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发帖数: 555 | 1 Silicon Carbide: Smaller, Faster, Tougher
Meet the material that will supplant silicon in hybrid cars and the electric
grid
Illustration: Anatomy Blue
Some technological revolutions are flashy, and some are almost invisible. We
’re quite familiar with the flashy ones; they’ve given us powerful
computers we can hold in the palms of our hands, devices that can pinpoint
our locations by way of orbiting satellites, and the ability to bank and
shop without leaving our homes.
But none of these innovations would have occurred without the technology
that delivers power to them. Over the last half century, a more subtle
revolution in power electronics has provided us with compact and efficient
semiconductor devices that can manipulate, regulate, and convert electricity
from one form to another.
Silicon has long been the semiconductor of choice for such power electronics
. But soon this ubiquitous substance will have to share the spotlight.
Devices made from silicon carbide (SiC)—a faster, tougher, and more
efficient alternative to straight silicon—are beginning to take off. Simple
SiC diodes have already started to supplant silicon devices in some
applications. And over the last few years, they’ve been joined by the first
commercially available SiC transistors, enabling a new range of SiC-based
power electronics. What’s more, SiC wafer manufacturers have steadily
reduced the defects in the material while increasing the wafer size, thus
driving down the prices of SiC devices. Last year, according to estimates
made by wafer maker Cree, the global market for silicon carbide devices
topped US $100 million for the first time.
Within five years, we should see this market balloon as SiC devices find
their way into power electronics for hybrid and all-electric vehicles,
creating simpler and more efficient power systems. SiC power devices will
also become vital in solar and wind energy creation, by reducing the energy
lost as electricity is converted to a form that can be used on the power
grid. Eventually, silicon carbide could remake the grid itself by
eliminating the need for bulky substation transformers, thereby saving an
enormous amount of energy that is now wasted as electricity makes its way
from power plants and other sources to its final destination. Although the
field of SiC power electronics is still relatively immature, we expect it’s
in for a big growth spurt.
Silicon-based devices are so mature and inexpensive to manufacture, it might
be hard to believe that any material could shake silicon from its perch.
But silicon carbide is quite special. Many of the material’s most
attractive properties stem from a single physical feature: SiC’s bandgap,
the energy needed to excite electrons from the material’s valence band into
the conduction band. Silicon carbide electrons need about three times as
much energy to reach the conduction band, a property that lets SiC-based
devices withstand far higher voltages and temperatures than their silicon
counterparts.
One of the biggest advantages this wide bandgap confers is in averting
electrical breakdown. Silicon devices, for example, can’t withstand
electric fields in excess of about 300 kilovolts per centimeter. Anything
stronger will tug on flowing electrons with enough force to knock other
electrons out of the valence band. These liberated electrons will in turn
accelerate and collide with other electrons, creating an avalanche that can
cause the current to swell and eventually destroy the material.
Because electrons in SiC require more energy to be pushed into the
conduction band, the material can withstand much stronger electric fields,
up to about 10 times the maximum for silicon. As a result, a SiC-based
device can have the same dimensions as a silicon device but withstand 10
times the voltage. What’s more, a SiC device can be less than a tenth the
thickness of a silicon device but carry the same voltage rating, because the
voltage difference does not have to be spread across as much material.
These thinner devices are faster and boast less resistance, which means less
energy is lost to heat when a silicon carbide diode or transistor is
conducting electricity.
Because of these features, silicon carbide could be used to replace slow
silicon switches with alternative designs that are faster and more energy
efficient. To sustain voltages beyond about 200 volts, a silicon transistor
has to be quite thick. This added thickness boosts resistance, which in turn
demands impractically large devices in order to maximize current-carrying
capacity. To mitigate this problem, high-voltage silicon switches tend to be
bipolar transistors: They use both holes and electrons. The design carries
more current, but it takes time for all the charge carriers to fully exit
the device. When the transistor is being switched from its “on,” current-
carrying state to its “off,” voltage-blocking state, there is a period of
overlap where the remaining charge carriers are exposed to high voltage and
dragged through the device, dissipating heat.
