Outlook for GaN HEMT Technology

 Kazukiyo Joshin

 Toshihide Kikkawa

 Satoshi Masuda Keiji Watanabe

It is expected that the high electron mobility transistor (HEMT) using gallium nitride (GaN) as
its wide band gap semiconductor will be applied in diverse, green ICT systems because of its
high efficiency. The GaN HEMT utilizes high-density two-dimensional electron gas (2DEG) accumulated in the boundary layer between GaN and AlGaN through their piezoelectric effect andnatural polarization effect. This makes it possible to realize a low on-state resistance (Ron).
Combined with a high breakdown voltage, the GaN HEMT indicates a superb performance asa power device. After the development of GaN HEMT technology started for power ampliﬁers
of mobile base stations, it was expanded to a radar sensor application. Further expansion of
its application is expected in the field of power conversion, in equipment such as server powersystems. While the development of GaN HEMT technology has been promoted, focusing on“high output power” conventionally, further advantages such as high efficiency and low energy
consumption have been attracting much attention in recent years. In this paper, the outlook
for GaN HEMT technology is described in terms of its contribution to energy saving.

1. Introduction
Commercialization of gallium nitride (GaN) is
observed in various areas including LED lighting andgreen-colored lights of traffic signals. Also in terms ofelectronic devices, application of high electron mobility transistors (HEMT) using GaN in the field of green
ICT systems is considered very promising. Fujitsu
Laboratories is a forerunner in the development of
HEMT, globally. The development of compact-size antennas for broadcasting satellites by Fujitsu gatheredpublic attention in the 1980s and Fujitsu has been aleader in this technology area since then. The basic
configuration of GaN HEMT is shown in Figure 1. GaN
HEMT utilizes high-density two-dimensional electrongas (2DEG) accumulated in the boundary layer between GaN and AlGaN through their piezoelectric effectand natural polarization effect. This makes it possible
to realize a low on-state resistance (Ron) as its transistor
characteristic. Combined with a high breakdown voltage as its wide band gap semiconductor, GaN HEMT
shows a superb performance as a power device.
High-frequency GaN HEMT is an indispensable
138

component for power amplifiers for transmission of
wireless communication systems and radar systems,
which need high-output-power and high-efficiency performance. For instance, mobile base stations require
high-output and high-efficiency power devices of a
100 W class. To achieve power in the order of 100 W,
a parallel configuration of multiple devices is needed
if conventional GaAs MESFET or Si LD-MOSFET is used.
On the other hand, thanks to its high-output-power
performance, GaN HEMT can achieve 100 W or higher
with just a single device, making high-efficiency andpower-saving power amplification possible.
Recently, GaN HEMT R&D has been expanding
to power conversion electronics. Their operation frequency is less than several ten MHz in contrast to the
GHz operation of high-frequency power amplifiers.
While the development of GaN HEMT has been promoted, focusing on “high output power” conventionally,
its focus is shifting to further advantages such as highefficiency and low energy consumption.
In this paper, the contribution of GaN HEMT technology is described in terms of energy saving.
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K. Joshin et al.: Outlook for GaN HEMT Technology

To operate a transistor circuit with high efficiency,
it is ideal if either the current or voltage that runs during the transistor operation can be made zero at any
time. Then, the product of current and voltage (i.e.,
the power consumption of the transistor) becomes
zero, and you can achieve 100% operation efficiency.
However, in reality, there is always a finite value ofresistance, Ron, during the on-state of the transistor so
the power consumption cannot become zero. Besides,
parasitic capacitances of transistors make instantaneous switching impossible and cause a power loss.
Compared to the silicon (Si) transistors widely used inamplifiers and switching power systems, GaN HEMT
shows a lower power loss during the on state (on-state
power loss) with low Ron based on high-density 2DEG.

Source

Gate

Drain

n-GaN
AlGaN
2DEG

i-GaN
Buffer layer
SiC or Si substrate

Figure 1
GaN HEMT basic configuration.

