December 13, 1965 Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Electronics,
published 1930 - 1988. All copyrights hereby acknowledged.
The December 1965 issue of
Electronics magazine reported in multiple articles on the state of Japan's electronics
industry (see the table
of contents for other stories). Japan's indisputable lead today in many realms of
semiconductor, commercial, and consumer products proves successful implementation
of the strategy described in these articles. Per this piece's NTT employee authors, "In
one decade, Japan's semiconductor industry has become the world's second largest. Pioneering
engineers, a variety of unusual devices, and breakthroughs in miniaturization techniques
account for phenomenal growth." A notable claim is taking credit for inventing the ceramic
"pill" packaging format for high frequency transistors.
Japanese Technology - When You're Second, You Try Harder
In one decade, Japan's semiconductor industry has become the world's second largest.
Pioneering engineers, a variety of unusual devices, and breakthroughs in miniaturization
techniques account for phenomenal growth.
By Takuya Kojima and Makoto Watanabe,
Electrical Communications Laboratory, Nippon
Telegraph and Telephone Public Corp., Tokyo
Nippon Electric Co.'s multiple diffused base transistor (left) compared
to a conventional planar transistor at right. By widening the base area with a second
diffusion, NEC reduces base spreading resistance, thus increasing maximum frequency
Emitter mesa transistor built by the Nippon Electric Co. (left) withstands
drive-in effect which had destroyed conventional mesa transistor (right).
Large scale production of semiconductor devices is the nucleus of the Japanese electronics
industry. More than 400 million transistors were produced last year, making Japan's semiconductor
industry the second largest in the world, trailing only the United States. Yet quantity
is not the industry's sole accomplishment. Japanese engineers have created some unusual
devices such as the passivated mesa transistor, a bidirectional twin transistor, the
Esaki diode, and a double-diffused pnp transistor of unique structure.
All this has happened in the last decade. The dominant force behind such rapid growth
has been Japan's pioneering - in the transistorizing of consumer products such as a-m
and f-m radios, tape recorders and television sets, now small enough to be called micro
The structure of the Japanese industry helped too. All the makers of semiconductor
devices in Japan - and the total number is less than 20 - also manufacture consumer products,
other electronic equipment or both. Because they are in the same company, information
flows rapidly between device builders and equipment designers.
Most of the semiconductors made in Japan are germanium devices, and go into consumer
products. New consumer products, however, require better quality devices. Thus, the transistorization
of large television reviewers, with screens up to 19 inches, demands high-frequency transistors
and high-power devices. Communication and industrial equipment also needs special-purpose
devices of high quality. Although silicon technology is new in Japan, its spread has
been rapid and most semiconductor suppliers produce both germanium and silicon devices.
Challenge of Higher Frequencies
As in the United States, there is great pressure in Japan to produce higher-frequency
devices. For example, television makers want transistors capable of operating up to 1,000
megacycles for ultrahigh-frequency receivers. For this application, Japanese suppliers
offer both germanium and silicon devices.
To boost operating frequency, Japanese firms are trying either to minimize the base
spreading resistance of their devices or to minimize the collector capacitance. The reasons
become evident from the equation for maximum frequency of oscillation of a transistor:
where rbb' is the base spreading resistance,
cc is the collector capacitance and τec is the carrier transit
time between emitter and collector. The base spreading resistance and collector capacitance
degrade performance. Base spreading resistance not only decreases the power gain and
output power but also degrades the noise figure.
To lower this resistance in silicon transistors, firms have introduced some novel
device structures. For example, the Nippon Electric Co., Japan's biggest microwave equipment
manufacturer, uses a multiple base diffusion process to add another area of impurities
in the 2SC288, 2SC289, and 2SC272 devices (shown above). After the usual diffusion has
formed a conventional base area, a second process diffuses impurities just outside the
emitter area, widening the base thickness and reducing the base spreading resistance.
The rbb'cc product of the 2SC288 is only 3 picoseconds; the base
resistance is less than 1/3 that of a conventional transistor.
NEC also achieves a low base spreading resistance with a second approach called emitter
mesa structure and shown in the figure below. This structure reduces the drive-in effect
in which impurities in the base region are driven toward the collector area, forming
a small projection in the collector junction plane.
