J A N U A R Y
2 0 1 2
I S S U E
N ° 4
I N D U S T R Y
Ramping production: EpiGaN is
aiming to begin producing 6-inch
GaN-on-silicon epitaxial wafers
in the first quarter of 2012.
(Courtesy of EpiGaN/Arnaud
Collette)
R E V I E W
GaN-on-silicon wafers: the enabler
of GaN power electronics
Low cost silicon substrates have brought the first GaN devices to market. AZZURRO
Semiconductors, EpiGaN, NTT Advanced Technology, Oxford Instruments Plasma
Technology and Powdec discuss what remains to be done to further penetrate into
the silicon-dominated power electronic world.
“G
aN in power semiconductors is one of
the buzzwords the electronics industry,
in terms of the numbers of people
talking about it.” So says Chris Hodson, product
manager for power semiconductors at semiconductor
equipment producer Oxford Instruments Plasma
Technology (OIPT), based in Yatton, UK. In fact, today
it’s more than just a buzzword – it’s a commercial
reality, even if the available GaN transistors are
only just beginning to wrest a first small share
in the power electronics market from silicon. El
Segundo, California’s International Rectifier and EPC
Corporation are behind the highest profile initial GaN
devices, rated up to 200 V, to stake claims in this
sector. And both companies’ offerings have a major
underlying characteristic in common that helps make
them commercially viable: they are produced starting
from comparatively low cost silicon substrates.
As well as keeping raw material costs to a minimum
for device makers, such substrates also allow them to
exploit the same tools currently used with silicon for
6
many back-end processes. This promise is attracting
many other companies, creating the “buzz”. But
even with the increasing popularity of GaN-on-silicon
technologies, are they really the ideal solution for
power electronics that they seem? What challenges
remain, and can any other substrate technologies
compete?
The investors backing Germany’s AZZURRO
Semiconductors were convinced enough in the
potential of the company’s GaN-on-silicon epitaxial
wafers to provide €14.5 million ($17.9 million)
funding in November 2010. It used that money to
establish a new site in Dresden, which it moved to
from its original Magdeburg facility in summer 2011,
and where it is now ramping up production to reach
maximum output in summer 2012. The company
also supplies epiwafers to the high-brightness LED
sector, but Erwin Ysewijn, vice-president, sales and
marketing, at AZZURRO noted that customers in the
power electronics industry have different needs.
P O W E R
D e v ’
I S S U E
Tokyo, Japan based NTT Advanced Technology
(NTT-AT) already supplies GaN-on-silicon
epitaxial wafers with breakdown voltages
up to 1,800 V for RF applications. “Now we
are at 6-inch, but we are developing 8-inch
epiwafers,” said Takashi Kobayashi, executive
engineer for the nanotechnology business
unit at NTT-AT. Kobayashi noted that with
breakdown voltages largely dependent on the
thickness of the epilayers, this should be readily
translated to power electronic applications. And
while most of the company’s power electronics
customers are still in development, he also
emphasises that through a manufacturing
collaboration with fellow Japanese company
Mitsubishi Chemical, NTT-AT already has ample
Expanding capability: having offered 6-inch
GaN-on-silicon wafers since 2005, AZZURRO
Semiconductors says 200 mm wafers should be
"technically feasible" in 2012.
(Courtesy of AZZURRO Semiconductors AG)
“The wafers which we ship need reasonably
large diameters because I don’t think that
to date there is any power semiconductor
maker who has any interest in 2-inch or 4-inch
wafers,” Ysewijn said. “They all want 150
mm and a reasonable outlook for 200 mm
and that’s what we can accommodate.” 200
mm GaN-on-silicon epiwafers from AZZURRO
will be technically feasible in the first half of
2012, Ysewijn said. “Commercial shipments
will depend on market requirements, which are
certainly different between the LED and highvoltage market,” he noted.
Hasselt, Belgium’s EpiGaN, an epiwafer
producing company spun out of nearby giant
semiconductor research center Imec, likewise
raised €4 million ($5.1 million) from investors
in July 2011. Marianne Germain, chief executive
officer at EpiGaN, explained that her company
was also using the funding to establish
production facilities, which will commence
operation in the first quarter of 2012.