Using silicon carbide instead of silicon in high-voltage devices will let
manufacturers replace slow silicon bipolar transistors with single-carrier,
or unipolar, devices such as metal-oxide-semiconductor field-effect
transistors, or MOSFETs. Very few charge carriers are left behind in such
devices, so the transistors can be switched quickly and far more efficiently
. The faster devices also have the added benefit of more-compact and less-
expensive packaging because they require smaller control circuitry.
For all its fine qualities, silicon carbide has been a difficult material to
master. One of the biggest hurdles to its widespread use in power
electronics has been in wafer manufacturing. When engineers first started
working with the material in the 1970s, they struggled to grow large single
crystals of the stuff—the silicon and carbon atoms had a habit of combining
with one another to form a hodgepodge of different crystalline structures.
micropipes
https://spectrum.ieee.org/image/1927174
micropipes 2
https://spectrum.ieee.org/image/1927175
Images: Cree Wafer imperfections: Defects such as these micropipes had to be
eliminated in order to boost yield and drive down the cost of silicon
carbide power electronic devices.
Over the years, researchers succeeded in creating larger and larger single-
crystal wafers. And in 1991, a few years after the company was founded, Cree
released the first commercially available SiC wafers. They were just an
inch across and used mostly for research, but it was a start. Since then
Cree and other manufacturers, including Dow Corning, SiCrystal, TankeBlue,
and II-VI, have made steady progress in boosting the size of the wafers;
these days 4-inch SiC wafers are common, and 6-inch wafers are on the
horizon. A larger wafer size means that more devices can be built on each
wafer, which drives down device costs.
At the same time, companies have been working to overcome another early
stumbling block: a high number of defects in SiC crystals. Unlike silicon,
SiC doesn’t have a liquid phase. As a result, SiC crystals are grown layer
by layer from vapor at roughly 2500 °C. This process is difficult to
control and can easily create tiny, tornado-like tunnels called micropipes (
shown at right), which arise from dislocations in the crystal early in the
wafer formation process.
Devices built atop these micropipes don’t perform as designed. Even a few
micropipes per square centimeter is enough to erode device yield and thus
boost costs. But as wafer producers fine-tuned manufacturing processes, they
also made steady strides in eliminating such defects. In 2005, the U.S.
firm Intrinsic Semiconductor Corp., later acquired by Cree, debuted 3-inch
SiC wafers with no micropipes, and 4-inch micropipe-free wafers are now
available.
Of course, wafers would be nothing if there weren’t devices to build on top
of them. In 2001, more than 50 years after the first silicon power
electronic devices emerged, Infineon Technologies, based in Neubiberg,
Germany, released the first commercial SiC device. It was a Schottky diode,
a simple junction made from metal and semiconducting material. Schottky
diodes rectify alternating currents in much the same way that a standard p-n
junction does, but the devices exhibit much faster response times. Although
they cost more than silicon diodes, SiC Schottky diodes offer a range of
benefits, including better energy efficiency and reliability and cooler
operation. They also eliminate the need for devices like snubbers, which
would otherwise be used to protect silicon circuitry from current spikes. In
less than 10 years, SiC Schottky diodes have all but replaced the silicon p
-n diodes in switched-mode power supplies for computers, particularly those
in large data centers. Manufacturers now offer SiC Schottky diodes that can
withstand voltages as high as 1700 V, more than five times the maximum
voltage of comparable silicon devices.
SIC diode wafer
https://spectrum.ieee.org/image/1927141
450-ampere test module
https://spectrum.ieee.org/image/1927142
Photos: Oak Ridge National Laboratory Cut Devices: SiC diodes cut from a
wafer [top] can be peeled and packaged in modules, such as this 450-ampere
test module [bottom] developed at the University of Arkansas.
But to truly revolutionize power electronics, you need a second component:
transistors. These more sophisticated devices have taken longer to realize
in silicon carbide. It wasn’t until 2008 that the first SiC transistors—
junction field-effect transistors (JFETs) manufactured by Mississippi-based
SemiSouth Laboratories—finally hit the marketplace. The number of
transistor offerings has since boomed. SiC transistors with a range of
architectures are now offered by the likes of Cree, Infineon, Rohm, and
TranSiC. Each design has its advantages, and the jury’s still out on which
one will get the biggest share of the market, but the competition is clearly
heating up.
