I am still looking into this. Checking with
sources at Lucent and VTSS. Because neither
company works directly on this its hard
to find somebody that can evaluate. I'm not
selling VTSS.
In the threads a Patent Atty.
maybe at IBM gave me dire warning to sell.
He and a few others seem the only ones to
be concerned. Lucent expects to be buying
VTSS chips for years to come. Thier products seem
to be improving with time, I'm told.
The Wall Street Analysts that understand
this and aware of it are still betting
VTSS.
Jerry
THE CASE FOR SILICON GERMANIUM
The semiconductor industry has become legendary for making chip
designs smaller and faster.
But the new transistor technology based on silicon germanium offers
additional speed for
telecommunications applications without further shrinking the
circuitry on a bipolar chip. Once
a disappointing material that simply fell apart due to internal
structural problems, silicon
germanium has reached the
stage of being a manufacturable product.
Virtually every major telecommunications company is working on
silicon germanium
technology. The technology holds great promise for reducing the cost
of consumer products
(cellular telephones and direct broadcast satellite entertainment
services), improving business
applications (telephone network transmission), and helping make
possible new applications
(collision-avoidance automobile radar).
Another reason the technology is important is that it offers
additional performance at lower
cost than some competing high-speed circuit technologies such as
gallium arsenide. "In terms
of cost, silicon germanium fits in between gallium arsenide and
silicon, but it's closer to silicon,
which is its appeal," says John Long, a professor of electrical and
computer engineering at the
University of Toronto.
Long is doing collaborative circuit design research with NORTEL
(Northern Telecom) using
silicon germanium for telecommunications applications.
When IBM began looking at silicon germanium in the early 1980s,
semiconductor chips used
mostly bipolar technology. Each year, those
chips were expected to advance in speed. But IBM scientists foresaw
a
time, in about 1990, when scaling conventional ion-implanted base
transistors would become increasingly difficult.
"The rate at which silicon performance in bipolar was progressing
was
recognized to be unsustainable," says Dr. Bernard Meyerson, an IBM
Fellow at IBM's T. J.
Watson Research Center in Yorktown Heights, New York. As the
circuitry on chips was
scaled down to achieve greater speeds, it became clear that the
performance characteristics of
bipolar silicon chips would eventually break down at very tiny
dimensions.
"The catch is that when you make a device smaller by simply
vertically shrinking the profile,
the amount of charge in the base of a bipolar transistor must remain
fixed," Meyerson says.
"So the density of the charge in the layers goes up and up.
Unfortunately, the result is that it
becomes easier for electrons to leak through that region. The
electrical charge leaks like a
leaky faucet. That means you encounter a point, as the device gets
smaller, where the
transistor no longer
works cleanly because there is significant leakage of current.
Reliability goes down and power
consumption goes up. The resistance in the base also rises, and that
causes problems with
electronic noise."
An alternative technology was needed if bipolar silicon was to
continue to advance in speed.
IBM's answer was silicon germanium, a compound which, while
compatible with silicon chip
manufacturing processes, had some important differences. "Instead of
making things go faster
by making them smaller, we needed to change the basic physics of the
material in the device
so we could get additional performance without a reduction in size,"
says Myerson.
During this time, IBM was also looking for a way to get around the
compromises inherent in
designing bipolar silicon chips to go even faster. Enhancing speed
usually meant compromising
some other chip characteristics. Silicon germanium, it was hoped,
would permit device
designers to simultaneously increase speed, reduce electronic noise,
and increase voltage.
It had been known in the industry since the late 1950s that putting
small amounts of
germanium in silicon would produce design benefits similar to those
found in the
semiconductor compound gallium arsenide, which provides fast circuit
speed but is more
expensive to produce than silicon. But producing silicon germanium
had remained a challenge.
"It was too hard to grow the thin films of silicon germanium,"
Myerson says. "The germanium
atom is 4 percent larger than the silicon atom, so if you put too
much germanium in the silicon,
the silicon fell apart." As a result, the early silicon germanium
devices didn't work. They failed
to show either high speed or acceptable operation, owing to the poor
quality of materials
available at the time.
Myerson's solution was to create a technique for growing silicon
germanium thin films that
used both ultra-high vacuum (UHV) and chemical vapor deposition
(CVD). This clean
system, called UHV/CVD, allowed the films to be grown at much lower
temperatures than in
the past. "While some silicon processes required temperatures of
more than 1,000 degrees
centigrade, germanium UHV/CVD is done at below 500 degrees
centigrade," says Myerson.
