January 1965 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Electroluminescent (EL) devices
were patented by General Electric back in 1938, but it was not
until the 1960s that the fabrication process, involving copper-doped
zinc sulfide (ZnS) as the light-emitting
compound, had developed to the point where high volume production
was feasible. Early EL displays exhibited short lifetimes and
low efficiencies. EL panels are also referred to as light-emitting
capacitors because of their construction geometry. Some of the
first commercial applications for such EL panels were as back
lighting in automobiles. Electroluminescence can also be obtained
in semiconductors in the III-V group class like indium phosphide
(InP), gallium arsenide (GaAs), and gallium nitride (GaN).
Electroluminescence: Theory and Practice
By Lester W. Strock, Ph.D. / Sylvania Electric Products Inc.
The cold light of electroluminescent devices is being
put to ever increasing use in home and industry. Here is the
theory behind them coupled with an explanation of how they are
fabricated into various types of lamps and display items.
The phenomenon of electroluminescence (EL) has made possible
the commercial development of a true area cold light source.
This has been a significant technological advance - both in
the lighting and display-device areas. In the lighting area,
applications are presently confined to those requiring brightness
in the 0.5- to 100-footlamert range. The impact of EL has been
significant in the display-device field because of the wide
variation in sizes and geometrical shapes possible with EL.
Individual letters and numbers are available in sizes from 1/4"
up. Special units of segmented lamps, where the segments are
combined at will by appropriate switching for forming any letter
or number, are commercially available for incorporating into
larger information display boards, such as airline information
areas, for flight position markers, ad computer-fed displays,
for example, in financial market centers.
Decorative wall panel woven from colored
strips of EL lighting tape.
Electroluminescence is the phenomenon whereby light is emitted
from a crystalline phosphor (zinc sulfide in present lamps)
placed as a thin layer between two closely spaced electrodes
of an electrical capacitor. One of the electrodes must be transparent.
The light output varies with voltage and occurs as light pulses
more or less in phase with voltage pulses. They are thus operated
on a.c. with light output strongly dependent on power frequency.
Solid-state modules designed to translate
a four-bit binary code to a numeric-type readout on the front-mounted
Since performance of the lamps is determined primarily by
the characteristics of the phosphor, a detailed description
of EL-phosphor properties and the electroluminescence mechanism
is essential to an understanding of the unique character of
various types of EL devices.
Photoluminescence in ZnS
The two chemical elements, zinc and sulfur, are familiar
materials in science, industry, and the home. Sulfur was known
to the ancients, while zinc was first prepared as a free metal
in 1746. These elements readily combine to form a compound ZnS
(zinc sulfide), which occurs naturally as the mineral zinc-blends
or sphalerite - a major natural source of Zn (zinc) metal.
The addition of small amounts of certain metals to ZnS, as it
is prepared in the laboratory, converts this compound into a
very important electronic and luminescent material. ZnS is the
host crystal for a large family of phosphors. When excited by
electromagnetic radiation of energy ranging from near-infrared
wavelengths (2 microns) down to gamma rays, these phosphors
emit light which collectively covers the entire visible spectrum.
Chemically, the color variations are achieved by progressive
substitution of Zn by Cd (cadmium) and of S (sulphur) by Se
Essential, however, is the addition of much smaller amounts
(0.0001-0.1% range) of specific metals, of which Cu (copper)
is a prominent and practical example, but also including Ag
(silver) and Au (gold) as "activators." Likewise useful as "co-activator"
is a halogen (prominently Cl, chlorine) or some normally trivalent
material like Al (aluminum), Ga (gallium), In (indium), or rare
earths. Methods of preparation also greatly influence the characteristics
of a particular phosphor.
ZnS phosphors have been known for a long time for their response
to cathode rays as well as to 3650 Å ultraviolet excitation.
An electron beam striking the face of a vacuum tube coated with
ZnS phosphor generates the visual image of its path on oscilloscope,
radar, and TV screens. Phosphate- and silicate-based phosphors
are used on the inner walls of fluorescent lamps where the exciting
radiation is of shorter wavelength (2536 angstroms in this particular
Electroluminescence in ZnS
G. Destriau reported excitation of zinc sulfide phosphors
by an electric field as a scientific phenomenon in 1936, but
the light emitted was so faint that some scientists cast doubt
on the existence of the phenomenon. The first practical demonstration
of electro luminescence was given by Sylvania at the I.E.S.
technical conference in 1950. This clearly demonstrated that
ZnS is a very important material having both interesting and
practical electronic and luminescent properties. Thus it is
that an increased knowledge of an old and previously used material
like ZnS has become the essential component in new solid-state
light sources and devices.
