This is PAGE 1 of the 1970s exhibit. Go here for PAGE 2.
1970-1979: THE LED COMES OF AGE
This is an unopened package containing an unused General Electric SSL-190 numeric (digital) LED display that was most likely made in the early to mid 1970s.
Most LED displays of this time used several very small, individual LED chips inside each segment. A display like this would have been common to see
in LED calculators and LED wristwatches. The digits themselves were quite small, so a small bubble magnifying lens was mounted above each digit.
While this limited the viewing angle, it did make the numerals large enough to be easily discernable.
Then along comes this critter. Instead of having many individual pinpoint LED chips in each segment, this display uses a single, very long and narrow
LED chip for each segment.
This is an extreme closeup of one. I stripped the color information from this picture because the details showed up much more clearly without it.
You can see the hair thin bond wires attaching to each chip in two places. The top ohmic contact is a strip that runs the entire length of each chip, with a bond wire
attaching at each end. Presumably, if any one thing went wrong inside one of these, the display would still be fully functional and not have to be replaced, which would have
been an expensive proposition back then. Even the LED used for the decimal is connected by two bond wires.
The display on the left is not lit; you can only see the top ohmic contact strip on the surface of each segment's LED.
The display on the right is lit. The purplish tint is caused by the camera not rendering red correctly. These displays were most likely GaP (Gallium Phosphide) types; the red lens
not only filters out the green emission, but improves the overall contrast of the display making it easier to read. This is still done to this day.
All photos of the SSL190 except the package itself courtesy of Paul Schick..
Now why would somebody put a visible red LED in an expensive TO-18 can?
These metal cans are normally associated with invisible LED types, such as infrared, and modern ultraviolet models.
I don't know the answer here, but maybe Hewlett-Packard does. This LED is marked "HP 2-4405 308" on the outside
of its gold-colored can, which indicates it's clearly a Hewlett-Packard part, but the date is not known. Construction appears to be 1970s technology, but there's no way I can pin down the exact year without additional information.
(Update 02-03-06): I received an email from a website fan containing the following information:
I'm originally from Hampton, VA and lived next to NASA Langley while growing
up (50+ now). My father's company once contracted to make some very special
capacitors for the space program and they were mostly gold and platinum for
reliability. I would guess that the early space program might have used
TO-18 cans for indicator diodes for reliability. It's just a guess, but
maybe someone from NASA could shed more light on the subject (pun intended,
The red glowing LED chip inside is your classic "original formula" GaAsP on an opaque GaAs substrate, and it seems to use an early variation of the center ball bond like more modern LEDs
Here's a closeup of the LED's die. It really is transparent blue, like the sapphire used for making gemstones. I've never seen a red LED like it, and am now at a loss to
explain its operating chemistry.
This is the LED die when lit at a very low current level. It's not broken, but it appears to be completely enclosed inside an air bubble.
Spectrographic analysis of this LED.
These Motorola axial LEDs came from the early 1970s. The model shown here is a rather
weak IR model, emitting somewhere around 900nm. Even when driven hard, they barely illuminate a
chemical IR detector card, and don't get "bright" enough to overload my digital camera to the point of image blooming like any modern IR LED does.
These small LEDs have a wide emission angle, and probably served as part of a coupled emitter/detector pair , not unlike the devices
one might find in the encoder wheel assembly for a trackball. A slotted wheel would be positioned between the LED and detector,
and when turned, a circuit counts how fast the beam is being broken & reestablished by the turning wheel.
A set of two IR LED/IR detector pairs would be used in a trackball; one for the X axis and one for the Y axis.
These could have also been used as tape detectors in a magnetic tape based device - video, audio, or data storage; and to determine
the position of movable elements inside a piece of machinery.
And these green Motorolas are the visible light counterpart to the IR model shown above.
These too are quite dim, but would be bright enough to be seen on panels or on computer PCBs in areas with low to moderate illumination.
A high percentage of these LEDs did not work. Some have only pieces of the die on the slug, others have no LED chip at all inside, and
others have the LED chip mounted on the wrong side of the leadframe - so the light comes from the bottom of the LED instead of out the top!