Even before that facility opens, EpiGaN claims
it can already make GaN-on-silicon substrates
capable of producing devices that reach the
breakdown voltage seen as pivotal in driving
uptake of GaN. “Today we deliver 6-inch
600 V wafers,” she said. “This is where we think
it will be the most easy to outperform silicon,
and this is for us the highest priority. From there
we have two development routes: one to move
to 200 mm, and one to go to voltages more
than 1,000 V.”
P O W E R
D e v ’
N ° 4
J A N U A R Y
2 0 1 2
market within the next year. “In order to obtain
high breakdown voltage between the drain and
the silicon substrate, a thick AlGaN/AlGaN multilayer buffer, typically 4 to 5 μm thick for 600V
class, is needed,” Kawai said. “Such thicker
buffers induce high epitaxial cost.”
Aggressive on passivation
Kawai feels that there are other, even bigger
problems, to be faced too. “Current collapse
is a severe issue encountered in today’s highvoltage GaN devices, and not yet resolved
enough,” Kawai said. “The large electric field
near the gate edge kicks channel electrons into
both the surface and the GaN layer, resulting
in an increase of the channel resistance. Metal
“Today we deliver 6-inch 600 V wafers. This is where
we think it will be the most easy to outperform silicon,
and this is for us the highest priority,”
says Marianne Germain, EpiGaN.
production capacity to support them when they
reach the commercialisation stage. “We’ve
transferred manufacturing technology so that
they can produce epiwafers with the same
quality as NTT-AT, but we NTT-AT can sell them,”
Kobayashi said. “Together, we can produce a
few thousand wafers a month right now.”
But Hiroji Kawai, president and co-founder of
the Yokohama, Japan, GaN wafer producer and
device developer Powdec, is doubtful that
600 V voltages and higher will reach the market
quickly. While his company has developed
1,100 V breakdown voltage GaN transistors, he
doesn’t expect commercial production on large
diameter silicon wafers until 2013 or 2014. He
also says that he is “conservative” on the chances
of any 50 A, 600 V GaN devices appearing on the
field plate technology associated with a high
quality passivation film could accommodate the
collapse, but it’s not yet enough in practice.”
With passivation, where oxides or nitrides are
deposited as protection onto devices, potentially
part of the solution to this problem reaches
beyond epitaxial structure. As a supplier of
etching equipment for GaN in the LED industry
that are readily converted to power electronics
processes, OIPT is well equipped to supply
the means to solve device processing issues.
It also sells plasma-enhanced CVD tools for
passivation in high-power silicon devices, and is
now seeing its ALD equipment gaining interest
in association with GaN. “An ALD dielectric can
provide a high interface quality leading to lower
leakage and higher breakdown in devices, and
Crushing current collapse: Japan’s Powdec has developed 1,100 V transistors with a superjunction
field plate using a GaN/AlGaN/GaN polarization junction concept (left) which is more effective at
preventing current collapse than a conventional metal field plate (right). (Courtesy of Powdec)
7
J A N U A R Y
2 0 1 2
I S S U E
N ° 4
it’s being used for GaN-on-silicon based devices
for surface passivation also, but also to provide
some isolation for the higher voltage devices,”
Hodson said.
While etching and passivation steps might
usually be considered as processes undertaken
by device makers, Germain said that passivation
is part of the offering available in EpiGaN’s GaNon-silicon wafers. “After properly controlling the
stress in the material, the second thing that we
consider as important is the passivation,” she
explained. “We cap the wafer with in-situ SiN,
deposited by MOCVD as part of the epiwafer
growth process and this gives a strong advantage
in terms of device performance, controlling the
traps, and also in terms of device reliability.
There are very strong indications that the insitu SiN is increasing the material’s robustness.
Stressing the device at high temperature leaves
the material completely unchanged if you cap
it with SiN grown in-situ. Whereas if you have
an AlGaN/GaN HEMT structure with a GaN
cap at some point it starts relaxing and you
destroy your channel properties. This is a key
advantage, which we will believe will solve one
of the biggest problems, the dynamic behaviour
of the devices.”