Though the effect is more pronounced in a silicon mesa transistor, where the impurity
is gallium, than in a planar transistor where the impurity is boron, It becomes critical
in any high-frequency transistor. That's because a high-frequency device has an extremely
narrow base width which is a bottleneck in the base region between the area immediately
beneath the emitter junction and the area outside the junction. The bottleneck causes
an appreciable increase in the base resistance and disturbs the uniform carrier flow
in the base area.
In the emitter mesa structure, a mesa formed by a vapor etching process prior to diffusion,
offsets the drive-in effect. The height of the mesa is just enough to compensate for
the depth of the projection that would be formed in the junction plane by the drive-in
phenomenon, Thus an ideal flat junction structure results.
There is one other advantage of the emitter mesa
structure: it eliminates unwanted parasitic capacitance and carrier injections around
the vertical outside edge of the base. Although these can be ignored in an ordinary device,
they are appreciable in a high-frequency transistor whose emitter width is 5 microns
or less. The parasitic capacitance decreases the high-frequency amplification factor
in the small-current region of the emitter; the excess carrier injection at the edge
decreases the current amplification factor in the large-current region of the emitter.
By using the emitter mesa structure NEC increases the gain by 3 db throughout the
range of emitter current and decreases noise by 0.5 db.
From the equation for the maximum frequency of oscillation of a transistor (above),
it is clear that frequency can also be increased if collector capacitance is reduced.
In the base mesa transistor designed by NEC, the geometry lowers this characteristic.
In the structure (p. 83), the base area is defined by a deposited layer of silicon dioxide.
Since only a small region of the base is needed to make contact with the metallization
of the electrode, the capacitance of the metallized portion to the collector is negligible.
Such low collector capacitance makes the device well-suited for application in wideband-amplifiers
- and especially in amplifiers with automatic gain control because circuit capacitance
changes less with changes in voltage stemming from the gain control.
It seems clear that all three techniques - multiple diffused base, emitter mesa, and
base mesa - could be applied to one device, to produce even better transistors capable
of handling higher frequencies.
At the Matsushita Electronics Corp., the semiconductor producer of the big Matsushita
Industrial Electronics Co., another approach to reducing collector capacitance has been
taken with extended base planar transistors. A highly doped area just beneath the extended
base electrode shields the electrode from the collector. In the Matsushita 2SC562 series,
the base-to-collector capacitance is as low as 0.15 picofarads.
Minimum base-to-collector capacitance eliminates several bothersome effects. By definition,
in an extended base electrode device, a metallized contact to the base is extended along
the silicon dioxide layer on top of the collector bulk semiconductor region for easier
bonding of the base lead wire. If the device has an extremely small base area, the parasitic
capacitance between the extended base electrode and the collector bulk semiconductor
region is comparable to the capacitance of the intrinsic collector junction. Such a high
capacitance makes it impossible either to increase the power gain of the transistor in
ultra-high-frequency ranges or to stabilize transistor operation at lower frequencies
where capacitance can cause feedback. In addition, if the intermediate frequency stage
of an amplifier is equipped with automatic gain control, high capacitance causes the
bandpass characteristics to change with the gain of the transistor.
Most of the high-frequency devices Matsushita has developed are going into television
sets. The 2SC562 is used in the control stage of television i-f amplifiers with forward
gain control. The 2SC563 goes into the output stage of i-f amplifiers. And the 2SC593,
with a power gain of 20 db at 450 Mc and a cutoff frequency more than 1,500 Mc, is for
Because silicon devices cost considerably more than germanium ones, there is still
a lot of interest in germanium devices in Japan, even for high-frequency applications.
Japanese engineers use mesa, planar and alloyed diffused types of germanium transistors
in high-frequency applications. One example is the 2SA448, a double-diffused pnp transistor,
shown on page 84, developed by the Sony Corp. The mesa surface is divided into two steps
of equal area, separated by a space of only one micron. One step is the base contact
metallization region; the other is the emitter contact metallization region.
One micron or less separates the emitter electrode (top) and base
electrode (bottom) of Sony's double diffused germanium pnp transistor. Used for high-frequency
applications, it can be fabricated easily.
