LEDs get brighter and more colorful

LED.NOV–Dialight–pm

LEDs get brighter and more colorful

Recent gains in LEDs show brightness up, power consumption down, and the
elusive blue finding its way to the market. Many of these gains will be at
the expense of incandescent and neon lamps

BY GARY DURGIN Dialight Corp. Manasquan, NJ

LEDs consume less power, are smaller, and have better than a 50-fold
improvement in life expectancy over incandescent and neon lamps. They also
have high resistance to shock, vibration, moisture, and temperature
extremes, with good visibility even in direct sunlight. It is no wonder
that more and more LEDs, in a sparkling array of colors, are performing
functions previously handled by incandescent and neon lamps. Since their
introduction in 1969, LEDs have found their way into automobiles, indoor
and outdoor large-area advertising and information displays, railroad
crossing signals, and expressway signs. Electronic and computer
applications range from portable radios to mainframes, with LEDs mounted
on pc boards in discrete, surface-mount, or value-added board-indicator
arrays. From the beginning, automakers were eager to replace the
incandescent bulb from dashboard to taillight with bright LEDs that last a
minimum of 100,000 hours (11 years). The enormous automotive marketing
potential triggered fierce and determined development by manufacturers to
come up with brighter, more efficient LEDs. Such aggressive activity has
indeed spurred a marked improvement in high-brightness red, green, amber,
and yellow LEDs, with blue still lagging despite intensive efforts in
materials research. Discrete LEDs can have their light output directed to
the desired panel location by bending their leads. Surface-mount LEDs must
include secondary optics, such as light pipes, to guide the light path,
which adds the cost of an extra component and its assembly. Recently
introduced right-angle surface-mount LEDs, such as those offered by
Dialight, eliminate these drawbacks by combining the LED and its optics in
one tiny package. Manufacturers around the world are actively producing
LEDs in a wide variety of sizes, shapes, and colors as well as specials
for flame-proof and waterproof applications. Leading U.S. makers include
QTC, Dialight, Hewlett-Packard, and Cree. European and Far East firms
include Siemens, Stanley, Liton, and Rohm.

Incandescents and neons Of course, the LED's competitors have their
advantages, too. The incandescent lamp consists of a thin filament mounted
between electrodes in an evacuated glass bulb. When connected to a current
source, the filament is heated to its incandescence: the point at which
radiant energy or light is emitted. To improve efficiency and spectral
response, a small amount of inert gas is added to the evacuated bulb. The
size, length, and material of the filament can be varied to accommodate a
wide range of applied voltage and power requirements. At present, the
incandescent lamp is brighter than neons and LEDs. Another major advantage
of the incandescent lamp over its neon and LED rivals is its wide spectral
response, allowing its use with filters or lenses to provide any color
display. The incandescent lamp's disadvantages begin with its being a
filament that is steadily burning itself out. Its relatively limited life
span is further reduced when the applied voltage rises; a 10% applied
voltage increase slices the expected life of a 1,000-hour bulb to less
than 400 hours. Unexpected increases in temperature also markedly
influence the component's reliability. Because the delicate filament is
mounted on a relatively weak structure, the incandescent bulb is subject
to damage from excessive shock and vibration often experienced in portable
equipment. And the relatively short lifespan of an incandescent, typically
several thousand hours compared to over 100,000 for LEDs, dictates the
need for sockets to allow for the anticipated replacement. Incandescent
lamps with over 100 combinations of voltage, current, and light output are
offered by such firms as GTE, Chicago Miniature, and Oshino with base
types including screw, flange, wedge, and wire terminal. Neon glow lamps
are a popular on/off indicator in small home appliances like coffee makers
and irons. They consist of a pair of electrodes mounted within an envelope
containing neon gas. When a high voltage is applied through a
current-limited resistor, an arc is created that ionizes the gas and
produces a reddish-orange glow; the addition of a small amount of mercury
changes the light output to a bluish tint. The breakdown voltage for
high-brightness neon lamps ranges from 95 to 135 Vac, suitable for
line-voltage operation using a small dropping resistor.
Conventional-brightness neon indicators are available for operation from
either ac or dc, with breakdown voltages extending from 65 to 90 V. Neons,
because of their relatively sturdy electrode structure and absence of a
thin filament, are less prone to shock and vibration failures than
incandescents and have a life expectancy approaching 25,000 hours.