It has been amply demonstrated that electroluminescence,
as a phenomenon distinct from photoluminescence, is dependent
on some unique structural peculiarity of the ZnS crystal, as
well as its specific activator composition. This becomes evident
from the fact that the application of thermal and mechanical
stresses can be quite important in converting photoluminescent
to electroluminescent phosphors. These stresses produce strains
within the crystal, forcing a rearrangement of its atoms. For
a brief review of the present theories of electroluminescence,
refer to the boxed copy on page 26.
Light Output of EL Phosphors
The characteristics of EL phosphors are evaluated in the
laboratory in demountable cells. Such cells may present a square
or circular lighted area of 2 to 3 square centimeters. The viewing
side is made of conducting glass plate which serves as one electrode
of the electric capacitor. The powdered phosphor (small crystals,
10- to 30-micron size) is suspended in some liquid dielectric,
such as castor oil, in a ratio of 1 cc. oil to 1 or 2 grams
phosphor. The bottom electrode is made of metal (brass or steel)
in some convenient and substantial design so as to provide a
3- to 5-mil spacing for the oil/phosphor mixture. Both electrodes
are connected to a variable frequency and voltage a.c. power
supply. Voltages in the 50- to 500-volt and frequencies in the
60- to 6000-cps range permit a thorough evaluation of phosphors
for both scientific and industrial purposes.
Flexible electroluminescent lighting tape
can be made in lengths up to 150 feet. Strip is 1/32" thick
with lighted width of 1 1/8".
Electroluminescent numbers are used at O'Hare
Field in Chicago in order to identify flight numbers and arrival
Light output can be measured with a photomultiplier detector
and suitable current-reading instrument. Evaluation of the commercially
significant parameter of life must be made on finished commercial
products, as other factors also influence the life of the device
using EL phosphors.
Initial testing of electroluminescent numeric
Design of EL Devices
Unique features of EL lamps include: (1) their being an area
light source, and (2) their lack of catastrophic failure. Both
features are responsible for their special applications in areas
of novelty lighting, instrument lighting, and the broader field
of display devices. Device applications make use of another
feature of EL lamps, namely their versatility in matters of
size and shape. Being light sources, their immediate function
in whatever circumstances is to render something visible. This
may be accomplished by reflected light, in creating a silhouette,
or by its own luminosity. One thing is certain: an EL device
is a solid-state light source and, as such, be combined with
other solid-state components to provide an all-solid-state system.
The fact that EL light is available from luminous areas of a
few square millimeters to a square meter or more, gives EL a
broad application range from tiny readout lamps to very large
information display boards.
The actual construction of the EL lamp is determined by its
intended application. There are, at present, three types.
Metal-Ceramic & Glass-Ceramic Construction
The major application of the metal-ceramic construction is
in lamps intended as low-level light sources. Their essential
components are shown in Fig. 1. Present commercial products
include: nighttime position markers (Nite Lites); switch plates,
luminous background advertising items; clock faces; telephone,
radio, and TV dials; automobile instrument panels; highway signs
and markers; and decorative panels (in a variety of colors)
for walls, ceiling, and space dividers. This lamp is of rugged
construction and long life (in the neighborhood of 10,000 hours
or more when operated at 60 cps). It can be made to operate
in the range of 50 to 1000 volts to produce surface brightness
from a few tenths to 100 foot-lambert's. Most current commercial
units fall into these types: 120 volts, 60 cps yielding 1 to
1.5 foot-lambert's; 250 volts, 400 cps yielding 5 to 6 foot-lambert's,
or 10- to 30-footlamberts range at 600 volts. By increasing
voltage and frequency (e.g., 600 volts, 2000 cps), brightness
in the neighborhood of 100 foot-lambert's results. No lamp should
be operated above manufacturer's ratings.
Fig. 1. Example of five-layer, metal-ceramic
A. typical brightness-voltage characteristic is shown in
Fig. 2. Increasing the frequency will not harm the lamp, but
unless rated for high-frequency operation, no spectacular increase
in brightness may result. Fig. 3 illustrates, in the case of
a particular EL phosphor, the brightness at three different
operating frequencies, as well as the shift in color toward
blue at higher frequencies. This is a result of the more rapid
rise of output by the blue component in a two-band (blue-green)
phosphor. Since phosphor life is closely proportional to operating
frequency, obtaining high brightness in this manner is at the
expense of life.