"That was a key improvement,
because germanium typically separated from silicon at the higher
temperatures. The
lower-temperature process also prevented other materials, called
dopants, from diffusing
away from where they were placed. The benefit of that was that you
could grow tightly
controlled, uniform, and therefore manufacturable films of this
alloy. Once you could do that,
you could include that alloy in the heart of the transistor to get
additional speed from a
same-sized bipolar
device."
The first silicon germanium IC ever built at IBM was a digital-
to-analog converter completed
in 1992. It had 3,000 transistors and 2,000 passive elements, such
as resistors and
capacitors. In September of 1996, IBM became the first semiconductor
company to qualify
what is called "a silicon germanium heterojunction bipolar
transistor" for manufacturing. While
in most telecommunications circuits the effective maximum speed of
commercially available
silicon transistors is 15 gigahertz of unity current gain frequency
(fT) and about 15 gigahertz
maximum available power gain (fMAX), silicon germanium transistors
being manufactured by
IBM have a speed of 46 gigahertz (fT) and 65 gigahertz (fMAX).
"This technology also improves chip yields," says Dave Harame, who
manages silicon
germanium product development for IBM. "We measure
our yields by using 4,000 transistors in chains-that's 4,000 0.5 x
2.5æ2 emitter area transistors
connected in parallels-and we have gotten as much as 78 percent
average yield."
The potential economic benefits of such a technology are enormous,
because silicon germanium chips will be made with the same tools as
silicon chips. This means millions of dollars won't have to be
invested in new semiconductor
tools, as is typically the case when a shift is made from one
generation of chip technology to
the next. "You're just changing one aspect of bipolar transistor
technology," Meyerson says.
"Because silicon germanium technology uses the same dimensions as
the previous generation
of silicon technology, you can reuse existing silicon tools. It is
the first time a profound change
in the performance of a technology has occurred without a profound
investment."
In the early 1990s, IBM converted its mainframes from bipolar to
CMOS
technology. As a result, IBM decided to focus the silicon germanium
technology on bipolar applications in the communications industry,
for
which IBM felt it was ideally suited. "In computing," Myerson says,
"CMOS has an inherent advantage over bipolar because it uses
significantly less power, and
thus generates less heat. But bipolar is a lower-power alternative
to CMOS at high
frequencies (found in
telecommunications). In very high-frequency operations, where the
transistor is switching constantly, the speed advantage of bipolar
lets it operate at lower power
in continuous mode than CMOS can." The other advantages silicon
germanium bipolar brings
to high-frequency
applications are that it has higher gain and less noise than its
silicon counterparts.
Larry Larson, an engineering professor at the University of
California in San Diego, formerly
of Hughes Electronics, said he believes silicon
germanium could extend the performance of silicon bipolar technology
by anywhere from 50
to 100 percent. Ted Kamins, a scientist at Hewlett-Packard
Laboratories in Palo Alto,
California, believes "a 50
percent increase in speed is the type of ballpark figure we are
talking about, although it could
be a factor of two." Larson and Kamins are referring to the cut-off
frequency of the bipolar
transistor.
Another approach is to take the implanted base silicon bipolar
transistor and insert a silicon
germanium base in the device without any optimization. "Right now,
silicon germanium offers a
20 percent improvement in performance over straight silicon with the
same design rules," says
John Long, at the University of Toronto. "That means 20 percent
faster digital performance, or
20 percent wider bandwidth, in whatever analog circuit you're
designing." IBM has gone
further by imbedding silicon germanium into its advanced silicon
technology, so it can take full
advantage of the benefits of silicon geranium.
According to Kamins, the real gains from silicon germanium will be
from high-end
applications. "The main benefit is going to be extending silicon
technology into the frequency
range that, up to now, has been covered by more expensive compound
technologies like
gallium arsenide." Kamins, who was a project leader in HP's silicon
germanium development
effort from the late 1980s until about two years ago, says that
while silicon germanium is fast,
its biggest asset is that chips using it can be built using standard
silicon processes. "Silicon
germanium will not get up to the highest speeds that have been
achieved with compound
semiconductors using gallium arsenide and indium phosphide, but it
can extend silicon
technology somewhat into that range. And it will allow you to
leverage the overwhelming
investment in silicon technology."