At Oak Ridge National Laboratory, in Tennessee, we’ve been exploring how
well SiC diodes and transistors work as the power electronic devices for all
-electric and hybrid electric vehicles. After the battery, electronics are
the key added cost to these vehicles. Electronics are needed to convert wall
power into battery power, to recharge the battery from the engine or from
the brakes, and most important, to operate the traction drive, which
transforms battery power into electricity that can run the motors that
propel the vehicle. Of all the electronics in an electric vehicle, the
traction drive draws the most power.
The drive has two main parts: a boost converter that increases the DC
voltage from the battery and an inverter that converts this electricity into
the three-phase AC needed by the motor. The three-phase inverter in turn
consists of six diodes and six transistors. In computer and laboratory
simulations at Oak Ridge, we’ve shown that simply swapping silicon diodes
with SiC Schottky diodes cuts the inverter’s energy loss by 33 percent,
consistent with other estimates. The reduction doubles if you also replace
the silicon transistors with SiC transistors. This boost in efficiency
results mainly from SiC’s lower resistance—which means it loses less power
to heat—and from faster, more efficient switching.
But SiC’s advantages don’t end there. Because it takes extra energy to
kick electrons into the conduction band, SiC’s wide bandgap also makes the
material much more heat resistant than silicon. Excess heat can excite so
many electrons that it can interfere with a device’s operation. For silicon
, this thermal failure occurs at around 150 °C, but SiC devices can
withstand considerably more than twice that temperature. This thermal
resistance makes SiC attractive for a range of rugged applications,
including military systems and electronics for oil wells, geothermal plants,
and robotic spacecraft.
In hybrid and all-electric vehicles, SiC’s operating temperature is high
enough to obviate the need for one of the bulkiest engine components: the
liquid cooling system. Hybrid vehicles need two cooling loops—one for the
gasoline engine, which runs at 105 °C, and another to cool the power
electronics and traction motor. Because silicon-based electronics stop
performing above roughly 150 °C, and because some physical space separates
the electronics from the coolant, this second loop needs to run even colder
than the engine loop—at roughly 65 to 70 °C.
Liquid cooling adds significantly to the overall size of the engine, and if
the liquid leaks out, it can destroy the electronics. Our simulations
suggest that SiC inverters, because they can operate at higher temperatures,
could reduce the size of the cooling system by 60 percent. If we combine
these inverters with other high-temperature components like high-temperature
capacitors, we might be able to eliminate the second loop altogether and
simply cool the electronics with air. First, though, the packaging and
peripheral components—the capacitors, control circuits, and drivers that
turn transistor gates on and off—must also be made to withstand high
temperatures. We’ve slowly been making progress on this front and have
built drivers from scratch that work at up to 200 °C.
How efficient could SiC ultimately make electric vehicles? Electric traction
drives already convert more than 85 percent of their power into usable
mechanical energy, more than double the raw efficiency of a gasoline engine.
But the U.S. Department of Energy (DOE) has set some ambitious goals (pdf)
for boosting the efficiency even further. By 2015, the agency says that
drives should convert 93 percent of their power into mechanical work and by
2020, more than 94 percent. In other words, it wants future drives to lose
half as much energy as present-day drives. These efficiency targets wouldn’
t be hard to reach by themselves, but the DOE also expects that electric
traction drives in 2020 will be half the size and less than a fifth of the
cost. These ambitious targets will be all but impossible to hit with silicon
alone, but we think SiC has the potential to get us at least most of the
way there.
One area where SiC devices are already making inroads is solar power.
Photovoltaic panels, whether they’re mounted on a roof or spread across
hectares of land, need inverters to convert the DC electricity made by the
panels into AC electricity that can be fed into the power grid. This
conversion process is already quite efficient: Silicon-based inverters lose
just 2 to 3 percent of the energy they process. But inverters containing SiC
diodes and transistors can easily cut that loss in half. Over the 20-year
lifetime of a 10-megawatt solar plant, that could add up to hundreds of
thousands of dollars in savings.