The power loss at switching between the On state andOff state is also low based on low parasitic capacitances. These two characteristics make it possible for
GaN HEMT to operate with lower power consumption
than conventional Si transistors. In power electronics
applications, GaN HEMT is considered very promising
as an electronic component for a switched-mode powerdevice. The issue to be addressed in this applicationis the “ringing” that occurs during the switching due toits high-speed response property [Figure 2 (a)]. It isimperative to suppress this ringing, which is a causeof unnecessary power consumption. Meanwhile, inhigh-frequency amplifier applications of several hundred MHz or higher, the device should be operated
with high efficiency by controlling a load impedance sothat the product of current and voltage of the transistor
becomes zero. For instance, in the case of the inverse
class-F operation, where the load impedance is tuned
to be infinite at even harmonic frequencies of the signal frequency (fundamental frequency) while it is tuned
to be zero at odd harmonic frequencies, the waveformof transistor drain current becomes rectangular andthat of the drain voltage becomes half-wave rectified[Figure 2 (b)]. Because the waveform of the drain current is rectangular and the peak current level is low, it
is possible to reduce power consumption and voltage
drop due to transistor Ron or resistance in the feed line.
This makes it possible to realize excellent efficiency.
On the other hand, the transistor should have highbreakdown voltage because the peak value of the drain

00.1

Ringing

0.2

0.3

0.4

0.5

Drain voltage VD (V)Drain current ID

Drain voltage VD

VDID

VDS = 20 V, Frequency = 2 GHz

60

0.3

VD
ID

40

0.2

0.1

20

0

0

0.2

0.4

0.6

0.8

Time (µs)

Time (ns)(a) GaN HEMT for switching power supply(b) GaN HEMT for high frequency amplifier

1.0

Drain current ID (A)2. High-efficiency operation

0

Figure 2
Waveforms of drain voltage and drain current.
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K. Joshin et al.: Outlook for GaN HEMT Technology

voltage may become three times as high as the DC
supply voltage (VDS). Because a high breakdown voltage is one of the superior characteristics of GaN HEMT,
it is an ideal electronic component for high-efficiency
operation. Comparing the switching waveforms reveals
that an overlap of current and voltage remains in highfrequency amplifiers where the efficiency remains atapprox. 70% normally, while it is possible to achieve
90% or higher in the switch-mode power supply.

3. High-frequency GaN HEMT technology
As examples of high-frequency GaN HEMT monolithic microwave integrated circuits (MMIC) technology,
a 10 GHz one-chip transceiver and a 6–18 GHz wideband MMIC amplifier are explained below.
Recent wireless communication systems andsensing systems use various frequency resources andtheir transceivers are composed of different devices
respectively. If a communication chip that integrates
the functions of such transmitter and receiver is developed, the system size and power consumption canbe reduced. To handle high-output-power transmitter
signals and weak receiver ones on a single chip simultaneously, it is essential to efficiently switch between
the transmitter signal and receiver signal with minimum loss while preventing the transmitter signals frominterfering with the receiver signals. It was difficult
to meet both these requirements with conventional
technologies.
Our innovative transmitter/receiver switch using
GaN HEMT can address this issue. Despite its compact
size (1.8 mm × 2.4 mm), the device achieved the highest performance in the world with a transmission loss
as low as 1.1 dB in the range of 0–12 GHz. Drastic size
and weight reductions (1/10 or less) were achieved
compared to the conventional transmitter/receiverswitch system using a magnetic substance.
Further, the transistor is shielded against unnecessary signal radiation through arranging groundvia holes around the transistor, and unnecessary
signal interference was suppressed by appropriately
arranging signal wiring based on 3D electromagnetic
simulation. These innovations have ensured stable operation of a high-output-power circuit without causing
erroneous oscillation. Using these technologies, a single-chip transceiver MMIC was developed on a pilot basis(Figure 3). In this device, a transceiver switch, a140transmitter amplifier and a receiver amplifier are integrated. While realizing an output of 6.3 W in the
10 GHz band, the chip size could be miniaturized successfully to 3.6 mm × 3.3 mm, which is 1/10 of theconventional devices using multiple chips.1)
Many conventional wireless systems including
aircraft radars use multiple frequency resources anddevices for each frequency band. However, if a single
amplifier can cover a wide frequency range, then energy saving, size miniaturization and weight reductionof radar systems or wireless communication systems
can be realized. In the case of aircraft radars, thesystem is switched among multiple bands as below: Cband (4–8 GHz) suitable for rainy weather conditions;and X band (8–12 GHz) and Ku band (12–18 GHz)
where objects can be measured with high accuracy. If
there is a single amplifier covering the range from the
C band to the Ku band, a multiple-unit configuration
becomes obsolete and system downsizing as well as
energy saving is achieved. In the conventional system,
an amplifier circuit comprised of a parallel connectionof multiple GaAs transistors is used. While the output
power level can be increased by increasing the number of transistors, this results in larger wiring loss due
to prolonged circuit wiring. Therefore, it is difficult tocover the bandwidth up to 18 GHz with a system using
GaAs amplifiers.
By developing a circuit of power divider and combiner with a super-wide bandwidth on a semiconductor
substrate, an output power level of as high as 12.9 W
could be realized in a wide bandwidth (6–18 GHz) ina GaN HEMT MMIC technology. The figure of 12.9 W
is twice that achieved by the conventional approach.2)
Further, by developing a receiver amplifier using GaN