Fig. 2. Brightness versus lamp voltage for
a 600-volt EL device. Note also how increasing excitation frequency
The designation metal-ceramic refers to the use of an iron
sheet as one electrode and the ceramic (low-melting-point glass)
phosphor embedment material in its construction. A second ceramic
layer can be placed between the iron plate and phosphor layer.
The second electrode should be transparent. It is applied in
the form of a conducting tin-oxide layer sprayed onto a thin
glass layer over the phosphor layer, which is, in turn, usually
covered by a second glass layer for moisture, mechanical, and
electrical protection. Special spraying techniques for forming
thin and uniform layers have had to be developed by manufacturers
of EL units, and for sintering glass frits into transparent
glass layers. The active phosphor layer of such lamps varies
from about 2 to 8 mils depending on rated operating conditions.
Fig. 3. Effect of frequency on wavelength
distribution of the EL emission of a two-band phosphor peaking
at 4600 and 5200 A.
For space lighting applications, special blends of blue,
green, and yellow phosphors have been incorporated into lamps
for producing various tones of white light. Fig. 4 shows the
frequency response of such a white blend designed for 60-cps
operations (voltage affects color in only a minor manner).
Fig. 4. Light emission in different color
bands of an EL phosphor blended to produce white at 60 cps.
Note frequency effect.
The various slopes for the different color components, as
described previously, cause a decided color shift in emitted
light when a lamp is operated at different frequencies.
Besides the metal-ceramic makeup, glass-ceramic construction
is also employed.
This is the construction favored for lamps used in display
applications, since it provides a flat surface for displaying
information. In it, the active phosphor layer is placed directly
on the conducting surface of the glass plate without an intermediate
ceramic layer. This decreases the diffusion of light within
the lamp and leads to sharper readout patterns. Finally, the
back electrode is advantageously made of an evaporated metal
film which adapts itself to segmentation for the purpose of
applying voltage to selected localized areas of the lamp. This
is done by vaporizing the back electrode through appropriate
masks. These lamps are normally made to operate at 250 volts
and 400 cps to produce an over-all brightness of approximately
Flexible Plastic Construction
In this type of construction (see this month's cover), the
initial electrode is again of metal; normally a thin aluminum
foil. It is first coated with a thin uniform layer of white
high-dielectric material, followed by a mixture of phosphor
in a high-dielectric organic material which is converted to
a tough plastic film on baking at relatively low temperatures.
The second electrode and light-escape side of the lamp is a
thin conducting glass paper cemented to the phosphor layer.
The entire EL unit is then sealed in a moisture and electrical
protection envelope. The assembly is approximately 1/32-inch
thick, with a 1" to 5-mil separation between electrodes. At
120 volts, 60 cps, this construction is a 5-footlambert light
source which increases to 30 foot-lambert's at 120-volt and
This construction is suitable for the production of lamps
in long strips, in widths up to 8 or 12 inches. A commercial
version of this construction is the "Tape Light," recently made
available by Sylvania in 1 3/4-inch widths. The potential application
of this flexible lamp in lengths of many feet would seem to
be great, considering that it will take the shape of a variety
of surfaces employed in contemporary design and construction.
Like the other lamp types, phosphors are available for producing
"Tape Lights" in a variety of colors (green, blue, white, yellow).
The various types of construction just described may be used
primarily for lighting purposes, or in special device applications.
In practice, the display applications are at present largely
confined to the glass-ceramic construction for reasons already
stated. There remains to briefly summarize the manner in which
the operation of lamps is altered, compared to their more simple
use as light sources, when the intended use is as a display.
The problem is to convert the uniform area light into a field
of contrasting brightness so as to present a visual message
to an observer or objective detector. Basically, this means
varying voltage across the active phosphor layer from place
to place in the lamp.
In practice, this means operating the lamp in localized segments
at either of two voltages, namely, at zero (or below threshold)
and some finite value which will excite electroluminescence.
The "on-off" operation scheme is widely employed. The mechanics
of doing this are provided by appropriate segmentation and design
of the back electrode (metal film applied through masks). The
problem is simple if a fixed stationary display is the goal.
But more important, EL devices make possible variable displays.
This adds complexity to their construction since various combinations
of component elements of a design must be selectively activated.
This requires switching of the operating power via wiring circuitry
connected to thin metal films between segments of, at times,
very small areas.