That’s just for starters. Infineon has estimated that improvements in power
electronics could eventually reduce electricity consumption by as much as
30 percent. To get there, the U.S. National Science Foundation funded the
creation of the FREEDM Systems Center in 2008, a corporate and academic
partnership that is researching ways to build a smart, flexible power grid
using wide-bandgap devices. Last year, the DOE’s Advanced Research Projects
Agency–Energy also put money toward revamping power grid electronics. Two
grants went to teams led by Cree and GeneSiC Semiconductor that are
exploring ways to make SiC devices that can operate at more than 10 000 V,
up to 15 000 V—well beyond the capabilities of silicon devices.
Remaking the power grid calls for SiC components that don’t yet exist,
including high-voltage bipolar transistors and p-n diodes. But if the
research succeeds, it will pave the way for new devices that can connect
distribution lines to higher-voltage transmission lines. At present, that
job is performed by massive, multiton transformers, which dominate power
substations. Someday, though, utility companies could replace these
behemoths with far more efficient solid-state transformers, each the size of
a suitcase.
Of course, that’s still a long way off. One key technical hurdle will be
continuing to improve the quality of SiC channels. Today’s SiC transistor
channels carry charges a factor of 10 slower than their theoretical limits,
but modifications, such as better surface quality, should help.
Right now, silicon carbide is experiencing the same sorts of growing pains
that silicon did in the 1950s and 1960s, when physicists and engineers saw
it as a replacement for germanium. Despite the fact that SiC devices are
still relatively new and more expensive than their silicon counterparts, the
material has already demonstrated clear advantages over the alternatives.
As more and more such devices come to market and their capabilities expand,
they could start a revolution of their own.
This article originally appeared in print as “Smaller, faster, tougher.”
About the Authors
Burak Ozpineci and Leon Tolbert are both IEEE senior members. Ozpineci heads
the power electronics and electric machinery group at Oak Ridge National
Laboratory, in Tennessee. Tolbert is the Min Kao professor of electrical
engineering and computer science at the University of Tennessee, in
Knoxville. The two began working on silicon carbide devices in 2001, when a
friend of a friend of a friend sent along some of the first SiC Schottky
diodes. “We were hooked,” Ozpineci says. | z***7
发帖数: 555 | 2 What Happened To GaN And SiC?
Early predictions were overly optimistic, but these technologies are
starting to make inroads.
About five years ago, some chipmakers claimed that traditional silicon-based
power MOSFETs had hit the wall, prompting the need for a new power
transistor technology.
At the time, some thought that two wide-bandgap technologies—gallium
nitride (GaN) on silicon and silicon carbide (SiC) MOSFETs—would displace
the ubiquitous power MOSFET. In addition, GaN and SiC were supposed to pose
a threat to higher-end, silicon-based insulated-gate bipolar transistors (
IGBTs). Power MOSFETs and IGBTs are the workhorse chips in power electronic
systems.
Compared to silicon-based devices, GaN and SiC power chips operate at higher
voltages, frequencies and temperatures, helping to eliminate up to 90% of
the power losses in electricity conversion. Wide bandgap refers to higher
voltage electronic band gaps in devices, which are larger than 1
electronvolt (eV).
As it turns out, power MOSFETs and IGBTs are moving towards their limits.
But today, the two technologies continue to dominate the landscape in
applications from 5 volts to 6.5 kilovolts. In contrast, GaN-on-silicon
power chip shipments are lower than expected amid a multitude of challenges.
And SiC MOSFETs are shipping, but SiC also suffers from high wafer costs.
“At one time, in International Rectifier’s promotion for GaN, the company
said that within 10 years the topology would be that you use silicon for
anything below 5 volts,” said Stephan Ohr, an analyst with Gartner. “You
would use GaN for anything from 5 volts to 600 volts to 1,000 volts. And you
would use SiC for anything above 1,000 volts.”
That prediction didn’t pan out. “I am not seeing that happening now,” Ohr
said. “I don’t think you can buy a GaN part today. They are all on
allocation. But if you go to a distributor, you can find SiC. SiC got to the
market faster than GaN.”