Even though high precision is required in manufacturing, the fabrication of the 2SA448
is relatively simple. First, a coating of silicon dioxide is deposited uniformly over
the entire face of a germanium wafer. Then gallium is deposited on the oxide coating
and diffused through it to form the emitter layer of p+ material. Trenches in the SiO2
are formed by a photolithographic process. The p+ material below these trenches is etched
out to form deeps whose bottoms reach to the p-material. Then the SiO2 layer
is removed, leaving a surface of alternating p+ and p- stripes. At this point, the device
is a p- wafer with parallel ribbons of p+ material along its upper surface.
In the next step, the base diffusion of n-type material takes place. A layer of n-type
material forms at the base of the trenches and under the p+ ribbons because the diffusion
constant of the n impurity is 1,000 times that of gallium which was the p+ impurity.
But, because the quantity of n impurity is much smaller than that of gallium, the p+
region stays a p+ region. Aided by geometry, the n impurity extends further into the
p- region at the bottom of the trenches than under the p+ region. Since the n layer under
the p+ layer is the base region of the finished device and the n layer at the bottom
of the deeps is the base lead attachment region, the finished transistor has a thin base
and low base spreading resistance.
After the second diffusion, a shadow evaporation process forms the aluminum base and
emitter contact regions. In this process, the entire base and emitter contract regions
are metallized with only about a micron spacing between them. No precision positioning
is required since the step in the structure provides a built-in mask.
Finally, the wafers are diced and individual pellets mounted on tabs for mesa masking
and mesa etching. Mounting, lead attachment and sealing are conventional.
Built this way, Sony's 2SA448 has a power gain of 8 db at 1 gigacycle. Noise figure
at this frequency is 7 db in the common emitter connection.
The considerable effort to produce high-frequency devices has not been duplicated
with high-power units. Though many companies make power transistors, both silicon and
germanium, most are conventionally designed.
How Sony's germanium transistor, 2SA448, performs at high frequency.
Its performance is good up to 1 gigacycle.
Epitaxial or triple-diffused silicon power transistors are manufactured with capacities
ranging from 10 to 150 watts - not exceptional when compared with devices made in the
United States with power ratings up to 300 watts. Currently the 2SD137 made by Kobe Kogyo
has the highest collector breakdown voltage of any device made in Japan: 300 volts. Recently,
both Kobe Kogyo and Toshiba (Tokyo Shibaura Electric Co.) started manufacturing overlay
transistors which have higher power capability in the high-frequency range.
In entertainment and industrial applications, alloy drift and diffused base germanium
transistors are still used almost exclusively. In audio-frequency amplifiers, horizontal
deflecting systems for tv picture tubes, and regulated power supplies, they have proven
to be free of secondary breakdown. Many people wonder whether silicon will ever replace
germanium for such applications.
The Passivated Mesa
Although the planar structure is clearly the most widely used for silicon transistors,
it has one serious limitation: the breakdown voltage of the collector is low. After examining
the probable causes of this limitation, Hitachi Ltd., has developed an improved passivated
mesa transistor which has a better collector junction.
In Japan, as in the United States, the causes of collector breakdown in planar structure
are not clear. Partially, it's caused by geometry: the electric field is concentrated
at the corners of the diffused area. Some researchers believe that a large amount of
impurities in the base region cause surface breakdown. The surface of the base has a
greater concentration of impurities than the region adjacent to the horizontal collector
junction because diffusion produces a graded layer with a higher concentration of impurities
near the surface.
At other times, a poor silicon-silicon dioxide interface seems the cause. Or, if the
silicon-silicon dioxide surfaces are. separated by an n+ surface layer, breakdown can
Hitachi's new process produces a mesa structure that has a high collector breakdown
voltage, low noise figure, small leakage current, and a high current amplification factor
in the small current region.
The process is applied to a completed mesa transistor. After silicon dioxide is deposited
on the transistor by the thermal decomposition of organic oxysilane, a thin film of lead
is deposited onto the oxide layer. Finally, the device is exposed to high temperature
so the lead and silicon dioxide can combine to form a protective glass whose composi-tion
is lead oxide and silicon dioxide.
Many kinds of transistors treated this way are available for entertainment and industrial
applications. For example, the Hitachi 2SD190 is a silicon device with a BVcbo
of 300 volts; the 2S280H is a twin transistor for low-level differential amplifiers and
it has an excellent reliability record.