The simplest variable display device is a seven-segment numeric
readout, such as the one shown in Fig. 6A. This is commercially
available to give numbers ranging in size from 3/8" to 8" in
height. Connecting these seven segments electrically (via the
back electrodes of the lamp) in various combinations forms all
numbers 0 through 9. This simple arrangement of segments cannot
center the digit "1." In order to center the digit "1,"a 9-segment
device is available (Fig. 6B) which can be operated as an 8-segment
lamp. The switching arrangement is only slightly more complicated.
A far greater range of readout applications is provided by
a 14-segment lamp (Fig. 6C) with which letters of the alphabet
as well as digits can be displayed. This is the familiar alphanumeric
readout. The added segments, however, substantially increase
the switching, electrode contacting, and connecting problems.
By increasing the number of segments and changing from rectangular
to circular segments, more aesthetically pleasing number and
letter displays are obtained. The familiar 35-dots-per-letter
in a 5 x 7 array is the best-known example. Others have been
developed. The practical problems become formidable with these
multi-segmented lamps, both in circuitry, switching, and in
applying the segmented electrode itself. Display units have
been developed which carry several numerics on one lamp. This
provides space economy, reduces hardware, and provides clearer
Fig. 6. (A) Seven segment readout, (B) nine
segment readout, and (C) 14 segment readout. These are alphanumeric
Fig. 5 shows the complex alphanumeric pattern using the 35-dot
5 x 7 array, along with a sample of the display obtainable by
its use. Increased visibility of display is possible by use
of neutral gray (60-70% transmission) glass in the lamp. Surface
reflections are reduced by application of anti-reflection coatings
to the glass. The use of a glass-surface lamp makes it possible
to apply a honeycomb filter for purposes of reducing the effect
of high ambient light. This, however, restricts the viewing
angle to approximately 30°, but makes it possible to use
it in sunlight.
Fig. 5. A complex display pattern using 35
dots in a 7 x 5 array can be used to produce a complete set
The switching component requirements of EL-display devices
depend on the speed demanded by the application. Mechanical
switching is employed in electronic accounting and scaling equipment
and in production-line status boards. For more rapid switching
requirements and in conversion of intelligence from a computer,
electronic switching is employed through the use of silicon
controlled rectifiers, neon-photoconductors, reed switches,
and transistor circuits.
With the versatility in size, geometrical form of information
display elements, and switching speeds now available, the possible
applications of EL-display devices seem almost unlimited.
Producers of EL devices naturally stimulate those interests
by continued assistance and developments. Efforts are continually
being made to develop phosphors and lamp structures which will
substantially increase the brightness of present EL light sources,
so that lamps may eventually be as bright on domestic power
operation as now obtainable at higher voltages and frequencies.
Brightness up to 100 foot-lambert's, sufficient for ceiling
and wall panels incorporated into the architectural structure,
are not unrealistic expectations to those engaged in product
development, and perhaps more so to those of us in basic research.
Review of Present Theories of Electroluminescence
A ZnS crystal consists of alternating layers of zinc and
sulfur atoms. Much lower fields (factor 10 or more) are required
to produce light when the field is applied parallel to these
layers. Since high conductivity also exists parallel with the
"easy" EL direction, light emission in this direction is evidently
dependent on the flow of external electrons. In the perpendicular
direction (called the c-axis) light emission involves displacement
of electrons already present in the crystal. An EL crystal thus
seems to emit light by two mechanisms: (1) low-field/high-current
for a field parallel to atom layering or perpendicular to crystal
c-axis, and (2) high-field when the field is perpendicular to
This directional dependence or anisotropy (different properties
in different directions) has been explained by the author as
resulting from an isolated atom displacement disorder. Such
a disordering of the otherwise regular repetition pattern of
Zn- and S-atoms of the ZnS crystal causes a sulfur atom to move
under the influence of local strains into a nearby and alternate
(but unoccupied) position of the crystal lattice. As a result,
a covalent chemical bond (electron pair) by which this displaced
sulfur atom was attached to one of its normal four Zn-atom neighbors
has been broken. The geometry of the immediate site, after creation
of this disorder, is such that a single charged copper ion (Cu+'-ion)
can be exactly fitted into the space next to the broken band
on the displaced S-atom. This "quasi"-free Cu+1-ion
restores a more normal local energy situation.
This seemingly trivial matter of moving an isolated S-atom
for a very short distance of about 4 A (angstroms) to a neighboring
site, followed by entry of a Cu+1-ion adjacent to
it, provides a model useful far developing the details of a
light-emission mechanism from an entirely different viewpoint
than previously attempted.