An example, according to Kamins, is the difference between the IBM
and
HP approaches. IBM's version of silicon germanium was compatible
with
CMOS integrated circuits, while the HP design was compatible with
existing HP bipolar processes. "The deciding factor in success is
going to be the degree to
which the device process is compatible with a company's particular
integrated circuit
technology. Companies are likely to design devices to be as
compatible as possible with
existing processes."
Silicon germanium is likely to make many radio frequency
applications
more affordable than they are today, says Ronald Finnila, Director,
Corporate Technology at Hughes Research Laboratories in Malibu,
California. Among the
potential applications for silicon germanium, according to Finnila,
is expanding available
bandwidth in both wired and wireless local area networks (LANs) for
personal computers.
"Silicon germanium has the potential for lowering the costs of
existing products or integrating
more functions on a semiconductor chip, so you can use fewer chips."
For telecommunications applications, IBM combines bipolar silicon
germanium with standard IBM CMOS to miniaturize and thus drive down
the cost of communications components. For example, the technology
could be used in
cellular phones to combine the functions of several computer chips
onto a single chip. That
might result in cellular telephones using three to five times fewer
individual chips than they do
today. This consolidation of chips could result in less expensive
handsets for cellular
communication and new Personal Communications Services (PCS), an
all-digital alternative to
cellular.
Long, whose research for NORTEL uses IBM's silicon germanium
technology, said the
appeal of using silicon germanium in cellular phones is that it
would save money on expensive
chip packaging by combining the functions of many chips onto just a
few. This would also
reduce the drain on the cellular phone's battery, because much of a
phone's electrical power
use today is for communicating between individual chips. Eliminating
inter-chip
communications would make phone batteries last longer.
Other cellular phone applications for silicon germanium include
power
amplifiers (which boost the signal for phone transmission) and down
converters (which handle received phone signals). "Silicon germanium
is very high-speed and
hence makes a very high-efficiency power amplifier," Larson says.
"For the down converter, a
silicon germanium device has low noise-which means the device itself
doesn't add a lot of
noise to the signal-and that's important because you're trying to
receive the tiniest signal you
can."
Silicon germanium also might be used in products that consumers use,
but never see, such as
the driver chips that turn lasers on and off inside fiber-optic
telephone lines. According to
Myerson, lower costs for silicon germanium could result in its
displacing other compounds
currently used for those driver chips, such as gallium arsenide and
indium phosphide. Long
also sees a role for silicon germanium devices in fiber-optic
telephone lines. "A silicon
germanium chip would allow you to run the fiber-optic cable at
higher speeds, giving you
higher call capacity." Larson is enthused about future uses of
silicon germanium. "I think the
potential of the technology is really enormous. I once spearheaded
an effort to bring that
technology in for applications such as collision-avoidance
automobile radar and
telecommunications."
Silicon germanium could make collision-avoidance radar for
automobiles economically
feasible for the first time. According to Myerson, "Silicon
germanium will allow products that
formerly were accessible only to the military to have consumer cost
points." Finnila adds, "Car
radar is certainly something that's being looked at. For some, but
not all, car radar
applications, silicon germanium appears to be a good candidate for
lower-cost solutions." He
says that, at the moment,
silicon germanium is being considered for side- and rear-viewing
radar.
There also may be military applications of the technology. Long says
that gallium arsenide "has
a captive market in the military because it offers many things that
are at the leading edge in
semiconductors, but basically because it is fast. Silicon germanium
is also fast and VLSI, and
it might be possible for it to make some inroads in that area."
Another appealing application for silicon germanium may be direct
broadcast satellites, which
beam data and video programs to small home satellite dishes. Long
says that because the
receiving dishes are consumer products, any cost reduction made
possible by the new
transistor technology would be welcome. "If there's one thing
silicon germanium needs," says
Long, "it is a high-volume product to drive it. Something like
direct broadcast satellite TV,
which is a mass consumer item in the US and Europe, could provide
that kind of volume."
While IBM is the first with a manufacturable silicon germanium
technology, there are
companies close behind, according to Long. Still other companies are
believed to be
interested, but are keeping a low profile. So far, according to
Long, IBM holds the leading
position in silicon germanium.
Myerson estimates that more than 50 companies worldwide are working
with silicon
germanium, either in connection with circuit design or fabrication
equipment. "It has become a
substantial effort," he says. |