Transmitter
amplifierAntenna

Receiveramplifier
Chip size: 3.6 × 3.3 mm

Transmitter/receiverswitch system

Chip configuration

Figure 3
GaN single-chip transceiver MMIC for 10 GHz band.
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K. Joshin et al.: Outlook for GaN HEMT Technology

HEMT, the highest level of performance in the world
could be realized with a signal gain of 16 dB and noise
figure of 2.3–3.7 dB with a size as small as 2.7 mm
× 1.2 mm in the 3–20 GHz range. Besides, a highoutput-power, compact size transceiver module wasdeveloped by using MMICs with the aforementioned
properties (Figure 4). While miniaturizing the transceiver module size to 12 mm × 30 mm, 10 W output ina wide bandwidth (6–18 GHz) was realized.3) By using
this technology, multiple frequencies can be used withjust a single transceiver module. System integration
is promoted for radar and wide-band communication
systems where multiple frequencies are used, and miniaturization, weight reduction and energy saving of thesystem will become possible.

4. Power GaN HEMT technology
Because the Power GaN HEMT shows a lower Ronand the switching speed is faster compared to the conventional Si power devices, it is expected to improve
power conversion efficiency and contribute to systemminiaturization. Specifically, if the switching frequency
for the power supply can be set to a range that exceeds
1 MHz from the conventional range of less than several
hundred kHz, miniaturization, weight reduction andenergy saving of capacitor and magnetic components
such as transformers and inductors will be achieved.

GaN transmitter amplifier

GaN receiver amplifier

12 mm

30 mm

Pre-amplifier

Figure 4
GaN HEMT transceiver module for wide band.
FUJITSU Sci. Tech. J., Vol. 50, No. 1 (January 2014)

Ron of power GaN HEMT can be reduced by applying a positive voltage on the gate electrode to increase
the 2DEG density. However, in the above-mentioned
high-frequency GaN HEMT configuration, its gate positive voltage is limited to as low as about 2 V, because a
Schottky barrier junction is adopted (i.e., a gate metalelectrode is formed directly on the semiconductor) andits large gate leakage current. To overcome this issue,
an insulated gate configuration was developed, wherea dielectric insulation layer is inserted between the interface of the gate metal and the semiconductor. As aresult, a gate voltage higher than 2 V could be appliedand a low Ron characteristic could be achieved. In addition, tolerance for high peak voltage of the unexpectedringing could be improved at switching operation.
Al2O3 with a wide band-gap property was adopted as a material for the gate insulation layer, andthe atomic layer deposition (ALD) method is used forlayer formation. The ALD method is an approach toobtain a flat and uniform insulation layer by controlling layer thickness at an atomic-layer level. Because
the conventional compound semiconductors such as
GaAs or InP have a large amount of electron traps on
the interface between the insulation layer and semiconductor, it is very difficult to adopt an insulated gateconfiguration such as Si MOSFET. On the other hand,
while GaN shows relatively few interface traps, it is still
inadequate to adopt an insulated gate configuration.
The interface trap impacts the stability of voltage thatturns on GaN HEMT (Vth). When the gate voltage is applied to positive, electrons are trapped in the interface
between the insulation layer and the semiconductor,
which results in shifting of Vth.4) We discovered that themain factor of this Vth shifting is the AlOH impurities in
Al2O3 and Ga oxide in Al2O3/GaN interface. By removing
these substances, it was possible to reduce the Vth shifting to 0.25 V from about 2 V.5)
Reducing the production cost is also an important factor for power GaN HEMT commercialization. In
concerted efforts with Fujitsu Semiconductor, we havedeveloped a GaN-on-Si technology that allows cost
reduction by adopting a large diameter Si substrate.
Especially, we have developed an epitaxial growthtechnique of high-quality GaN on a 6-inch Si substrate
as well as an optimal electrode configuration thatcan suppress Ron elevation at switching operation. A
prototype power supply circuit was developed for an141