Hitachi claims the process can be applied to other semiconductor devices, too.
Beginning of Field Effect Devices
Among Japanese engineers, the field effect transistor is still a novelty whose application
is very limited. Only five companies supply them at present: Toshiba, Hitachi, Fujitsu,
Kobe Kogyo and Mitsubishi. Typical of these devices is the Toshiba 2SJ13, a p-channel
junction FET with a transconductance of 3.5 milliohms. The Mitsubishi 3SK15 series is
a depletion mode metal oxide semiconductor device for general purpose use. The Hitachi
3SK11 is a depletion mode n-channel MOS fabricated by a technique called field cooling
Depletion mode, enhancement mode and even nonuniform channel MOS devices can be made
by the field cooling process. A small quantity of movable impurities, such as sodium
ions, are impregnated in the silicon dioxide layer. An electric field applied between
the gate and bulk crystal at high temperature causes the impurities to drift through
the oxide layer, changing the surface potential of the silicon appreciably. When the
surface channel has reached the desired conductance, the field is removed and the device
is cooled, fixing the impurities in the oxide layer.
Making the Esaki Diode
Unquestionably the best known Japanese semiconductor development is the Esaki or tunnel
diode, invented by Leo Esaki at the Sony Corp. in 1957. After a resounding acceptance,
particularly because of its apparent high speed, the tunnel diode turned into a big disappointment.
One reason was the incorrect use of the device in circuits. It is a diode and cannot
replace transistors or other multi-lead devices. But another reason was reliability.
Initially, every manufacturer fabricated Esaki diodes by a conventional alloy-etching
process. It produced a diode whose structure resembled a boulder balanced on a point,
and the device was not very rugged.
In addition, performance requirements were in conflict with each other. For a high
cutoff frequency, the junction diameter has to be about 5 microns or less; but for high
reliability, the final junction diameter cannot be smaller than the initial junction
diameter before etching. It turned out that a 5-micron diameter area - needed for high-frequency
cutoff - was too small for lead attachment.
Because the Esaki diode was a truly Japanese development, Japanese companies continue
to work with it. To build more reliable devices, some of them have switched to a mask
technique. At Sony, where the device was developed, a process called the bridge technique
was developed, using a combination of evaporated mask and etching methods.
Applications of Passivated Mesa Transistors
Low-drift differential amplifier uses two pairs of twin passivated
mesa transistors. Voltage gain is 40 db; drift is 10 microvolts per degree centigrade.
In the output stage of a home radio. a high voltage passivated mesa
transistor is protected by a silicon varistor.
In the new Sony process, after a germanium slice has been coated with silicon dioxide,
a trench about 20 microns wide is cut in the oxide coating by photolithographic etching.
Then two regions, 50 microns by 50 microns, on each side of the trench are metallized.
An alloy dot bridges the two metallized areas over the trench, forming a junction at
the bottom of the trench and ohmic contacts to the two metallized regions. A final etching
process brings the diode to the desired characteristics of peak current and peak-to-valley
In a diode made this way, the etched junction is only slightly smaller than the original
junction. But the junction does not have to contribute to mechanical support; rather,
the ohmic contact region supports the junction.
Besides being stronger, the new diode has better electrical characteristics. One which
Sony produces has a cutoff frequency of 10 to 21 gigacycles, self resonant frequency
of 14 to 22 gigacycles, and a capacitance-to-peak current ratio of 0.1 to 0.25 picofarads
Other High-Frequency Diodes
In Sony's new method of fabricating tunnel diodes, a dot of alloy
material bridges the trench between two metallized areas. The result is a more rugged
Because of Japan's interest in and use of solid state microwave, there has been a
lot of activity in developing high-frequency diodes for communication systems. Among
the first Japanese semiconductor developments was the Kita diode or silver-bonded diode
developed at the Electrical Communications Laboratory of NTT, and now manufactured by
Nippon Electric Co.
The Kita diode has outstanding characteristics when used as a parametric amplifier,
upconverter or frequency multiplier at microwave frequencies. The reason is the small
capacitance of the depletion layer, typically less than 0.5 picofarads, and a low series
resistance, less than 10 ohms. Although the device was first developed in 1954, its greatest
applications have appeared in the past two or three years. Now new ones are being discovered
in high-speed switching, clamping and clipping.
