

80
Ratio of SM1/SM2 (%)

100

SM1

Forward current (A)Breakdown voltage (V)A Novel GaN Device with Thin AlGaN/GaN Heterostructure for High-power Applications

1.E-02FESBD
1.E-03

Conventional
SBD

1.E-04

Schematic drawing of FESBD
1.E-05Figure 13 Breakdown voltage versus SM1/SM2 ratio.

4.

CONCLUSION

The normally-off FET and the low on-state voltage diodehave been developed using a GaN-based HFET structure
on a Si substrate. The reduced-thickness AlGaN structure
has been employed to obtain the normally-off FET with athreshold voltage of 0 V. Its pinch-off characteristics aresuperior and the breakdown voltage is over 300 V.
Moreover, the reduced-thickness AlGaN structure has

On-state voltage

0V

Figure 14 Comparison of FESBD and SBD for forward
characteristics.
1.0E+00
1.0E-01
1.0E-02

Ir (A)

1.0E-03
1.0E-04
AlGaN 20 nm (SBD)

1.0E-05
1.0E-06
1.0E-07

AlGaN 5 nm (FESBD)
AlGaN 5 nm (SBD)

1.0E-08
1.0E-09
1.0E-10

0

20

40

60

80

100

Vr (V)

Figure 15 Difference in reverse characteristics of several
structures.
10

1
Current (A)ratio gradually increases until about 90 %, little change
maintaining the nearly equal value as for the case of SM2
only. This behavior indicates that even a small fraction of
SM1 is sufficient to maintain breakdown voltages bymeans of pinch-off effects. At the ratio of 95 % and higher,
however, the breakdown voltage tended to decrease. The
forward current dependence was also investigated indicating a linear relationship increasing with the area ofSM1, so that it is desirable for the SM1/SM2 ratio to be aslarge as possible. Considering the trade-off relationshipbetween the breakdown voltage and forward current, theoptimum value for the ratio would be approximately 90 %.
Figure 14 compares the forward currents of conventional SBD and FESBD. It is seen that in the conventional
structure the current begins to rise at around 1 V due tothe offset caused by the height of barrier, while in the caseof FESBD the current rises at approximately 0 V. On theother hand, Figure 15 shows the reverse characteristics ofthree different structures: conventional normally-on type
SBD using an HFET epitaxial substrate with a 20-nm
AlGaN layer; SBD using SM2 only with an AlGaN layer
reduced to 5 nm in thickness; and FESBD with a 5-nm
AlGaN layer. It can be seen that a reduction in the leakage
current of about one to two orders of magnitude has beenachieved in comparison to the case of 20-nm AlGaN layer.
This is considered to result from the difference in the voltage required to pinch off the 2DEG channel. Thus, thevalidity of FESBD principles and the usefulness of the
reduced-thickness AlGaN structures have been confirmed.
Figure 16 shows the forward characteristics of the large
device with a Schottky electrode width, i.e. corresponding
to gate width, of 200 mm. The current begins to rise at
approximately 0 V and reaches a maximum of around 7A, demonstrating that a large-current operation is also
possible with the FESBD structure.

Forward voltage (V)

0.1

0.01

0

0.5

1

1.5

2

2.5

Voltage (V)Figure 16 Forward characteristics of large current FESBD.

been applied to the FESBD of Furukawa's proprietary
structure, thereby realizing a high breakdown-voltage andhigh-current operation. Hereafter, we intend to apply
these devices to various circuits in order to promote
developments aimed at realization of high-efficiency
power supplies.

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A Novel GaN Device with Thin AlGaN/GaN Heterostructure for High-power Applications

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Normally-off operation GaN HFET using a thin AlGaN layer for low
loss switching devices.” Mater. Res. Soc. Symp. Proc. Vol.831,
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ISPSD, pp.58-61, 2003.
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