All told, GaN and SiC will grow faster than silicon-based power semis over
the next decade. But in total, GaN and SiC are projected to have a combined
share of only 13% in the overall power semiconductor market by 2024,
according to Lux Research. Silicon-based power semis will continue to
dominate with an 87% share by 2024, according to the firm.
Still, OEMs face some tough decisions today. Silicon-based power semis
continue to work, but OEMs still want to have smaller, faster and more
efficient devices and for good reason. The power losses in today’s systems
range from 8% to 15%, according to experts.
So, the questions are clear. Are silicon-based power MOSFETs and IGBTs on
their last legs? Will GaN and SiC power devices eventually fulfill their
promises and displace silicon? And, of course, which technology will provide
the ultimate performance?
The contenders
Power semis are used in the field of power electronics. Basically, power
electronics make use of solid-state electronics to control and convert
electric power. The conversion is performed with various semiconductor-
switching devices.
The perfect switch would have infinite speeds, zero on-state resistances and
infinite off-state resistances. Unfortunately, the perfect switch doesn’t
exist. So, engineers must look at several factors when evaluating chips,
such as voltage, current, switching speed, load and temperature.
Today, there are several devices to choose from. On the transistor front,
the entry-level market is served by traditional power MOSFETs, which are
used in 10- to 500-volt applications. Developed in 1976, power MOSFETs are
based on a double-diffused (DMOS) architecture. They are vertical structures
, meaning the current flows from the source at the top to the drain at the
bottom.
Power MOSFETs are cheap and here to stay. At best, GaN and SiC could make a
tiny dent in applications below 500 volts. In any case, the big and hotly
contested market is now taking place in two voltage segments—600 volts and
1,200 volts. In these areas, four basic technologies are competing for some
large markets, such as adapters, automotive, switching power supplies and
solar inverters.
In this segment, there are two silicon-based solutions—super-junction power
MOSFETs and IGBTs. Super-junction power MOSFETs, which are souped-up
versions of power MOSFETs, are used in 500- to 900-volt applications. Super-
junction power MOSFETs are vertical devices. They also consist of pillar
structures in the body, confining the electric field in the epi region.
The IGBT, meanwhile, is a three-terminal device that combines the
characteristics of MOSFETs and bipolar transistors. IGBTs are used for 400-
volt to 10-kilovolt applications.
Then, there are the two wide-bandgap technologies—SiC and GaN. Based on
silicon and carbon, SiC has a bandgap of 3.3 eV. Silicon has a bandgap of 1.
1 eV. SiC FETs are targeted for 600-volt to 10-kilovolt applications.
Another technology, GaN, is a binary III-V material. In the power arena, GaN
-on-silicon chips are used in 30- to 600-volt applications. GaN has a
bandgap of 3.4 eV.
The best technology?
IGBTs, SiC and other technologies are geared for the niche-oriented markets
at 1,700 volts and higher. But what is the best technology for the larger
600- and 1,200-volt markets? It’s not a simple answer. “You will likely
have a co-existence of all technologies. But it also depends on the
applications of the voltage range and how much a customer is willing to pay
for a device, whether they will go for a silicon-based solution, GaN or SiC,
” said Roland Rupp, project manager for SiC devices at Infineon, the world
’s largest power semi vendor. Infineon sell chips based on all of the
technologies—MOSFETs, IGBTs, GaN and SiC.
Indeed, there are tradeoffs between the technologies. For example, both
super-junction MOSFETs and IGBTs are ramping up on 300mm wafers, making them
less expensive than GaN and SiC. In comparison, SiC MOSFETs are ramping up
on 100mm wafers, while GaN-on-silicon is running on 150mm substrates.
In addition, super-junction power MOSFETs and IGBTs continue to improve in
terms of performance. For instance, in some hard-switching applications,
super-junction devices are closing in on GaN or SiC. “With respect to
manufacturing cost, the IGBT is clearly superior to all other power switch
technologies and has the lowest T-dependence of conduction losses,” Rupp
said.