Another hurdle that Powdec’s Kawai says
GaN-on-silicon power devices must overcome
is attaining adequate yields for high power
High mobility: NTT-AT supplies 6-inch GaN-on-silicon wafers with an AlGaN/GaN HEMT
epitaxial structure. (Courtesy of NTT-AT)
though the reverse current is larger than that
of its SiC counterparts,” he said. “However, this
can be resolved by implementing the ‘junction
barrier’ structure same as the structure
employed by SiC.”
And OIPT’s Hodson noted that before GaN-onsilicon technology broke through, freestanding
GaN substrates had been considered a potential
starting point for making power devices.
“We’ve had conversations with the people on
power semiconductors, and there’s interest in
“Current collapse is a severe issue encountered in today’s
high-voltage GaN devices, and not yet resolved enough,”
Hiroji Kawai, Powdec,” said Hiroji Kawai, Powdec.
devices. “Surface defect contamination during
MOCVD growth is one of the major factors
determining die yield, with the die cost
increasing rapidly with die size,” he said. “The
size of an LED die is smaller than 0.1 mm2,
whereas transistors capable of about 10 A
have a die size as large as several mm2. I
do not know whether the present and future
commercial reactors can produce such a large
die with high yield.”
Powdec, by contrast, is bringing a GaN Schottky
diode to market that is deposited on a sapphire
substrate. His company grows the GaN layers
using an epitaxial lateral overgrowth method
that it says delivers a dislocation density several
hundreds of times lower than conventional
approaches. “Our purely vertical diode sustains
more than 800 V with very small on-resistance,
8
freestanding GaN if it’s cost-effective,” he said.
“But that’s always been the barrier, they’re
great, but if you’ve got to buy a 2-inch GaN at
a market price in excess of $2,000, it doesn’t
make commercial sense usually.”
As well as GaN-on-silicon, NTT-AT also supplies
epitaxial wafers up to 4-inch diameter deposited
on sapphire and SiC and 2-inch diameter on
freestanding GaN. GaN-on-silicon is really the
only choice for Kobayashi’s power electronic
customers, however. “GaN-on-SiC is used in
power amplifiers in base stations, but the SiC
substrate price is very expensive,” Kobayashi
said. “We don’t think it’s used in power devices
because the competitor is silicon. Usually to
compete with silicon, they try to use large
diameter substrates, at least 6-inch.”
Kobayashi also suggests that defects in GaNon-silicon aren’t such a problem for yield, but
admits there could be concerns for long-term
performance. “2-5 x109 is our typical dislocation
density,” he said. “That level does not affect
device performance or I-V characteristics, but
we are not sure if it affects reliability – that’s
still in development.” Optimising GaN growth
conditions also helps deal with the dynamic
behaviour problems seen in the devices.
“Current collapse is influenced both by epilayer
Deviation minimised
In 2008 OIPT acquired Silver Spring, Maryland,
based TDI Inc. and its HVPE technology, which
rapidly deposits thick GaN layers. Now, Hodson
says, it is preparing to exploit this approach
in tools intended to make GaN wafers more
accessible. “We’re pursuing a multi-wafer HVPE
approach to produce freestanding GaN with the
potential for low dollar-per-wafer numbers,” he
said.
Wafer homogeneity: thickness mapping of an
EpiGaN wafer’s uniformity obtained by in-situ
metrology. Uniformity is 0.6% over the 150 mm
III-Nitrides layer, with 5mm edge exclusion.
(Courtesy of EpiGaN)
P O W E R
D e v ’
I S S U E
Power GaN main players and related products
(Power GaN report, Yole Développement, Feb. 2012)
Company
AZZURRO (GE)
BeMiTec (GE)
Dowa Electronics Materials (J)
EPC (US)
EpiGaN (B)
Furukawa (J)
GaN Systems (CA)
International Rectifier (US)
MicroGaN (GE)
NTT (J)
OnChip Power (US)
Panasonic (J)
Powdec (J)
Sanken (J)
Sumitomo SEI (J)
Toshiba (J)
Transphorm Inc. (US)
GaN epiwafer
X
GaN device
FL
X
X
X
X
X
X
FL
X
X
X
X
X
X
X
X
X
FL
X
X
X
X
X
X
FL: Fab-less business model. Design only
Power GaN pure-players
quality and device technology,” Kobayashi said. “We have improved
epilayer quality to prevent current collapse.”