Indicator assemblies and lenses Incandescent indicator light assemblies,
with connection terminals at one end and a lens cap at the other, come in
round (since round holes are easier to punch) and square shapes for
aesthetics. A variety of front- and rear-panel mounting configurations
have been devised to allow fast bulb replacement when required. Lenses–to
concentrate or diffuse light output–may be integrated into the indicator
body or offered as a separate assembly that either screws into or snaps
onto the indicator body. Lenses come in a variety of shapes–ranging from
dome, stovepipe, torpedo, and cylindrical–to maximize light output and
side visibility (see Fig. 1). When low profile is desirable, flat, convex,
or faceted lenses are employed. Filters are best suited to incandescent
lamps, which cover the visible spectrum and thus allow any color filter or
tinted lens to be placed in front of it. When filters are placed between a
lamp and its lens, color appears only when the lamp is energized. A
translucent rather than transparent lens is often used with large
incandescent lamps to conceal the filament structure when the lamp is not
operating.

LEDs brighten up
The LED (see Fig. 2) is a simple semiconductor device offering
advantages of small size, shock and vibration resistance, as well as fast
switching speed (up to 100 MHz). When electrons in the n-region of a p-n
LED diode recombine with a hole near the junction area, light is generated
at a wavelength determined by the difference of energy levels during the
recombination process. This, in turn, depends on the semiconductor
materials used. Early calculators and watches used GaAsP to produce red
(655 nm) displays. More efficient devices evolved as a mixture of GaP,
rather than GaAs, was used as a substrate to improve transparency. GaAsP
produces a wavelength of 650 nm (red), 560 nm (green) for GaP; SiC
delivers 470-nm blue, while infrared wavelengths of 940 nm are created by
GaAsP and 880 nm with GaAlAs. Moreover, a range of colors can be achieved
in GaAsP LEDs by altering the ratio of arsenic to phosphor and the doping
of the epitaxial layer. Improvements in efficiency have also resulted
from the conversion from the traditional vapor-phase process to a
liquid-phase epitaxial process. High-efficiency blue LEDs, using various
semiconductor materials, including ZnS and ZnSe, are still elusive because
of the high-resistance p-n junctions created, which dictate the need for
high operating voltages. Recent efforts for blue LEDs are concentrated on
SiC, which has a bandgap less efficient for light generation, but is more
easily doped. Once all three primary color high-efficiency LEDs–red,
green, and blue–are available, full-color RGB display using LEDs can be
created in the same manner that a three-gun CRT produces color TV
pictures. LEDs come in a wide variety of packages (see Fig. 3). The 3-mm
(T-1) and 5-mm (T1-3/4) lens diameters are by far the most popular. These
types may be diffused to offer a wide angle of radiation (+/- 35 degrees).
For direct viewing, or a non-diffused narrow-angle (+/- 12 degrees) for
back lighting. Low-cost LEDs provide a monochromatic (one-color) output
(red, green, yellow, and amber) determined by the semiconductor chip it
contains. Two LED chips in the same two-leaded package as the single LED
are available with a combination of red/green connected in reverse
parallel. When biased in one direction, the first color is emitted. The
second color is emitted when the voltage is reversed. By applying an ac
voltage and varying the duty cycle, color combinations can be achieved
between the two LED die. The drive circuitry can be simplified by going to
a three-leaded, common-cathode LED. These bicolor LEDs are particularly
useful where board space is at a premium. One LED can display three
different states: red, amber, and green. Panel and system designers are
no longer restricted to square and round holes on front panels to gain
access for bulb replacement when LEDs are employed. Aesthetically pleasing
enclosures using backlighting can be fashioned with pc-board-mounted LEDs
and the newly offered right-angle surface-mount LEDs. Since LEDs are
available in a variety of colors, they need no filters like those used
with incandescents and neons. LEDs usually include lenses to maximize
light output. Until very recently, most of the progress in very
high-efficiency LEDs have taken place in the red and infrared materials.
GaAlAs material was developed over the last eight years to the point where
cost-effective and reliable red LEDs are offered with luminous intensities
exceeding 1,000 mcd. Transparent-substrate versions (TS GaAlAs) are now
available in red with intensities of over 15,000 mcd. Many automotive
manufacturers have already replaced the incandescent lamp used in the
center brake light with these super-bright LEDs. The breakthrough in
super-efficient materials beyond red arrived early this year with
Hewlett-Packard's introduction of its AlInGaP material (see Fig. 4). HP is
currently offering an amber (590 nm) device delivering over 8,400 mcd.
“Amber LEDs create additional opportunities–for example, red LEDs can be
used for the third brake light, while amber LEDs can be used for turn
signals. Different colors add meaning,” states Hannah Suen, of HP's
Optoelectronics Div. Stanley Electric's Motohito Watanabe indicates that
his company is pursuing AlInGaP to develop high-brightness yellow (590 nm)
and green (565 nm) LEDs for availability in the near future. Rohm Corp.,
according to its optoelectronics project manager, Ray Ponkey, is also
developing an AlInGaP amber LED to produce 2,000 mcd with a 10 degrees
viewing angle targeted for the automotive and outdoor display markets. A
major thrust at Rohm, according to Ponkey, is the use of metal organic
chemical vapor deposition (MOCVD) laser technology to grow LED die. The
goal is to achieve LEDs with over 1,000-mcd output in all colors at 20 mA;
wavelengths will include 700, 670, 635, 585, 565, and 555 nm. Advantages
of the -MOCVD, adds Ponkey, include large 8-in. wafer processing, slower
crystal growth for better yield, and greater control over optical
characteristics. Efforts in blue LEDs at Cree Research have resulted in a
line of SiC devices with typical luminous intensities of 20 mcd. Stanley's
Watanabe says that attention is being directed at materials with direct
transition light-emission structures such as ZnSe and GaN for
high-brightness blue LEDs. He predicts such devices will be available by
the mid 1990s. The steady progress in LED efficiency over its 25-year
growth offers not only higher brightness levels, but also the option of
operating the LED at much lower power to achieve adequate output (see Fig.
5). Thus, designers can pack more LED chips into a tiny package to display
greater amounts of information. For example, recent bright amber and red
products can operate below 0.5 mA and provide the same intensity as a
10-mA LED. An exciting application for such devices is for displays in
cellular phones to replace hard-to-read LCD counterparts. Although the
LED may appear deceptively simple, many users run into design and
manufacturing problems. The aesthetic appearance of the device is
extremely important to the OEM because it is usually an important
marketing feature of their equipment. Among the common problems
encountered when a piece of equipment uses an array of LEDs are mechanical
alignment and the matching of intensity, hue, and epoxy tint. The matching
problem is particularly bothersome because many of the parameters, such as
epoxy tint, cannot be specified on the data sheet. The industry lacks
exact second sources, and the user must be careful about mixing vendors.
Several companies, including Dialight, have addressed the matching issues,
provide devices with housings to ensure proper mechanical alignment, and
assist the user on the proper application of LEDs. These value-added
suppliers can often substantially reduce the time to market because of
their technical expertise in the application of LEDs and in-house plastic
tooling and molding capabilities.