Making the diode is relatively easy; the big difference is in the method of bonding.
In a conventional diode gold wires are used. In the Kita device, the tip of a silver
whisker, containing a small amount of gallium, contacts a bulk crystal which has been
highly doped with n type germanium or silicon. Applying a large current pulse produces
a very small area of p+ material on the crystal, completing the fabrication of the diode.
As an indication of Japanese activity producing a variety of diodes:
• Nippon Electric Co. produces high frequency Zener diodes with low junction
• Fujitsu Ltd., the Nippon Electric Co., and the Mitsubishi Electric Corp. make
silicon diffused varactors for solid state microwave systems of 2, 4 and 6 Gc. The Mitsubishi
MVE6006 can deliver an output of 3 watts at 4 Gc when used as a frequency tripler. That's
the highest output at this frequency of any Japanese diode.
• The New Japan Radio Co., Ltd., Fujitsu Ltd., and the Sanyo Electric Co. make
variable-capacitance diodes with a retrograded junction, a device which is also called
a hyper-abrupt junction diode. These devices are used as a tuning element which covers
a wide frequency range and as a modulator in f-m communications systems.
• Fujitsu Ltd., has also developed a new gallium-arsenide light emitting diode
that throws a narrow beam of non coherent light through a transparent window at the top
of the mounting. It has been used in a micromanipulator which accurately positions tools
driven by a pulse motor.
Special Purpose Devices
Balanced modulator has a symmetrical twin transistor (in color) built
by the Nippon Electric Co. This configuration has high conversion efficiency and small
carrier leakage because the saturation resistance and voltage are appreciably smaller
than those of diodes used in conventional modulators.
A look at some of the special purpose devices developed in Japan helps understand
both the spread of Japan's semiconductor industry and its electronics industry.
One unusual device is the V-203, a bidirectional twin transistor, built by the Nippon
Electric Co. for balanced modulators. A unique junction structure and a controlled epitaxial
technique produces symmetrical characteristics (see circuit below).
Another device is a high-speed four-layer diode developed by Mitsubishi. A two-terminal
silicon device, it has a break-over voltage of only 3 volts and a switching time of 20
nanoseconds. Most probably application is in fast digital circuits.
And still another new device is the gate-turnoff silicon controlled rectifier produced
by Toshiba. Labeled the M8392, it has a turnoff gain of 8; that is, a gate current of
500 milliamps can turn off a current of 4 amps.
Power Handling Devices
Although both power equipment manufacturers and transistor makers make power handling
devices - silicon rectifiers, silicon controlled rectifiers, and silicon symmetrical
switches (bidirectional four-layer diodes) - the development effort doesn't begin to
compare with that in the United States. In general, SCR's, for example, are expensive
and are not yet used widely. Until recently, Japanese SCR's did not have the large current-carrying
capacities of those available in the U. S. and Europe.
The situation is changing and some new devices supply the strongest evidence. A new
SCR developed by Nippon Electric Co. uses a silicon slice 1% inches in diameter; it's
the biggest SCR developed in Japan. Called the V-179, it has a mean forward current of
700 amps, repetitive peak reverse voltage of 2,350 volts, and a surge current rating
of 9,000 amps.
One not so large is the CJ-021 built by Hitachi for ac-dc conversion in a 2,200 kilowatt
electric locomotive. Ratings of this SCR are: a peak reverse voltage of 1,200 volts and
a mean forward current of 390 amps. Because so much of Japan's extensive railroads net
is electrified, there is likely to be an increased use of SCR's for conversion
and speed control as the manufacturing volume increases and decreases the cost.
Hitachi has one other interesting SCR, the CR-93VE, a small high speed device. It
takes only 3 microseconds to turn on 1,000 amps, and 6 microseconds to turn off 10 amps.
But it can handle 1,000 amps only for short surges.
Silicon symmetrical switches are a specialty of the Shindengen Electric Manufacturing
Co. which makes several series of them. Its KXB series contains two terminal bidirectional
switches with breakover voltages of 100 to 200 volts. The K17B-10 and K17B-20 have a
rating of 150 amps, bidirectional rms current and the K5B can handle 12 amps.