But AZZURRO’s Ysewijn feels that delivering consistent epiwafer
quality is even more important than “only” minimising defect density
– although this is still very important. “Yes, the better the quality of
the material, the higher the breakdown voltage, the lower the onresistance,” he commented. “AZZURRO’s first target on a specification
is to get a good performance on all the parameters on all the wafers
with minimum deviation. It’s better to have a stable quality meeting
90 per cent of all the parameters than to have one wafer meet 100 per
cent of the parameters but the next five wafers will not meet any of
the parameters. The GaN world already wants to produce thousands
of devices reproducibly. We supply stable materials with quality control
material to the device makers.”
Such a focus underlines that the ability to grow GaN epitaxial layers
on silicon substrates is well established and becoming increasingly
commercialised. And while device design problems remain to be solved
the sheer weight of researchers and companies studying them mean
that the question must be not if, but when, GaN-on-silicon power
electronics will be successful.
Andy Extance for Yole Développement
N ° 4
J A N U A R Y
Marianne Germain, Co-founder and CEO, EpiGaN
Germain received the M.S. degree and PhD degree in
Electrical Engineering from the University of Liege (ULg)
in 1993 and 1999, respectively. In 2001, she joined IMEC
in Leuven (BelgiumSince 2004 and 2010, she was Program
Manager of IMEC’s “Efficient Power” program, which
aimed at developing high-efficiency solutions for power
applications beyond the Si limits enabled by GaN-on-Si
technology. She pursued training management course in Vlerick Management
School (Gent) in 2008/2009. In may 2010, with her colleague Joff Derluyn
and Stefan Degroote, she co-founded EpiGaN, a spin-off based licensing imec
technology which manufactures and sells GaN-on-Si epiwafers for electronic
devices.
Chris Hodson, Product Manager for ALD
and power semiconductors, Oxford Instruments
Plasma Technology
Hodson has worked at Oxford Instruments for over 10 years
specialising in deposition process techniques including
PECVD, ICP-CVD and ALD. Chris is responsible for the Power
Semiconductor products and processes which include etch
and deposition processes for silicon based devices as well as wide-bandgap
devices.
Dr. Hiroji Kawai, Co-founder and President,
Powdec K.K.
Dr Kawai co-founded Powdec in 2001 to produce high
quality GaN wafers and to develop next-generation GaN
power devices with drastically lower power consumption.
His expertise lies in compound semiconductor devices,
starting GaN research in 1994. He also specializes in high
quality growth techniques such as MOCVD and equipment
design. Dr Kawai spent 30 years of his career at the Sony Research Center in
Yokohama, prior to the founding of Powdec, where he served as the General
Manager and Principle Research Scientist. He received his PhD in electrical
engineering and Bachelor of Science in chemical engineering from Shizuoka
University, Japan.
Takashi Kobayashi, Executive Engineer,
NTT Advanced Technology’s Nano-Technology
Business Unit.
Kobayashi graduated from Tohoku University in Sendai
Japan in 1981. Then he joined NTT Laboratories to study
III-V compound semiconductor materials such as GaAs,
InP, and GaN for electric device applications. He moved
to NTT Advanced Technology Corp. (NTT-AT) in 2008 to manage its GaNbased epiwafer business for power electronics applications. Now he is now
responsible for the GaN epi business section in NTT-AT.
Erwin Ysewijn, Vice-President, Sales and Marketing,
Azzurro Semiconductors
Ysewijn started his marketing and sales career in 1989
working at Hitachi Semiconductors, getting familiar with
Japanese and international business culture. Since then
the spent most of his time in Asia, covering among others,
Korea, India, Taiwan and China, working on marketing,
sales and strategic M & A projects, excelling in intercultural topics and growing
the business. In his last position before joining Azzurro he was heading the
global marketing activities at Lantiq.
Protecting performance: an ALD dielectric, produced using Oxford
Instruments Plasma Technology’s FlexAL tool shown here, can provide a
high interface quality leading to lower leakage and higher breakdown in
devices. (Courtesy of Oxford Instruments Plasma Technology)
P O W E R
D e v ’
2 0 1 2
9