Surface-mount LEDs
Designers moving from through-hole to surface-mount component
counterparts find a wide selection when they seek resistors, capacitors,
chokes, and other components. Until recently, this has not been the case
for LEDs. The development of surface-mount-compatible optical devices has
been hindered by the need for optical materials that can withstand the
high vapor phase (215 degreesC) and IR convection (260 degreesC)
temperature stresses involved in surface-mount assembly. In addition, to
be compatible with tape-fed automated pick-and-place machinery, the
surface-mount LED requires flat surfaces rather than the conventional
rounded bodies for device pickup. A pressing requirement by customers has
been the insistence for post-process quality at a 50-ppm level, because
incoming inspection of tiny tape-and-reel supplied parts is impractical.
Also, the reworking of expensive, densely packed surface-mount boards
merely to replace a defective LED is unacceptable. Last year, Siemens and
Hewlett-Packard co-developed a surface-mount LED following a B-type
tantalum chip capacitor outline and footprint, with termination pads
similar to those on a chip capacitor (see Fig. 6.)
The LED chip sits in a well that directs the light output to a small
reflector lens. Light is emitted in a 120 degrees cone perpendicular to
the circuit board on which the LED is mounted. However, many applications
require the light output be directed toward a front panel. In through-hole
applications, it is relatively simple to route a long-leaded discrete LED
through openings to achieve the desired goal. Board-mounted surface-mount
LEDs need some means to direct the light output to its intended location.
One solution is a light pipe, using a section of a clear material to guide
the path of light from the surface-mount LED to its panel display.
However, this involves an additional component and assembly step, upping
production costs. Another approach to a right-angle surface-mount LED by
Dialight relies on a prism-type lens formation to bend the light path (see
Fig. 7). The device uses a surface-mount LED chip with its flat top and
sides mounted on a base of a long-chain polymer epoxy that can withstand
the thermal stresses of surface-mount assembly. The red AlGaAs (645 nm)
unit delivers 29 mcd, while the yellow (590 nm) and the green (567 nm)
versions offer 5 and 6.5 mcd, respectively, at 10 mA. All types are
available in tape-and-reel format. On-board diagnostics and backlighting
of front-panel legends are the major applications for such devices.