ZnS crystals are typical "wide-band-gap" semiconductors,
and previously proposed theories have been based largely on
energy-band-gap models. The band gap of ZnS is 3.76 electron
volts (ev.). This means that energies of this amount are required
to excite electrons associated with atoms on lattice sites of
the crystal (valence band) to the conduction band where they
may move for a very short time about the crystal as "conducting"
electrons. Activators in a ZnS phosphor add new levels in the
"forbidden" gap of ZnS at levels up to 1 ev. above the valence
band. Since light absorbed by the activators is normally of
shorter wavelength (higher photon energy) than that emitted,
the phosphor has served as a wavelength or color converter.
In the case of the ZnS:Cu-Cl EL phosphor most widely used, absorption
of 3650 A leads to emission of two wide bands: the blue peaking
at about 4600 A and the green peaking at about 5200 A. These
represent electron transitions of 2.70 and 2.4 ev. between activator
levels and trap levels a few tenths of an electron volt below
the conduction band.
Although satisfactory for the description and calculation
of photoluminescence phenomena, the energy-band model has not
been able to treat all of the features of electroluminescence.
Mainly there is the question of how the energy of the electric
field, applied to lamps at levels of 50 to 100 volts per mil,
is transferred to the phosphor to cause light emission. Even
if the prime characteristic of electroluminescence is just a
special mechanism for triggering off photoluminescence which
then proceeds via the collision-ionization mechanism commonly
accepted in electroluminescence theory, the question of the
source for initial electrons remains. They do not seem likely
to be created by direct-field ionization of luminescence centers
because this will require fields acting on individual Cu activator
atoms, e.g., to produce blue emission, in excess of that generally
available by a factor 2280 in case of a 1-mil-thick device operated
at 120 volts. To produce blue emission, energy in excess of
2.7 ev. must be applied across a Cu-atom of 2.7 A diameter representing
a field of 1 volt/A or 108 volts/cm. The available field is
actually only 4.4 X 10-4 volt/A. Concentrating the
120 volts available into a very small fraction of the active
layer thickness; e.g., 120 A will produce a field adequate for
direct ionization for purpose of supplying the initial electrons,
which are then accelerated to velocities sufficient to collision-ionize
other centers. The assumption of the existence of such barriers
has been a prominent feature of previous EL theories. The field-release
of electrons from donors or from traps a few tenths of a volt
below the conduction band has been proposed in order to account
for the release of electrons at low values of field intensity.
Atom-Ion Pair Model
It is also possible to explain the above-mentioned anisotropy
of EL emission by the isolated atom displacement model. Its
essential feature is that in the EL center there exists an atom-ion
pair oriented with its join line perpendicular to crystal c-axis.
EL emission results when an electron oscillates between the
atom and ion in phase with an a.c. field.
The extent to which this electron exchange process produces
light directly by optical transitions and indirectly by virtue
of furnishing electrons which escape the center into the surrounding
host crystal, where on acceleration they ionize normal photoluminescence
centers, is still unknown. Assuming the essential correctness
of the EL-center model, the chemical entity of the atom-ion
pair will determine the color of EL emission. If some electrons
escape from the center to trigger off the characteristic photoluminescence
of the host crystal, the light emitted will be a composite of
the output of the two mechanisms involved.
EL and Photoluminescence
The close similarity of some bands of light emitted by an
EL phosphor under ultraviolet light and field excitation has
long been a point of interest which, on the basis of the model,
indicates that a substantial portion of electrons from the center
do escape to the surrounding crystal, especially at low frequencies.
However, as frequency increases, the number of electrons escaping
the center will decrease, confining. light emission more and
more to the primary EL mechanism, with less from the host crystal.
That this does happen receives support from the known frequency
characteristics of the widely used blue-green ZnS:Cu-Cl EL phosphor.
Here, the blue component emission increases steadily with frequency
and at a much faster rate than does the green component. Further,
green emission soon reaches a plateau. In line with the theory,
the blue component is the prime EL emission generated by the
EL center; while green is photoluminescence originating at luminescence
centers of the surrounding crystal.
Attempts to convert yellow photoluminescent phosphors to
EL phosphors have revealed that frequently any EL emission created
is blue. Most EL yellow phosphors contain a blue component.
In this case, electrons do not escape to yellow centers in the
surrounding crystal. The study of EL phosphors in the laboratory
is thus a very essential step in the development of commercial
Posted February 27, 2014