Another supplier is Hitachi, whose FR-01 is a 5-layer switch with one control gate
electrode. A control current, either positive or negative, of 100 milliamps can fire
the switch in either direction, regulating an rms current of 16 amperes.
Still a small part of the Japanese semiconductor industry is the manufacture of high-voltage
rectifiers, capable of handling reverse voltages of 3,000 and 4,000 volts. The Hitachi
HO3-DA has a peak reverse voltage of 3,000 volts and a rated mean forward current of
470 amps. A device made by the Sanken Electric Co. has a breakdown voltage exceeding
4,000 volts; mean forward current is 150 milliamps and the forward voltage drop is only
one volt when maximum forward current flows.
A Neat Packaging Idea
At the Nippon Electric Co., miniature high-frequency
transistors are assembled on rolls of Kovar material to simplify manufacturing and handling.
The transistors are mounted in tiny ceramic headers called Micro Disks which also minimize
parasitic capacitances and inductance created by conventional single-ended packages.
Assembly is simple and automated. Leads are stamped from a continuous flat strip of
Kovar. Silicon dies are mounted on the collector leads and interval leads are attached
between the base and emitter and the leads on the strip. Tiny ceramic disks, recessed
like an ashtray in the center are coated with low-melting glass, then attached from both
sides of the strip. When the assembly is heated, the glass melts and a hermetic seal
is formed. The leads are cut out from the strip and the devices separated from each other
for final testing.
Shindengen makes an avalanche rectifier diode, the S5Z-50, with a reverse surge power
rating of 2.5 kilowatts for 10 microsecond pulses. In the S5Z series, peak reverse voltages
range from 400 to 1,200 volts; mean forward current is about 20 amps.
Any survey of the Japanese semiconductor industry would not be complete without mentioning
several processing techniques which have been developed.
Many of the high voltage devices made in Japan receive a special surface treatment
called ONV, which means oxidation by nitrogen dioxide vapor. The treatment, developed
at the Electrical Communications Laboratory, consists of two processes: cleaning the
silicon surface in an atmosphere of hydrogen fluoride and nitrogen dioxide; and oxidizing
at a low temperature. Such treatment raises breakdown voltage, minimizes leakage current
and steps up the surge power rating.
Though germanium devices far outnumber silicon devices produced by Japanese semiconductor
makers, more research effort is being applied to silicon technology because it is newer.
For example, the Oki Electric Co. has perfected a simple process for depositing polycrystal
The company has made a tiny diode with an upper ohmic contact formed by depositing
polycrystal silicon. The polycrystal material is deposited in a window cut into oxide
masking. During fabrication, it acts as an impurity source for diffusing the p-n junction
beneath it, and afterwards as a protective coating and contact to the completed junction.
This technique supplies a rigid, reliable contact that is simple; no ball or fancy contact
structure is required as it is with many kinds of silicon diodes.
Another application of silicon polycrystal produces isolated silicon islands in integrated
circuits in a process similar to Motorola's EPIC process - but simpler.
In Motorola's process, a silicon crystal is etched to a waffle-like pattern and then
oxidized. Polycrystal is deposited over the waffle-like face; the bulk of the single
crystal material is removed by grinding and lapping until the waffle-like projections
are a group of oxide-isolated islands supported by polycrystal silicon.
In the Oki process, the starting material is a two-layer structure of thin silicon
single crystal on a polycrystal bulk. In the etch that produces the waffle-like structure,
the single crystal is cut down to the supporting polycrystal. The structure is then oxidized
and polycrystal silicon deposited just as it is in the EPIC process. But the original
poly crystal silicon is removed, leaving a group of oxide-insulated islands supported
by polycrystal silicon. What makes this process simpler is that the polycrystal material
is removed easily.
After receiving his Ph.D. from Osaka University, Takuya Kojima (left) started his
engineering career by developing tubes for wideband amplifiers. He switched to semiconductor
devices in 1955 and now heads solid state engineering work at the Electrical Communications
Ever since he graduated from the University of Tokyo in 1953, Makoto Watanabe (right)
has had an interest in semiconductors. As a staff engineer at the Electrical Communications
Laboratory, he develops high frequency germanium and silicon devices.
Posted August 24, 2018