K. Joshin et al.: Outlook for GaN HEMT Technology

evaluation of the developed power GaN HEMT andsuccessful results were obtained in a power conversion
efficiency exceeding those of the conventional Si powerdevices. Further, the server power supply system developed by Nakao et al., installing power GaN HEMTs in its
power-factor-correction (PFC) circuit,6) demonstrated ahigh output power operation up to 2.5 kW (Figure 5).
The total efficiency level with an input of AC 230 V andoutput of DC 12 V reached 94.3% at its peak. These
achievements make the high-power GaN HEMT apromising solution for green ICT system applications.
Further, a contribution to the low carbon society may
be expected by using this technology for power conditioners used in photovoltaic power generation, airconditioner power supplies and so forth.

5. Future prospects
In addition to the miniaturization and energy saving of the above-mentioned wireless communicationand power conversion systems, the use of technologies based on GaN HEMT is expected to cover wider
areas in future. While no reference is made in thispaper, expansion of this technology in the machineto-machine (M2M) field is expected through the use ofcloud networking based on various sensing functions.
In the area of sensing technology, diverse applications
are being examined including those in environment

research, biotechnology and structural analysis bypositive use of high-speed, high-sensitivity characteristics of the GaN HEMT. Through penetration in the
M2M and ICT fields, GaN HEMT is expected to make agreat contribution in new areas by supporting the intelligent social infrastructure from the device level. A
smart city system, vehicle/traffic systems, food industry,logistics, agriculture, healthcare/welfare industries,
environment systems and education systems are someexamples where this technology is expected to make acontribution.

6. Conclusion
In this paper, the frontier of GaN HEMT technology in high-frequency amplifier and power conversion
applications was reported while focusing on its advantage of “energy saving.” Because of its low on-state
resistance and high-speed switching performance, GaN
HEMT has a possibility to miniaturize device size andminimize power consumption with low cost throughfurther sophistication of the GaN HEMT technology andpromotion of circuit development that maximizes theseadvantages. This will open up a bright future for thistechnology in novel fields. We are committed to realize
a low carbon society by making proposals to improvevalues of various ICT systems by using this device.

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95.0

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AC-DC power output (W)Figure 5
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FUJITSU Sci. Tech. J., Vol. 50, No. 1 (January 2014)

K. Joshin et al.: Outlook for GaN HEMT Technology

Kazukiyo Joshin
Fujitsu Laboratories Ltd.
Mr. Joshin is currently engaged in researchand development of GaN HEMT.

Satoshi Masuda
Fujitsu Laboratories Ltd.
Mr. Masuda is currently engaged in research and development of GaN HEMT.

Toshihide Kikkawa
Fujitsu Laboratories Ltd.
Mr. Kikkawa is currently engaged in research and development of GaN HEMT.

Keiji Watanabe
Fujitsu Laboratories Ltd.
Mr. Watanabe is currently engaged inresearch and development of GaN HEMT.

FUJITSU Sci. Tech. J., Vol. 50, No. 1 (January 2014)143