Whoever was responsible for quality control where these were being made must have been an eggroll short of a poo poo platter.
When these LEDs were made, the LED dice were purchased from Monsanto, and the parts were molded into LEDs in East Phoenix at 52nd St. and McDowell.
Red axial lead LEDs similar to these were also made; I'll have a couple of more pictures showing them soon.
In the early 1970s, a lot of lessons were learned.
The lesson taught to the people who made this wide viewing angle red LED was "don't go overboard".
The die is bonded to a Kovar transistor header, and a glass to Kovar hard-seal bonded the LED leads.
The leads and all the guts also appear to have been gold plated.
Because these Motorola parts were too expensive to make, very few, if any, were ever sold.
Early 1970s Motorola MOR-10 7-segment displays
The first of these three pictures of a completely new & unused unit show
the date code 7231, indicating these are 29 years old and in this particular
case, having never tasted electricity until last night when these pictures
The common cathode is smack in the center; all of the leads on the outer
edge are anodes for the individual segments & decimal point.
This was also a VERY expensive part to make. Notice all of the dice used
per segment; each "dot" is an entirely self-contained, individual LED
enclosed in an intricate mask; this mask is supplied with a single, common bond wire.
The picture on the left shows the unit in use, displaying a "4".
These two LEDs are very early green models.
There is no bond ball whatsoever on the LED shown on the left - looks like they just laid the wire on the surface
of the die, smashed it down with tweezers (or other similar instrument), and dumped plastic all over it.
The crude anvil and the careless manner in which the die is mounted is identical to the 1969 red LED from England you saw in the last exhibit.
The green LED on the right was actually assembled using a broken chip. Fully 5% (or even more) of the chip is totally missing, and this apparently occured after the top bond was attached, as part of it is also missing in the same area. LEDs were so expensive to make though, the broken (but still functional) LED was assembled anyway, and sent on its merry way.
Here's another view of that green LED without a ball bond or ohmic contact. The wire looks like it was just squashed onto the face of the LED's light emitting crystal.
And the same green LED when lit. There is an awful lot of reds and oranges coming from the sides of the chip; this was not only evident in the picture, but with my
own eyes as well.
Spectrographic analysis of the unbroken yellow-green LED.
Same as above; newer spectrometer software & settings used.
USB2000 spectrometer graciously donated by P.L.
Here is an orginal chemistry (gallium phosphide on gallium arsenide) red with an unusual chip carrier. The die appears to be an elongated
shape, but it looks more like a mask formed by the top ohmic contact - used in conjunction with a true ball bond.
Best guess: they attached the bond wire to the contact ring to avoid cratering and resulting LED failure.
It worked - every specimen I have seen is fully functional to this day.
This LED appears to be an early 950nm gallium arsenide LED from the very late 1960s to early 1970s.
It is very dim (low output) and just to get this picture, it was cranked up to well over 70% of its rated input current.
Most modern IR LEDs overload my camera with just 1% or 2% of rated current.
This appears to be one of the earliest examples of large junction LED technology.
While most "ordinary" LEDs have emitting chips (dice) that measure about 100 to 150 microns on each side, this one appears to
be approximately 3,000 to 4,000 microns on each side!
Forward current of this early giant is said to be 1A continuous, and is fed to the large chip by three bond wires.
Each wire connects to a long, narrow contact extending the length (or width) of the chip surface.
Unfortunately, size alone does not give you a brighter LED.
At 200mA (the highest my old power supply will provide) the LED probably produces around 30-50mcd of light, or the equivalent
of a good strong semi-diffused panel LED from more modern equipment. But like almost all specimens in this museum, it does work. :)
Chemistry appears to be GaAsP on GaAs, this was determined both spectroscopically and by physical appearance of the chip (jet black when off - no transparency observed whatsoever, and a pure,
deep red color when on).
The part number for this suspected Monsanto LED was MV4 or MV4H.
This is PAGE 1 of the 1970s exhibit. Go here for PAGE 2.
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