But super-junction power MOSFETs hit the ceiling at around 900 volts. IGBTs
are plagued by slow switching speeds. “Both super-junction and IGBT
technologies are getting closer to their technological limits,” he said. “
There are still new ideas to further improve the trade-off between static
and switching losses and keeping short-circuit ruggedness, but they are
fighting with the fact that performance improvements are counterbalanced by
increased processing costs. The newly available 300mm wafer process
environment for such silicon-based power switches helps with respect to this
cost aspect, but will probably be the last significant productivity gain
for silicon-based power electronics for the next decade.”
So, there is a keen interest in GaN and SiC. Today, SiC diodes are used in
high-end power supplies for servers and telecom systems, but SiC MOSFETs are
still in the early stages of market penetration. Compared to power MOSFETs,
SiC has 10 times the breakdown field and three times the thermal
conductivity. “Neglecting the cost differences between the various
technologies would lead to a clear champion—SiC FET,” Rupp said.
But SiC also suffers from high wafer costs and low effective channel
mobility. In a move to address some of the issues, suppliers hope to reduce
the costs by moving to larger wafers. “We are doing production on 4-inch.
We want to go to 6-inch,” said John Palmour, chief technology officer for
power and RF at Cree, a supplier of SiC-based LEDs and power devices.
SiC MOSFETs are vertical devices. The channel structures also come in
various configurations, including trench and planar. Trench-based SiC
MOSFETs have lower conductivity loses than planar. But trench tends to
suffer from gate-oxide breakdowns, prompting some to devise double-trench
SiC MOSFETs.
Cree, for one, advocates the planar channel structure. In fact, Cree has
rolled out its third-generation SiC technology, which could address the
channel mobility issues. “It’s a die shrink,” Palmour said. “We’ve also
reduced the cost-per-amp.”
All told, SiC MOSFETs have some advantages over MOSFETs and IGBTs, but SiC
won’t displace silicon anytime soon. “IGBTs are not going away,” he said.
“They will be around for a while.”
Like SiC, GaN is also generating steam. A GaN high electron mobility
transistor (HEMT) is a lateral device. The current flows from the source to
the drain on the surface. Below the surface, AlGaN and GaN layers are grown
on a silicon substrate.
GaN-on-silicon is fast, but it also suffers from a lattice mismatch, making
it prone to defects in the fab. It also suffers from reliability issues and
low thermal conductivity. And there are also questions whether GaN-on-
silicon can scale beyond 600 volts.
“This assumes you can buy GaN parts,” Gartner’s Ohr said. “I have been
saying that GaN parts are still in development and experimental. But
assuming you can get one, you can reduce the size of your capacitors and
inductors that would go with your power supply or motor drive. But what is a
GaN part going to cost you? And does that pay for the smaller size and
incremental efficiency you can get from that?”
In fact, GaN has made slow progress. But one supplier, Transphorm, is making
some headway. In 2013, Transphorm acquired Fujitsu’s GaN IP. At the time,
Transphorm also announced a 600-volt GaN part, based on a cascode-type,
normally-off technology. The device reduces energy losses by 50%, compared
to silicon.
Then, earlier this year, the company moved into mass production. The chips
are being made on a foundry basis within Fujitsu’s 150mm fab in Japan. “
Transphorm is the only GaN provider who has announced and is actively
providing 600-volt qualified products to the industry,” said Primit Parikh,
president of Transphorm.
Parikh also dismissed the notion that GaN will run out of steam at 600 volts
. In the lab, Transphorm has demonstrated 1,800-volt devices. “It has
already scaled in demonstrations,” he said. “Our current focus is on mass
production and widespread commercialization of the first set of products at
600 volts. That will be followed by higher voltage devices at 900 volts and
1,200 volts.”
Transphorm isn’t the only GaN vendor, however. In fact, more than a dozen
companies have entered the GaN power chip fray in recent times. “One of the
reasons for that is clear. GaN is not just a technology. GaN is becoming
desirable as the power conversion platform of choice,” he added.
It’s unlikely that there is room for a dozen GaN suppliers. There also are
too many SiC vendors. Time will tell if the market will see a shakeout. But
clearly, GaN and SiC are shaking up the landscape. |
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