BOX

Light-output terms and factors

Luminous intensity is a measure of the light output produced by a light
source. Candlepower, previously used as the standard unit of measurement,
has been replaced by the candela. One candela is the amount of light that
shines through a 1/16-sq-cm hole in one side of a ceramic box that has
been heated to 1,772 degreesC. For a given candela, lumen and foot-candle
values can be calculated. The lumen measures the amount of energy in a
beam of light shining on a surface. The relationship between these terms
is as follows: a 1 candela light source produces a 1 lumen beam of light
which results in 1 foot-candle illumination on a 1-sq-ft area located 1 ft
from the source. The intensity of small light sources, such as LEDs, is
measured by looking directly at the light source on-axis; this point
source intensity is measured in millicandela (mcd.) Large-area light
sources are measured by total light output because it is difficult to
define the peak on-axis point. Measurement units are typically in foot-lamberts or candela per meter squared, with the relationship: 1 foot-lambert =
3.4 cd/m2. Some other points relating to luminous intensity
include: 1. The light intensity in mcd must double before the human eye
detects a noticeable difference. 2. Since the mcd measurement is angular
dependent, the viewing angle must be given when specifying luminous
intensity. A 10-mcd device, with a viewing angle of +/- 35 degrees, may
have the same total light output as a 200 mcd device at +/- 8 degrees,
and 3. Within a relatively broad range, luminous intensity is almost
linear with forward current. For example, a 10-mcd device at 10 mA reads
20 mcd at 20 mA. The viewing angle of a light source refers to its
relative output versus viewing angle displacement from its optical axis;
it is defined as the angle at which intensity is one-half its on-axis peak
value. Lenses, reflectors, and diffusers can be added to a display device
to modify its viewing angle. Spectral distribution relates relative light
output to wavelength. The human eye responds to wavelengths from violet
(400 nm) to red (700 nm) with the sensitivity peaking in the green (550
nm) region. Finally, to distinguish between the terms peak and dominant
wavelength, consider a device such as a green LED. The peak light output
by measurement might occur at 566 nm and this is the peak wavelength.
However, the eye, being an imperfect detector, might sense 569 nm as the
point at which maximum light occurs; this is called the dominant
wavelength.

CAPTIONS:
Fig. 1. Lenses come in a variety of shapes to focus or
diffuse light output.

Fig. 2. The LED is a simple semiconductor device offering advantages of
small size, shock and vibration resistance.

Fig. 3. LEDs are available in a wide variety of colors, sizes, and
configurations.

Fig. 4. Designated the HLMA series, the AlInGaP amber LEDs reach output
levels of up to 8.4 cd at 20 mA.

Fig. 5. The steady progress in LED efficiency has resulted in a much
greater light output for a given input current.

Fig. 6. A joint effort by Siemens and Hewlett-Packard resulted in
surface-mount LED following a B-type tantalum chip capacitor outline and
footprint, with termination pads similar to those on a chip capacitor.

Fig. 7. Another approach to a right-angle surface-mount LED by Dialight
relies on a prism-type lens formation to bend the light path.

The following companies supplied information for this article. For more
information, call the company or circle the reader service number.

Cree Research, Inc. Durham, NC Neal Hunter 800-533-2583

Chicago Miniature Location Contact and phone

Dialight Corp. Manasquan, NJ Joe Hurley 908-528-8961

GTE Location Contact and phone

Hewlett-Packard Co. Optoelectronics Div. Hannah Suen

Liton (spelled right?.)

Oshino Location Contact and phone

QTC Location Contact and phone

Rohm Corp. Rohm Electronics Div. Antioch, TN Ray Ponkey 615-641-2020

Siemens Components Optoelectronics Div. Cupertino, CA Rick Waltonsmith
408-725-3423
Stanley Electric Location Motohito Watanabe